1986 300SDL Glow Plug dash Board Light

[Marvin Smith]

I have a 1986 300SDL that is very difficult to stat in the morning due to a problem with the Glow plug system. I was told that the problem was due to a failed glow plug relay - which I had replaced to no avail. The glow plug light does not come "ON" when the ignition switch is first turned "ON". I reviewed this with the mechanic and he showed me that the light was good and that voltage was being applied to the glow plugs.

Despite this fact, the dashboard light that indicates glow plug activity does not illuminate. Exactly what does the light measure when the engine is first turned on?

Any suggestions as to what the problem might be and how to fix it?

[Ken300D]

Let us know if you have a voltmeter. Or even a 12-volt lightbulb tester. You need this minimal level of test equipment to troubleshoot the problem yourself. If you have that we can talk you through a few checks, such as:

1) Must have 12-volts at the high-amp battery supply lead to the glow plug relay. (You have to pull the plastic cover off the glow plug relay to access this and other leads and plugs - the glow plug relay is on the drivers engine compartment fender.)

2) Must have 12-volts on both terminals of the high-amp fuse (a strip of metal) and the fuse must not look burned or melted.

3) After pulling the multi-pin connector on the glow plug relay that contains the wires that actually go to the glow plugs, you must get 12-volts on all six glow plug terminals immediately after the ignition is turned to ON. This normally might last for 30 seconds before the glow plug relay removes the voltage.

Come back and tell us your results. It's not possible for us to guess the problem, but we can help you test for it.

Ken300D