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Old 08-21-2003, 10:08 AM
Registered User
Join Date: Aug 2003
Location: Cambridge, MN. USA
Posts: 2
Unhappy Fuel Temperatue guage problem


I recently acquired a 1985 300D with 186k on it and have little history of the car as the previous owner had it for only a year. So far I have been able (with the help of several competent mechanics) to get it working satisfactorily. Thanks to this forum for valuable advice!! The previous owner had the fuel/temperature/oil guage replaced (was leaking oil) and they all work except that the turn signal or emergency flashers cause both the temperature and fuel guages to increase their readings whenever the green turn signal indicator flashes. When I turn on the headlights both of these guages increase dramatically at a constant rate and the seat belt indicator flashes.

I've checked to make sure all the bulbs were the recommended type(one of the rear turn signals wasn't), checked and tightened the grounds by headlight, trunk and in the dash by the steering column(it was loose). I also pulled the electrical connection off the back of the fuel/temp guage and reinserted it. Since the old oil pressure guage was leaking there was still some engine oil present which I tried to clean up as best I could without removing the instrument panel. None of these helped the situation. The lights and turn signals all work but I'm worried this problem may ruin the fuel/teperature guages.

I am suspect of the garage that installed the guage as they left the hot wire for the clock dangling and it created a dead short on that circuit until I reconnected it. That circuit is now OK. The previous owner said this problem was not present with the old guages. Any ideas are appreciated and if I need to pull the instrument panel is it a difficult task?

Regards Mike
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Old 08-25-2003, 05:36 PM
Registered User
Join Date: Aug 2003
Location: Cambridge, MN. USA
Posts: 2
Voltage Relay


After looking into this further could it be the voltage relay? I see a small black plug-in on the back of the instrument panel that looks like it could be a voltage relay. Is it possible that the dead short that was occuring from the positive lead of the clock grounding out damaged this relay. Or maybe the garage that installed the instument panel used the old relay. Does anyone know if their is a voltage relay that regulates the voltage to the instument panel(teperature/fuel guage)? I see a voltage relay listed at an online MB parts supplier for around $25. Could this be the problem?

Regards Mike300D
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Old 08-26-2003, 09:58 PM
Registered User
Join Date: Jan 2001
Location: N.PALM BCH
Posts: 38

you have "leakage current" when you use your turn signals that is feeding back to the guages is my guess,probably thru ground. if there is a voltage relay for instrument lights,the "hot" 12 v from the clock would weld the contacts either open or shut,but i don't think there is a relay. the leaking oil pressure guage is what i would suspect. oil that was wiped off the circuit board still has slime that conduct voltage.
take the instrument cluster out,take all gauges out and spray everything with "electronic contact cleaner" then wipe with coffee filter,it's lint free.
72 250 CE-5spd-gone
79 280 CE
82 300TD
83 300 CD
83 380 SEC
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