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W460 Fuse Box Upgrade

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Suppatime
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Re: W460 Fuse Box Upgrade

Post#11 » Mon Apr 08, 2013 6:13 pm

Flanker525 wrote:Take a close look at your fuse box with a tester and see if you're getting the correct voltage across all the fuses.

I had a lot of the same problems that you're describing (Along with also having an incompetent previous owner....) Replacing the entire fuse box solved most of the problems and it took me less then an hour to do.

viewtopic.php?t=2598


Thanks Flanker. It seems we've shared similar electrical issues.

I'm thinking corrosion is a significant part of the problem. I've hit it several connections with Electrical Contact Cleaner and replaced all my fuses. But I don't exactly have a benchmark of what voltage I should be getting.

My electrical modifications to date are: Optima Red-Top Battery (900CCA), rubber-insulated 00 gauge battery ground with 1/2" connectors for starter motor and chassis (with a new tap and died chassis bolt -- the old one was seized and PB Blast + torch did nothing) and Bosch 115A AL-129X alternator (OEM on Saab 900) with the pulley swapped from the original Bosch 300GD alternator.

This is where I'm at with the fuse panel:

(L to R):

01) Blue 25A
02) White 8A
03) White 8A
04) White 8A
05) White 8A
06) Blue 25A
07) Red 16A
08) Red 16A
09) Blue 25A
10) Blue 25A
11) Brown ? (can't read it, don't know brown fuse specs as it's illegible)
12) Blue 25A

Engine Off, no key:

01) 11.5
02) 11.5
03) 02.1
04) 02.2
05) 10.9
06) 10.9
07) 11.8
08) 11.9
09) 00.0
10) 01.9
11) 01.9
12) 01.9

Engine On, headlights off

01) 00.0
02) 00.0
03) 00.0
04) 00.0
05) 00.0
06) 00.0
07) 13.8
08) 13.8
09) 13.8
10) 13.8
12) 13.8

Engine On, headlights on (low beam)
01) 12.9
02) 12.9
03) 02.4
04) 02.4
05) 12.2
06) 12.2
07) 13.1
08) 13.1
09) 13.1
10) 13.1
11) between 4.4 and 13.1; jumps constantly
12) 13.1

Current electrical issues: VDO clock is IN-OP, heater / air fan is IN-OP, rear fog is IN-OP, rear windshield defroster is IN-OP, gauge cluster turn signal lights work intermittently, low beam / high beam issues (seems to have resolved itself this afternoon... both sets work, though R-side city lamp is IN-OP).

This is about the extent of what I know how to do with vehicle electronics. I've found multiple stab-ins on positive connections at the headlight low beams, the VDO clock and the stereo. I've pulled all of them off and electrical taped the wires and it seems to behave better now (as many of these stab-ins had lose uncapped wires hanging all over the place... a butchery). I'm still having issues (though they've subsided) with my multigauge temp display and fuel level (both spike -- 1/4 up though sometimes pegged in the red for temp, 1/5 increase in fuel reading) with my headlight low beams turned on.

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vadimivanovich
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Re: W460 Fuse Box Upgrade

Post#12 » Mon Apr 08, 2013 8:38 pm

Right on Drew - getting organized!

I'd have to check my G to validate what you've measured but some of those low voltages seem fishy. Everything should be more or less 12-14V. You'll get some voltage drop across heavily loaded circuits but nothing major. Try an OEM fuse box first. Cheap & easy (relatively). I'm still suspecting a ground problem too, but you have to try something.

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bratwurst
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Re: W460 Fuse Box Upgrade

Post#13 » Mon Apr 08, 2013 9:52 pm

OK I'm taking a look at the wiring diagram and I'll try to analyze it a bit. No guarantees I'll be accurate if any EE's want to chime in to correct me I'll accept the help. I know enough to be dangerous. :)

In general, the fuse block should show either roughly 12v or 0v. By 12v I mean either 12.6v with engine off, and about 13.8v when running.

