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Old 02-22-2013, 10:28 AM
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Join Date: May 2011
Posts: 21
300SD - electrical problem with temp gauge and glow plug light

1980 300SD W116 with 425K miles.

Weird problem. The glow plugs work fine, the timer works fine when starting. The temp gauge works fine at low RPM.

Underway, the glow plug light randomly goes on and off, The temp needle bounces above the correct reading. Never below the actual temp. It jiggles above the correct temp and then settles back to where it belongs.

This is RPM related, and I think it is engine vibration rather than actual RPM.
Like I had a loose wire coming off the engine.
The problem has grown worse over time, used to be occasional, now the temp needle is dancing most of the time.

When I let off the gas and coast, the needle returns to normal.

Initially I thought it was instrument panel ground, cleaned up all those plugs and added an extra ground wire to the panel, on a speedometer mounting screw.

I replaced the temp sender.

I don't see any wires coming off the engine that should cause this. I cleaned up the plug to the alternator.

In the wiring diagram, I see that the temp signal passes through the glow plug timer mounted on the left fender. I cleaned up the plugs there.

I am starting to think that it is vibration passed into the fender and into the glow plug timer box.

I think my next bright idea is to dismount the glow plug timer and wrap it in foam or bubbles and see if I can isolate it from vibration.

Any ideas? Anybody have a source for a replacement glow plug timer assembly?

Thanks, the car runs perfect other than this, which means I drive it every day, which means I am bothered by the stupid flashing light constantly!
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Old 02-22-2013, 10:37 PM
Registered User
Join Date: Jun 2005
Location: Raleigh, NC
Posts: 13
The erratic temp gauge makes it sound like an intermittent wiring fault. The flickering glow indicator lamp seems at first glance to be an electronic fault within the glow relay however it could also be an intermittent wiring fault if 12V is being applied to the indicator lamp wire via a wiring fault.
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Old 02-23-2013, 08:47 PM
shadetree77's Avatar
Registered User
Join Date: Jul 2003
Location: Tennessee
Posts: 430
the glow plug relays are very very expensive for the W116 SD's and W123 D's of that era, if you can even find one. i converted mine to use a W126/W123 fast glow relay; i did a writeup on if you want to look at it. i think the fast glow relays can be had for about $100.
1983 300SD (daily driver)
1991 420SEL (work in progress)
1979 300SD (future replacement for 83 SD)

1983 300SD (totalled)
1986 420SEL
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