Question- electrical troubleshooting- oscillation temp gauge+ fuel gauge with turn

[ericgr]

I'm wondering if anyone has any thoughts on this.

In my previous posts I noted that I was having problem with my temperature guage. Aside from reading too high and flickering around sometimes, I have noted the following: When I use the left turn signal (and I think the right, I have to check again), I notice a small oscillation (up-and-down) of the temperature gauge needle and the fuel guage needle in synchronization with the turn signal clicker. That is, as the turn signal goes "click click" these needles on the gauge go slightly up-and-down (I'd say only about 1/8th of an inch).

As an aside, and possibly related or unrelated, but while touching the wiring harness beneath the radiator overflow tank (on the passenger side of the engine compartment, passenger side fender area), I received a very slight shock (almost like static electricity). I was actually looking in there for a short of some kind and just running my hand across it. I don't have any real electrical issues in the car that I know of, but an occassional miss at idle only, after considerable other troubleshooting (still underway), has me looking at the electrical system at this point as one cause (fuel injection system probably OK, vacuum loss still possible and still looking). A month or so ago I had removed the radiator overflow tank. When installing it, I noted how tightly it pressed up against the wiring harness and became concerned. This is why I was looking around this area. I can't see any issues in there though, the inside of the car looks very clean and the wiring looks clean, free of dry-rot, free of cracks for the most part, etc.

Everything, from an electrical standpoint, is stable in the car and normal. The only thing that doesn't work is the rear trunk light. There is no power to the connector (I verified that with a multimeter). I plan to look into that when I deal with the problem I described above.

What I'm thinking here is that the fuel/temp gauge circuit has gone bad. Any noise rejection circuitry in it, intended to separate-out any noise within the power system, may be shot. Of course it may also be a loose connector. I'll only know once I'm in there, which will require removing the steering wheel (see previous post). However, I'm wondering if I may be missing something there and if there may be a larger grounding problem in the car. If anyone has any thoughts on this, I'd be very interested thanks.

Below are some observatons. I looked at the Mercedes Benz Electrical Troubheshooting Manual for the 380 SL and noted the following and in addition some observations/assumptions:

1) On page 118 it shows the fuel/temp gauge connecting into component C151 which seems to be a 15 pin connector on the back of the instrument cluster. On page 116 it shows the turn indicators also connecting into this same connector. Makes sense I think. The instrument cluster takes input on C151 for these items and lots of others. The fuel gauge and temperature gauge are shown at connectors 4 and 3 on C151. On page 116 the left hand signal indicator is shown on connector 6, the right signal indicator is on connector 13.

2) On page 116, it would appear that fuses 5 and 12 are involved with power to the turn signal. On page 118 it shows, I think because I'm a bit confused there, Fuses 5 and 7 involved with the instrument cluster including the guages in question. That would mean Fuse 5 is common to both the turn signals and the gauges.

3) On page 118, the fuel gauge and the temperature gauge appear to have screw terminal connections to the same ground as does the turn signals-- G102. G102 looks fine in the car (simply looking at it, not tracing everything back-- G102 is by the passenger footwell, right sidewall next to the fuse box).

[Walrus]

I had (have) the same problem in a '88 560SL... refer you to my post titled instrument problems...

I think you are on the right track. I replaced a few fuses, and my problem mostly went away. Now on rare occasions, it will re-surface. Generally, a light thump on the dash pad rights everything...

I have been told loose or faulty grounds can also cause intresting problems...

Let me know how you fare in your repair of this item please...
Thanks

[ericgr]

Walrus, thanks for the information. By the way, I forgot to mention that I have a 1984 380 SL. I also found, like you, that thumping the dash makes the gauges come back down. My symptoms are very similar to yours except for the radio. I started looking closer into this when I was troubleshooting a cooling problem and noted that the gauge was inaccurate. Next, when troubleshooting a small intermittment miss at idle, I began to wonder if the idle computer/lambda processor wasn't taking a little hit here and there electrically, causing the fuel mixture to reset.

I'm diving in there today full-force with the exception of taking-out the steering wheel because, as mentioned in my previous post, I see some risks there based on previous work done in the car and/or how Mercedes put that steering wheel on. At this point the problem isn't bad enough for me to go that route though if I have to, I'll support the wheel (see my follow-up) and not rely on the wheel lock because I'm sure it'll break based on the torque (likely I should say) and will go to a garage with a power tool. Today I'm going to look at the wiring around the fuse box and passenger side, look for shorts to ground, troubleshoot the trunk light not working which may be a clue and basically trace-out what I can. I'll let you know if I make any progress and learn anything.

[ericgr]

I'm going to replace the voltage overload regulator while I'm in there since I have noted these go bad. I don't think it has anything to do with the problem; however, it has been linked to idle misses, etc. Since it does "get old" and it's relatively cheap I'm going to change it today along with a few other things around the engine.

[tecqboy]

I am not familiar with your specific vehicle, but all cars share some common electrical issues. The symptoms you describe are indicative of a bad "ground" somewhere in your wiring. I'd look at two areas for possible corrosion. Check the cable which runs from the battery negative post to the car frame, particularly where it bolts to the frame. Also check where this negative cable bolts to the engine itself. The bolts where these cables get connected corrode, especially the one near the battery. Have a real close look at the cables themselves, as the copper corrodes on the inside and often you can't see the problem under the insulation. Inspect those short "jumper" cables which go from the engine to the frame. Mercedes often runs return circuits (grounds) to a common tie block, particularly those in the instrument and dash area. Inspect your fuse blocks for corrosion. If you still have aluminum fuses, consider the newer brass type. Beyond these suggestions, you'll need a volt / ohm meter to measure voltage and resistance to ground at various points in the system.

[ericgr]

The problem in my temperature and fuel gauge (previously described) was caused by an intermittent short in the ground behind the instrument cluster. A competent mechanic identified this and wired around the ground, solving the problem and also making other features of the car work again that I didn't even know existed, namely the seat belt warning buzzer.

Note that before I owned this car (more than 10 years ago) someone had stripped the recessed bolt holding down the steering wheel. This combined with what was presenting itself as a complicated troubleshooting process had me give the car to the mechanic. How the mechanic got that bolt off of there is a mystery to me. He did it and installed a new one.

So my answer is that, while logically it would appear the gauge is going bad, it may be a bad ground. For further study is a task, for me, to determine how this intermittent ground has developed in the wiring harness. My major suspects are an alarm system installed in the car 10 years ago (I've deinstalled the "brain" but it's still hanging in the harness) as well as looking for any place where there may have been a short and melting, or anything loose, etc. This is a long-term look for me. At this point everything looks pretty good. The only thing semi-off at the moment is the hazard lights don't always come-on at the first hit of the switch, usually takes two its. I don't htink it's the switch, it was never a problem before. I believe it's related to the intermittent ground.

Note after this fix, the car reads a correct temperature and the fuel gauge is correct to within an about 10%