So looking at the "weird" voltages:

Fuse 3+4 are for the headlights. They are shorted together on hot side, therefore they are going to show the same voltage.

Fuses 10,11+12 are shorted on the hot side as well. Again, same voltages. I'm too lazy to go over all the traces, but the horn, fuel pump, and windshield cleaner nozzles are powered from these.

I'm going to guess fuse 10,11 and 12 should be at 0v when the motor is off. These fuses are connected directly to the ignition switch. I think these are switched by the ignition. You are measuring 2 volts when things are off because 10v in potential is being slurped up somewhere by some accessories which probably aren't related to the actual circuit. Since the circuit is switched and should be 0v - the power is being fed "backwards" to the fuse box and running to ground via this circuit. The reason it wants to run this way is that the culprit devices are not grounded properly anymore.

The problem with 3+4 is the same as 10,11,12 but possibly with a different source.

Some suspects:

Is the fuel pump still installed? Probably not, but if so, does it buzz when the ignition is on? Do your headlight washers work? Are they installed still?

The block 10,11 and 12 seem to connect on the load side through the turn signal cluster switch.... There is a ground wire from it, maybe its disconnected?

Did you already get the nasty ground connection near the trailer hitch? This usually shows up as issues on the gas gauge, and is pretty much guaranteed to be corroded if you haven't serviced it already.

Are there any brown wires under the dash not connected to the body? Brown = ground on Mercedes (logical Germans).

Anyways, no solutions yet but I hope that gave you some ideas.

Michael

Suppatime
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Re: W460 Fuse Box Upgrade

Post#14 » Mon Apr 08, 2013 10:16 pm

Michael:

I'll owe you a beer if ever we meet. I'll break your post down:

bratwurst wrote:In general, the fuse block should show either roughly 12v or 0v. By 12v I mean either 12.6v with engine off, and about 13.8v when running.


This was my thought as well... which is why it's so confusing that 1-6 show 0 volts when the engine is on.

bratwurst wrote:Fuse 3+4 are for the headlights. They are shorted together on hot side, therefore they are going to show the same voltage.


I know they're for the headlights, but I can't figure out how they're shorted together. I assume this would be behind the dash and not in the quarter panels independent of one another. I'll check it out, but there's a lot of dirty wires back there and I have big hands. From what I have seen, there aren't any tears in the shields for the wires.

My first sign of crappy electrical work was that there was a splice near the headlight switch where another 16-gauge wire was run... but snipped at some point in this truck's history. Problem with that is that I found it when I disassembled the steering column after a rather massive spark that happened when I flipped the switch and the bare copper touched the dash. My headlights didn't work the next day, but then a few days later, they started working again on their own. I've since removed all of the snipped spliced wires that were just hanging behind the dash... and there were a few of them.

bratwurst wrote:Fuses 10,11+12 are shorted on the hot side as well. Again, same voltages. I'm too lazy to go over all the traces, but the horn, fuel pump, and windshield cleaner nozzles are powered from these.


So this likely isn't a ground issue, but one of + power. The plot thickens.

bratwurst wrote:I'm going to guess fuse 10,11 and 12 should be at 0v when the motor is off. These fuses are connected directly to the ignition switch. I think these are switched by the ignition.


That was my understanding -- these are all operated by the key being in position 2.

bratwurst wrote:You are measuring 2 volts when things are off because 10v in potential is being slurped up somewhere by some accessories which probably aren't related to the actual circuit. Since the circuit is switched and should be 0v - the power is being fed "backwards" to the fuse box and running to ground via this circuit. The reason it wants to run this way is that the culprit devices are not grounded properly anymore.


Interesting. The only thing I installed that might be culprit here would be a Nakamichi MB-VI CD changer. The shop I had do the install did a piss-poor job. I rewired the + cable from the Nakamichi behind the fuse box to #10.

bratwurst wrote:Is the fuel pump still installed? Probably not, but if so, does it buzz when the ignition is on? Do your headlight washers work? Are they installed still?


