Curious glow plug relay behavior?
Thread Starter
Newbie
Joined: Feb 2011
Posts: 2
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From: Lubbock Texas
'84MB300SD
Curious glow plug relay behavior?
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.
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.
Thread Starter
Newbie
Joined: Feb 2011
Posts: 2
Likes: 0
From: Lubbock Texas
'84MB300SD
OldSinner111: Thanks for the advice. My car has a Hella GP relay that came in the car when I bought it last year. I ordered a Bosch GP relay to replace it as many have said I should, but the online vendor shipped me a HUCO (Made in Germany) GP relay. Should I return the HUCO and pursue a Bosch OR just put the HUCO in? Do you have any experience with the long-term reliability of the HUCO GP relays? Thanks
Senior Member
Joined: Jun 2006
Posts: 303
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From: Northern California
It changes all the time
The system is designed to make the light flash when you have a glow plug failure. To determine which plug has failed, remove the large plug from the global relay. With one lead of a digital ohmmeter grounded to the engine, probe each of the five connectors at the plug. The little number next to each connector corresponds with that glow plug reading from radiator towards firewall. Any glow plug that shows an open is obviously defective and needs to be replaced. You should also replace any glow plug that has a resistance much above one ohm.
