I have a well kept '84MB300SD with 90K miles. Starts and runs well as long as the temps are above freezing. I think I have some weak glow plugs and plan to replace them. My questions is about my glow plug relay: When I start the car the glow plug light comes on as it should and the car starts. What is curious is that the glow plug light comes on intermittently but often once the car is running. It has done this since I purchased the car Sept'10 and the only change is that the frequency of the on/off cycle has now increased to about 3Hz...the noise coming from the relay on the front fender is annoying. Now get this: turning the headlights on turns the glow plug light off & stops the noise reliably!

I read somewhere that the glow plug light can come on while the car is running, but the problem can be with the plugs & not the relay. Does this make any sense? Shouldn't off be off? My big question is should I replace the relay as well as the plugs? I have thought about just replacing the plugs to see if the above behavior changes, but do not want to damage my new plugs with a defective relay. Any advice will be appreciated.

 

hi; easy way to ck glow plugs , run insulated wire from hot wire {+} on battery tap the other end on g/p terminal if it sparks good if not replace , this is another's idea but great & cheep

 

I was an electrician for over 40 yrs if it has no resistance it will not spark resistance it will , a glow plug has a heating element basicaly same as turning on a burner on range , no resistance burner does not get hot same on glow plug , every one does not have a volt/ohm meter

 

I think you are both partially correct. Sonny_bo is correct in that if the glow plug is totally open, it does not work and there will be no spark. ForcedInduction is correct in that if it is NOT totally open, you will get a spark, but the resistance value might not be within the correct range for the glow plug to work well. Anyway, I have a Fluke multimeter and will be checking my glow plugs out this weekend. Thanks.

 

ForcedI: More solid advice, thank you

 

fluke = king n queen of equipment/testers

 

I just replaced my relay after replacing the glow plugs which did not help an intermittant light. The relay was the problem and now my 84 300SD starts and light works exactly as it should.