I was under the assumption that the ignition pump pulls fuel from the tank...is that correct? This truck was a gasser, but I'm not sure if an in-tank pump is present. There is no buzzing or hissing as if it were priming when I turn the key and warm up the plugs.

I have no clue about my headlight washers because my entire washer system was broken and/or missing upon purchase. The washer reservoir was a junkyard find VW tank, and the pump was broken off and disconnected. Also, I'm missing fluid lines that go to the hood, and the larger hose that ran past the radiator to the headlight washers was disconnected. So I honestly have no clue there.

bratwurst wrote:The block 10,11 and 12 seem to connect on the load side through the turn signal cluster switch.... There is a ground wire from it, maybe its disconnected?


Would this ground run through the combination switch wiring harness? If so, I can check. The big give-away for me is that there's an earth-connection mounted to the firewall just below the gauge cluster that has 6 male pins in 2 rows of 3... there's only one ground connected there, and I put it there yesterday.

bratwurst wrote:Did you already get the nasty ground connection near the trailer hitch? This usually shows up as issues on the gas gauge, and is pretty much guaranteed to be corroded if you haven't serviced it already.


I haven't inspected the ground back there near the hitch. There's surface rust on the hitch and under the rear door (well, it'll need new sheet metal), but the trailer electrical connection back there appears in good shape. I'll look behind it in the daylight.

bratwurst wrote:Are there any brown wires under the dash not connected to the body? Brown = ground on Mercedes (logical Germans).


This is where I was confused by a brown fuse. I'd never seen one before. As far as the ground cables go, the only ones I've seen without disassembling the dash all together are either connected to the dash, the firewall, or to each other (there's a couple male-to-male connector pins back there... and I'm not sure if they are OEM or not.

Suppatime
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Re: W460 Fuse Box Upgrade

Post#15 » Tue Apr 09, 2013 10:35 am

Well now I'm confused again. I shorted the 25A fuse #7 which powers my radio and I think the heater fan (I was mistaken before).

I've tried replacing that fuse multiple times on the way to work today and blew 4 fuses in about 5 minutes. There's too much power on fuse 7.

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Re: W460 Fuse Box Upgrade

Post#16 » Tue Apr 09, 2013 10:36 am

vadimivanovich wrote:Right on Drew - getting organized!

I'd have to check my G to validate what you've measured but some of those low voltages seem fishy. Everything should be more or less 12-14V. You'll get some voltage drop across heavily loaded circuits but nothing major. Try an OEM fuse box first. Cheap & easy (relatively). I'm still suspecting a ground problem too, but you have to try something.


Thanks Vadim. I'm not convinced the issue is at the fuse box. I have to do some serious investigating in the near future.

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vadimivanovich
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Re: W460 Fuse Box Upgrade

Post#17 » Tue Apr 09, 2013 1:25 pm

If fuse 7 is the blower motor, and you're popping that fuse all of the time my guess is that your motor is stuck. This happens from age and/or wear. Even a lack of use could do it. Usually the fix is a new motor ($). Don't power anything else off of this circuit (radio) because it's already very heavily loaded by the motor.

You're probably right about the real problems being elsewhere, but just make an assessment of the corrosion level on your fuse box contacts and act accordingly. Mine aren't bad, but I'm still planning to eventually put in breakers. Even my Unimog came stock with breakers!

Bratwurst, you da man... breakin' out the manual! Thanks!

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bratwurst
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Re: W460 Fuse Box Upgrade

Post#18 » Tue Apr 09, 2013 9:25 pm

Suppatime wrote:Michael:

I'll owe you a beer if ever we meet.


Well after dealing with enough electrical problems you just might be the right guy to get alcohol from, you might need it just to cope!

Suppatime wrote:I know they're for the headlights, but I can't figure out how they're shorted together. I assume this would be behind the dash and not in the quarter panels independent of one another.


