Glow Plug Lights Always On
 

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.

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

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.

Pre-glow relay
 

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

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.

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.

Check the DIY ....
 

That wizard of all posters, jbach36, wrote a great piece, explaining all about the glow plug process and changing them out. See the DIY post section in this forum.

jbach36 rocks!

New battery-Started car. Car started immediately. GP light is still on. Checked for voltage at the 5 pins. They all have voltage after 5 minutes. Time for a new relay? I read somewhere in the forum that the relay is repairable, something about changing a capacitor? Opened up the relay and nothing looks burned inside.

ground connection
 

check your ground gonnection

Have a reference for that?

could be the ground. I had the transmission rebuilt. Probably forgot to put the grounding strap. ACC blower doesnt work and the charging light is on also. Will check later.

The charging light being on suggests the voltage regulator assembly with the brushes is bad. Especially if it is barely one and goes off now and then, kind of as the engine speed picks up. Brushes wear out and then only intermittently make contact - adds resistance to the circuit and you don't have the juice needed to run the car off the alternator. Thus, the dash "charging" light comes on.

This is likely a separate problem from the one that has your glow plugs energized all the time. I would think the fuse would melt under those conditions though.

Jim

I second the "check the ground strap"

Furthermore if you do check the strap and it is ok, look into the alternator issue.

It could very well be the brushes on the regulator. I just replaced a regulator in my 300SD. AL117X, regulator: 1197311028. It went from having to "bump" the pedal to get anything out of the alternator to getting 14v when the car was started without touching the pedal.

As far as the relay goes, I suggest, if you have access to, going through the FSM's diagnostic for the relay.

Also, it might be wise to remove the relay and clean the contacts on top. I had to do this on one of mine to get it to work. I thought it had failed but I ended up sanding the fuse contacts and terminal 30 screw, using q-tips to get the crud out of the 4/5 pin connector areas and finally using battery terminal cleaner to freshen up the pins. Reinstalled, fired right up, no problem.