W108 electricals short problem - PeachParts Mercedes-Benz Forum

alabbasi · #1 05-07-2006, 12:44 PM

All

I'm wondering if someone could help me out here. This is on my 1973 Mercedes Benz 280SEL 4.5.

It appears that i have a pretty brutal effecting the following:

Right running light front and rear
Instrument cluster.

I keep blowing the number seven fuse.
I checked both the front and rear wiring harness and they appear to show no problem.
When testing the rear cluster, we would blow the fuse only when turning the running light. This would happen whether everything is connected or not.
The rear cluster appears to be fine when using the parking light (click light switch to the left) and not blow a fuse.
The front cluster was not connected at all, when we connected and tested it, it seems that the wiring is OK but I could not test the running light as the fuse would constantly blow.

The instrument cluster is out also (as one would expect).

Both harnesses on the front and the rear appear to be fine so I am not sure what to check. Can anyone give me any suggestions?

This car has always had electrical issues with the lighting even prior to the accident but the instrument lights did work.
Regards

Al

oxymoron · #2 05-07-2006, 01:30 PM

How about checking the license plate light and trunk light. I know on my 6.9, the trunk light holder is kinda wonky and it shorted out until I got it fixed right. This is of course a wild ass guess on my part.

Kenny

108 Fuse Map

1. Dome light, parking light, clock, passing signal, map light, glove compartment light. 8amp
2. Windshield wiper and washer, horn, cigar lighter. 25amp

3. (no discription). 8amp

4. Fuel pump. 8amp

5. Turn Signal indicator, back-up lights, fuel gauge, stop light, hand brake, warning light. 8amp

6. Heater Blower, automatic starting aid. 25amp

7. Tail light right, parking light right, license plate light right, dash board lighting, trunk light. 8amp

8. Tail light left, parking light left, license plate light left, fog lights right and left. 8amp

9. Head lights high beam right and indicator. 8amp

10. Head lights highbeam left. 8amp

11. Low beam right. 8amp

12. Low beam left. 8amp

From http://electric.articles.mbz.org/108fuses/

I'm sure #3 has a description... just dunno what!

Also for fuses under the hood:

electric sliding roof drive: 25A
Heated rear window: 16A
Electric window lifters: 2x 16A
Air conditioner: 16A
Additional fan (280 SE 4.5 / 280 SEL 4.5): 16A
Automatic aerial: 8A
Radio: 2A

From page 48 of the owner's manual.

kbannister · #3 05-07-2006, 02:35 PM

Don't over look the fact you may have a shorted out bulb

alabbasi · #4 05-07-2006, 03:12 PM

Bulbs appear to be good, they lit up on their own. when testing them. Even with tail ligt and front blinkers disconnected , it would continue to short.

I'm thinking that there is a short in the front harness or the insturment cluster.

Tomguy · #5 05-07-2006, 08:06 PM

Quote:

Originally Posted by oxymoron

1. Dome light, parking light, clock, passing signal, map light, glove compartment light. 8amp
7. Tail light right, parking light right, license plate light right, dash board lighting, trunk light. 8amp

Actually, according to my fuse map, the trunk light is on fuse #1 not fuse #7. And the license plate lights dont have side-specifics marked, #7 is for both according to my chart.

http://tomguy24.tripod.com/resources/fuses.jpg

alabbasi · #6 05-07-2006, 09:40 PM

Number 7 is the problem fuse.
Symptoms.
When wired to right tail light, blows fuse when running lights are turned on
When not wired to right tail light, blows fuse when running lights are turned on
When wired to right blinker blows fuse when running lights are turned on
When not wired to right blinker , blows fuse when running lights are turned on
When turning the parking light , the rear tail light lights up and does not blow a fuse.
When turning on the blinker, the blinker works without blowing the fuse (same bulb).

Even using a 100amp bulb, it will blow the fuse. All the connections look good, there does not appear to be any breaks at the ends of the wiring harness.

Tomguy · #7 05-07-2006, 10:59 PM

So if you take off the plug from the rear tail light it will still blow.

Have you tried checking the front marker? I forget, but I dont THINK it turns on when the side lights are on, just when the running lights are. The plug for it is behind the headlight and could be in bad condition (mine was)

alabbasi · #8 05-07-2006, 11:04 PM

Yeah, even with both lights disconnected, it still blows the fuse. But it does not blow the fuse when you have the parking light on (turning the light switch one click counter clockwise).

I even pulled the interior cluster out to see if there was any loose wiring, i found that that the cluster must have been replaced in the past as the wires are cut marked but everything appeared to be back together correctly with no loose wires.