The circuits for 3+4 are tied together (intentionally) before the fuses. So it won't be on the headlight side. Its somewhere between the turn signal stock and the fuse box.

Suppatime wrote:My first sign of crappy electrical work was that there was a splice near the headlight switch where another 16-gauge wire was run... but snipped at some point in this truck's history. Problem with that is that I found it when I disassembled the steering column after a rather massive spark that happened when I flipped the switch and the bare copper touched the dash.


Ugh!

Suppatime wrote:So this likely isn't a ground issue, but one of + power. The plot thickens.


If my theory is correct, technically it is a ground issue. Some mystery circuit (the culprit) is not grounded fully and some of that potential is grounding through circuits 10, 11, and 12.


Suppatime wrote:Interesting. The only thing I installed that might be culprit here would be a Nakamichi MB-VI CD changer. The shop I had do the install did a piss-poor job. I rewired the + cable from the Nakamichi behind the fuse box to #10.


If the changer isn't grounded properly, there could be voltage from the acc switched line on the changer going through the unit and grounding via the 10,11,12 circuits. That would only be the case if you wired the switched line up to an actual switched circuit, and that circuit had some sort of potential on it with the key off - which shouldn't be the case... but anything is possible at this point.

If you disconnect the changer power lead, and then measure the voltage on 10,11 and 12, if the changer is the issue they should be 0v with the ignition key out.

Suppatime wrote:I was under the assumption that the ignition pump pulls fuel from the tank...is that correct? This truck was a gasser, but I'm not sure if an in-tank pump is present. There is no buzzing or hissing as if it were priming when I turn the key and warm up the plugs.

I have no clue about my headlight washers because my entire washer system was broken and/or missing upon purchase. The washer reservoir was a junkyard find VW tank, and the pump was broken off and disconnected. Also, I'm missing fluid lines that go to the hood, and the larger hose that ran past the radiator to the headlight washers was disconnected. So I honestly have no clue there.


Yep that's right - the thing is the diesel doesn't need a pump (its built into the IP) but the gasser definitely did have a fuel pump. So the question is what did the guy who did the swap do with the fuel pump and its wiring? Are the wires just dangling and shorting out on the frame? Are they connected to a siezed pump?

Oh that sucks... Same thing with the headlight washers. The wiring might just be dangling, although this shouldn't be an issue unless you actually try to use them.

Suppatime wrote:Would this ground run through the combination switch wiring harness? If so, I can check. The big give-away for me is that there's an earth-connection mounted to the firewall just below the gauge cluster that has 6 male pins in 2 rows of 3... there's only one ground connected there, and I put it there yesterday.


Hmmm might do, would have to pull my truck apart to check though...

Suppatime wrote:I haven't inspected the ground back there near the hitch. There's surface rust on the hitch and under the rear door (well, it'll need new sheet metal), but the trailer electrical connection back there appears in good shape. I'll look behind it in the daylight.


I can almost guarantee you have issues there. In fact if that ground point looks all nice and clean, I'll buy you a beer! You'll need a short M6 bolt to replace the rusty disaster that I suspect is there now. The wires leading to this ground may be a corroded green mess. You may or may not be able to get away with just replacing the bolt and sanding down the connector eyelets. You should slather on some dielectric grease after you do this. I am not sure what MB was thinking putting a major ground point in an area exposed to mud and water, but they did and this is a common problem.

suppatime wrote:
bratwurst wrote:Are there any brown wires under the dash not connected to the body? Brown = ground on Mercedes (logical Germans).


This is where I was confused by a brown fuse. I'd never seen one before. As far as the ground cables go, the only ones I've seen without disassembling the dash all together are either connected to the dash, the firewall, or to each other (there's a couple male-to-male connector pins back there... and I'm not sure if they are OEM or not.


OK, not sure how far you've digged into it, but if you pull the instrument cluster, and the surround for the ignition switch and the cover on the opposite side of the steering wheel you can pretty much see everything worth seeing electrically without too much effort.

