Glow Plug Lights Always On - PeachParts Mercedes-Benz Forum

my123ca · #1 01-14-2010, 08:45 AM

Bad Relay? Is it repairable? Car just started running after more than 3 years. It wasnt like that before. Left it in a shop, got it back with lights always on now. They said they had a hard time starting it.

my123ca · #2 01-14-2010, 09:36 AM

The glow plug light is always lit when the car is running.

kerry · #3 01-14-2010, 10:56 AM

I'd begin with a standard diagnosis of the glow system similar to what is described on dieselgiant's site. I'd check for voltage the plugs with the key in glow and no glow position and check the ohm's on each plug. Report back.

Jeremy5848 · #4 01-14-2010, 12:34 PM

The glow plug light is controlled by a temperature-sensitive timer in the relay; it normally stays on for 5-10 seconds. The relay keeps the glow plugs on for about 30 seconds, that is, the glow plugs stay energized even after the light goes out. If it's quiet in the car, you can hear the relay click off and if you have the dome light on, it will noticeably brighten.

A glow plug light that stays on all the time suggests a faulty glow plug (or several) or pre-glow relay, confirmed by the shop's comment that the engine was hard starting. I would first check the 80 Amp strip fuse in the pre-glow relay and also the fuses in the under-hood fuse panel. The card in my '85 doesn't list a fuse for the electronic part of the pre-glow relay but I think there is one (can anyone confirm?).

The pre-glow relay does not always detect a single failed glow plug; you might have had a bad one for awhile and just didn't know about it -- the OM617 engine starts fairly well on 4 good glow plugs. If a second glow plug fails, the pre-glow relay will detect that and the engine also will become harder to start.

Jeremy

my123ca · #5 01-14-2010, 07:33 PM

Just finished testing resistance(?) of the glow plugs. They all have a range of 0.9 -1.1 ohms. Check for continuity also. They were good. Checked for voltage to the pins. They all have the same voltage. The strip fuse is also good. Just wondering if a weak battery can cause the GP lights to stay on. I had to jump start the car to bring it home. It wont start right now.

Graplr · #6 01-15-2010, 09:54 AM

Quote:

Originally Posted by my123ca

Just finished testing resistance(?) of the glow plugs. They all have a range of 0.9 -1.1 ohms. Check for continuity also. They were good. Checked for voltage to the pins. They all have the same voltage. The strip fuse is also good. Just wondering if a weak battery can cause the GP lights to stay on. I had to jump start the car to bring it home. It wont start right now.

No, a weak battery won't cause the light to stay on.

Since your battery is drained, I'm going to say your GP relay is bad and is always on which is causing the light to always be on. This would cause a drain on the battery.

kerry · #7 01-15-2010, 10:04 AM

x2. However, in the cases of the relay staying on that I recall being posted on this forum, the glowlight did not stay on. Perhaps this is a different kind of failure in the relay. It should be simple to test. Start the car and after 4 minutes or so, check for voltage at the glowplugs.

Graplr · #8 01-15-2010, 10:09 AM

Quote:

Originally Posted by kerry

x2. However, in the cases of the relay staying on that I recall being posted on this forum, the glowlight did not stay on. Perhaps this is a different kind of failure in the relay. It should be simple to test. Start the car and after 4 minutes or so, check for voltage at the glowplugs.

Agreed. However I had little luck finding my GP relay energized with a multimeter when my relay went bad. But my light would only flicker so perhaps he'll have an easier time diagnosing it.