This is the frustrating part of it. I am pretty certain that this was a problem that may have started before I bought the car. I found a wire running from the left market to the right marker. I'm pretty certain they ran that wire so that it could run both side markers when you turn the running lights on.

Tomguy · #9 05-07-2006, 11:07 PM

al: Check to see if the right side yellow rectangle marker lights up when you turn it one click CCW - I dont think it does, and the short may lie in there. It's either that or the cluster. The cluster itself can be diagnosed by unplugging it completely.

alabbasi · #10 05-07-2006, 11:12 PM

Thanks for that, none of the side markers light up. I will try by disconnecting them and seeing if i can get a result. Now if someone could explain to me how to get the grill star on, i'll be golden.

Tomguy · #11 05-07-2006, 11:51 PM

The side markers are NOT supposed to light when the switch is turned the opposite way - just checked. They only light when going the other way. Therefore, since those and the dash lights dont light when going that way the problem lies either in the right front light/wiring or the dash lights/wiring. The dash lights include the tranny light bulb (if console shift), heater levers, ashtray, and instrument cluster. Any of those can be the short.

Tristar1959 · #12 05-11-2006, 07:02 PM

Hi Al,

The "PaR" (first click left - Euro RH parking light) contact does not appear to involve the same chain via F7; in fact, it or "PaL" are only hot-fed "30" voltage via contact "P" when the ignition switch is in the off or "1" position - in common with the lead to the courtesy light switch. THAT factor isolates the two sources of feed to the lights - and why that feed doesn't backfeed to everything else on F7 will have to wait until I find how it's wired - perhaps a diode somewhere...My 280SE 4.5 wiring diag does not show how it does connect only to the R taillight and running light, but I'll try to find a similar setup in some other diagram. In any case - it's obviously NOT in the F7 chain/loop, which is why you can light them without blowing F7.

For the moment, I would suggest trying to isolate out two more likely candidates which share F7 - the ever-popular VDO instrument panel rheostat, whose coil can sometimes short out in many of its failing positions [you might even first try rotating it stop-to-stop and see if that changes anything], and the tach itself or the tach light. Unfortunately, simply pulling the multiplug off the back of the panel doesn't isolate which, but if that temporarily "cures" it, at least you're narrowing it down to the panel - then, where? I still vote for the rheostat and anything in the tach, first. The other oddballs in the equation are the tranny "shift light" which somehow involves the clock (which doesn't make apparent sense, on a "switched" fuse...) - now there's another good candidate to isolate - may have a diode feed that has failed.

That other lead across from the L side SIDE marker to R side marker is certainly interesting - but they all run off a supplementary fuse, and my diagram shows that crossover as normal; not sourced via the other lighting circuits. In my judgement, that has nothing to do with this problem.

PM/email me if you'd like me to email you my scans of what I have for a wiring diagram (537K diagram, 116K key)

Stan Braisted

Tristar1959 · #13 05-12-2006, 11:29 AM

Al,

On any of the same era diagrams I can access which do show the actual wiring for the L or R parking lights, they are separately fed, to a different bulb, than the usual "running" or "parking" lights - which explains why there isn't a backfeed problem to other elements on F7, as well as why you can get them, when the others are shorting out.

The "shift light" and clock connection must be for their sector lighting only,
but the tach light AND operation seem to both be off F7, as well. Each of those, as well as the overall panel/dash lighting via the rheostat, need to be individually isolated until you identify the (temporarily) missing one which no longer blows fuses.

Another approach would be to (ohm)meter through each of their individual leads/connections at a low resistance range setting (with the F7 wire source lifted/pulled) until you find one which goes directly to ground. But be aware that you may be able to read entirely normal low-resistance "grounds" through normal bulb filaments - just not zero.

Stan

Tomguy · #14 05-12-2006, 12:00 PM

Stan: The older light assemblies (With the red turn signal lens) had two bulbs for the running switch and were made of cast metal. The newer assemblies (with the amber lens) were changed for the US market - they're plastic and don't have that second bulb. I think the switch occured in the 1970 model year? All 4.5's have the single running bulb on the rear lens. I guess MB figured since it wasn't needed/used in the US it wasnt important to have the smaller wattage bulb that can remain lit for hours with minimal battery harm even in there.

I discovered this when I got a new assembly on eBay for my right rear. the wiring is very different on the red lens assembies - the brake light where the turn signal normally is on the ambers, and a few other differences.

alabbasi · #15 05-12-2006, 01:49 PM

Tomguy is correct, my wiring only has one bulb, which is a good thing because now I know that the wiring at the rear lights is good and the wiring on the front is also good (else the blinker wont work). So as it was suggested before, I need to check the wiring around the instrument cluster for short.

I get one of the Rovers this weekend to part out. Fun fun!