Regarding fuse 7, well gee why can't you just have problems on two circuits and not on another! I'm starting to see why you mentioned a new wiring harness, although I think that's still a bit drastic at this point.

on the wiring diagram I have (you can download from Vlad's site) it goes to:
- the warning flasher switch
- the headlight switch
- the optional two pole-power plug (is this a NATO plug? not sure..)
- and the headlight washer relay, and in turn the headlight washer motor (when the relay is on)

The wires from this fuse should be easy to find, they are solid red. Assuming fuse 7 is connected as I'm reading in the diagram, the problem with the blowing fuses may very well be that there is a red lead shorting out to ground at some point. If the headlight washer relay is engaged for some reason (hey anything is possible), and the headlight washer motor is missing from the truck, the wires that normally connect to the motor could be shorting to the frame and blowing the fuse. The wire from the relay to the motor is solid black and should go through the main firewall opening on the driver's side.

The reason the fuse blows is because too much current is running through it. Every circuit has devices on them with a characteristic resistance that limits the current through the circuit in normal operation. A motor that is not moving has almost zero electrical resistance/impedance which is why that can blow a fuse as well like Vadim stated. End of the day the reason you are blowing fuses is that there is low resistance to ground on that circuit, so most likely a short or seized motor.

So let's measure it... With the fuse removed, you should have 12.6v on one side of the fuse (hot side) and 0v on the load side. If you switch your meter to the ohms scale (disconnect the battery first, you can burn out the meter otherwise on the ohms scale) you should have maybe 50 or more ohms between the load side and ground. You might have infinite ohms if there is no load at all. The lower the number, the more current will pass through the circuit normally. If you have 0 ohms there is a full short, a few ohms might be a frayed wire or something.

The only caveat is that I don't have a list of fuses and positions so there's a possibility what I am reading in the diagram is wrong for your truck.

If there's a problem with the heater motor, its both expensive and a major PITA. I hope that's not the issue. The good news though is that I suspect you have at least three distinct electrical problems. I'm gonna guess you have two bad grounds (on 3+4 and 10,11,12) and a short on 7.

Hope this is helping.... keep the faith!

Cheers,

Michael
Last edited by bratwurst on Tue Apr 09, 2013 9:46 pm, edited 1 time in total.

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bratwurst
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Re: W460 Fuse Box Upgrade

Post#19 » Tue Apr 09, 2013 9:45 pm

Oh hey just noticed circuit 7 has a red wire which goes to your clock. So that helps to explain why it doesn't work...

Fuse 7 goes to the warning flasher switch via a solid red wire, this then goes via a second solid red wire to the clock. I bet someone tied off this line and made a mess of it.

Suppatime
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Re: W460 Fuse Box Upgrade

Post#20 » Wed Apr 10, 2013 12:49 pm

bratwurst wrote:Well after dealing with enough electrical problems you just might be the right guy to get alcohol from, you might need it just to cope!


I was in Old-Fashioned territory, not Absinthe territory.

bratwurst wrote:The circuits for 3+4 are tied together (intentionally) before the fuses. So it won't be on the headlight side. Its somewhere between the turn signal stock and the fuse box.


This makes sense as they're operated by the same switch, but what doesn't make sense to me is how they were working when the short occurred-- with the headlight switch all the way on, only one low beam worked. When I flipped on the high beams, that same low beam (driver's side) shut off as expected, but it's high beam didn't turn on... the passenger side one did (previously non-working low beam).


bratwurst wrote:If my theory is correct, technically it is a ground issue. Some mystery circuit (the culprit) is not grounded fully and some of that potential is grounding through circuits 10, 11, and 12.

If the changer isn't grounded properly, there could be voltage from the acc switched line on the changer going through the unit and grounding via the 10,11,12 circuits. That would only be the case if you wired the switched line up to an actual switched circuit, and that circuit had some sort of potential on it with the key off - which shouldn't be the case... but anything is possible at this point.

