Curious glow plug relay behavior?

OldFred · 02-28-2011, 11:15 AM

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.

To remove this ad, register today or login if you already are registered!

oldsinner111 · 03-07-2011, 04:09 PM

The newer upgraded relays are better.Also I believe you ignition switch need replacement.Its plastic and can crumble with age.

OldFred · 03-07-2011, 04:29 PM

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

hineywineries · 03-17-2011, 11:57 AM

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.

03-07-2011, 04:09 PM	#2
oldsinner111 Senior Member 1999 Mercedes-Benz S-Class Join Date: Jun 2010 Location: tennessee Posts: 494 Drives: s320 and 300sd	The newer upgraded relays are better.Also I believe you ignition switch need replacement.Its plastic and can crumble with age. __________________ 1999 S320 plus 125 nitrous shot hp,modified grille area,and cold air intake,wrapped exhaust manifold,wrapped air duct,cold air,ceramic pads,no cats Borla exhaust one muffler14.4 1/4 mile 1983 300 SD not stock Mobil One ATF diesel rated oil both cars

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Display Modes	Rate This Thread
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode	Rate This Thread:

03-07-2011, 04:29 PM	#3
OldFred Newbie Garage is empty, add now Join Date: Feb 2011 Location: Lubbock Texas Posts: 2 Drives: '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

03-17-2011, 11:57 AM	#4
hineywineries Senior Member Garage is empty, add now Join Date: Jun 2006 Location: Northern California Posts: 303 Drives: 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.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)