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).