If you disconnect the changer power lead, and then measure the voltage on 10,11 and 12, if the changer is the issue they should be 0v with the ignition key out.


A greater degree if disassembly will shed light on this.

As far as the changer ground goes, what constitutes a proper ground? I fear this may have been tied into a pre-existing ground or... hypothetically, some doofus at the install place saw a black cable and thought 'oh, that's ground' and just stabbed in. I'll check this.

bratwurst wrote:So the question is what did the guy who did the swap do with the fuel pump and its wiring? Are the wires just dangling and shorting out on the frame? Are they connected to a siezed pump?


The fascinating thing to me here is that some parts of the install were done very well. All the mechanical stuff that needs sockets and a torque wrench was within spec (with the exception of one of the solid motor mounts... it had come loose with vibration). I have a diesel filler neck for my fuel cap to tank. I have reinforced plastic fuel lines all the way back. It seems the mechanic was pretty good, but he knew very little about electronics. The 280GE wiring harness was spliced into, and left-over wires were zip-tied and tucked. Some of the grounds weren't connected. And then again, there was the main electrical problem when I purchased the G... the battery was constantly draining to the point if I didn't start it every day, it wouldn't have enough juice to turn the starter motor over.

This was the result of some no-name generic glow relay that was connected to a spliced parallel glow harness. There were two buttons on the console where the ignition is... unfortunately they were drilled in there, so I'll need to buy a new one now, but these two buttons were run in series. It seems one stopped working, and rather than replace it, they just rerouted the wires to a second switch on the other side of the steering wheel. The previous owner left me with a write-up of how they worked -- turn the key to position 2, flip the switch and wait 30 seconds then crank it. This didn't work very well at 70 degrees F ambient temp, and didn't work at all over the winter when much of Los Angeles was in the mid 50's during the day.

Opening it up and looking at it, neither of the buttons did a damn thing. The + power connection had severed off, rendering either button useless. The motor had been starting completely on it's own, which is surprising to me. Rebuilt Monarch injectors and pencil plugs always on about 50% seemed to do the trick after some cranking (5 seconds hot, about 30 seconds cold), which is pretty impressive for not knowing the mileage of the motor and knowing I have some minor blow-by issues.

I wouldn't be surprised if the fuel pump in the tank has been removed and blocked off, but I bet the wires are all back there still, held to the chassis with zip ties much like a rear brake line is. Side note -- I'd really like stainless steel brake lines.

bratwurst wrote:I can almost guarantee you have issues there [by the hitch]. In fact if that ground point looks all nice and clean, I'll buy you a beer! You'll need a short M6 bolt to replace the rusty disaster that I suspect is there now. The wires leading to this ground may be a corroded green mess. You may or may not be able to get away with just replacing the bolt and sanding down the connector eyelets. You should slather on some dielectric grease after you do this. I am not sure what MB was thinking putting a major ground point in an area exposed to mud and water, but they did and this is a common problem.


You win. I looked back there and it looks like a pile of crap. I actually noticed rust through one of the rear support beams under the rear door. This is going to get expensive rather quickly, from a body work standpoint.

I don't see any M6 bolts down there. What I do see is a lot of electrical tape, lose wires and rust.

Image

This G has seen a lot of time off paved roads.

bratwurst wrote:OK, not sure how far you've digged into it, but if you pull the instrument cluster, and the surround for the ignition switch and the cover on the opposite side of the steering wheel you can pretty much see everything worth seeing electrically without too much effort.


I've pulled out the glove box to get access to the center console in front of the heater core, pulled the dash section behind the steering wheel and pulled the back off the gauge cluster. I've tried to get the whole gauge cluster off, but as my old roommate left with both the torque wrench and breaker bar, I can't get the 27mm nut off the steering wheel at the moment. I'll have to buy a new one.

bratwurst wrote:Regarding fuse 7, well gee why can't you just have problems on two circuits and not on another! I'm starting to see why you mentioned a new wiring harness, although I think that's still a bit drastic at this point.


My thought is, this truck is 32 years old and I want to restore it and drive the hell out of it. I plan on long road trips, good mileage and not many fears through the canyons of Barstow, California and Sedona, Arizona. I intend to make this my daily driver so I can sell my Pathfinder.

A close friend of mine is an electrical genius, and he's gone over the wiring diagrams (both the proper 300GD and the 280GE) saying creating a new 300GD harness will be a piece of cake.

bratwurst wrote:on the wiring diagram I have (you can download from Vlad's site) [fuse 7] goes to:
- the warning flasher switch
- the headlight switch
- the optional two pole-power plug (is this a NATO plug? not sure..)
- and the headlight washer relay, and in turn the headlight washer motor (when the relay is on)


This is starting to make a lot of sense. Fuse 7, then, is the culprit for the majority of the electrical issues I've had. In the past, when flipping the hazard lights on, my stereo would cut out. My cigar lighter does not work (also tied in to this fuse, for some reason, 12-gauge speaker wire was used).

If the headlight washers are on the same circuit, I'm already giving it a load by adding the stereo and the cigar lighter. My stereo would cut out with the hazards and sometimes the turn signals themselves, and headlight operation has been trying about 1/8 the time.

bratwurst wrote:The wires from this fuse should be easy to find, they are solid red. Assuming fuse 7 is connected as I'm reading in the diagram, the problem with the blowing fuses may very well be that there is a red lead shorting out to ground at some point. If the headlight washer relay is engaged for some reason (hey anything is possible), and the headlight washer motor is missing from the truck, the wires that normally connect to the motor could be shorting to the frame and blowing the fuse. The wire from the relay to the motor is solid black and should go through the main firewall opening on the driver's side.

The reason the fuse blows is because too much current is running through it. Every circuit has devices on them with a characteristic resistance that limits the current through the circuit in normal operation. A motor that is not moving has almost zero electrical resistance/impedance which is why that can blow a fuse as well like Vadim stated. End of the day the reason you are blowing fuses is that there is low resistance to ground on that circuit, so most likely a short or seized motor.

So let's measure it... With the fuse removed, you should have 12.6v on one side of the fuse (hot side) and 0v on the load side. If you switch your meter to the ohms scale (disconnect the battery first, you can burn out the meter otherwise on the ohms scale) you should have maybe 50 or more ohms between the load side and ground. You might have infinite ohms if there is no load at all. The lower the number, the more current will pass through the circuit normally. If you have 0 ohms there is a full short, a few ohms might be a frayed wire or something.

The only caveat is that I don't have a list of fuses and positions so there's a possibility what I am reading in the diagram is wrong for your truck.


I'm getting 12.9V on the hot side 0.9V on the other with the battery connected and the fuse removed. Disconnecting the battery and switching the meter to Ohms, I'm getting anywhere between 3.77 and 37.4 on one side and 0.0 consistently on the other. On the first side (the hot side from the first measurement) is where the jump in Ohms occurs, usually between 3.77 and 10.0.

bratwurst wrote:If there's a problem with the heater motor, its both expensive and a major PITA. I hope that's not the issue. The good news though is that I suspect you have at least three distinct electrical problems. I'm gonna guess you have two bad grounds (on 3+4 and 10,11,12) and a short on 7.


It was working just fine a few weeks ago. The heater core was replaced in the past 2 years according to the previous owner. But I bet the fan wasn't touched. So maybe it's just tired.

I'm hoping the grounds are the issue as well.

bratwurst wrote:Oh hey just noticed circuit 7 has a red wire which goes to your clock. So that helps to explain why it doesn't work...

Fuse 7 goes to the warning flasher switch via a solid red wire, this then goes via a second solid red wire to the clock. I bet someone tied off this line and made a mess of it.


Yep, you're right. I didn't know they were connected, but the wire from the back of the VDO clock has been snipped and rerouted through some shrink-wrapped bundled of wires. It did work when I got the truck, but failed on me (because of electrical connections) in the last week.

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