Glowplug System - 91 350SD

[Robert L Meyers]

I'm a brand new poster so please pardon lack of familiarity with the protocols.

The glow plug indicator does not come on intially with the key turned to the run position. The engine starts OK (but its virtually new and the weather is warm now). Ten seconds or so after the engine starts, the glow plug lamp lights and stays on for a short while and then shuts off (probably its normal controlled timeout setting). I suspect the Preglow Relay module. Since a new one is pricey, I hate to just swap it without any other indication of a problem. I can't tell whether its a display indicator problem or if the glow plugs are actually not being turned on. My next step is to hook up a voltmeter to a glowplug and see what that says when I cycle the ignition key. Any other hints?

[darrellws]

Bob, you may have a couple of GPs that are bad that is causing the GP dash lamp to not come on. To test the GPS, unplug the GP wiring harness plug from the GP relay, with an Ohmmeter check each plug for resistance. The good GPs should read about one ohm, a bad one will read much higher depending what caused the GP to malfunction. Replacing the bad GPs should make the GP system operate properly again.

Darrell

[Tom Hanson]

Replace all your glow plugs.

[Robert L Meyers]

[QUOTE]Originally posted by darrellws
Bob, you may have a couple of GPs that are bad that is causing the GP dash lamp to not come on. To test the GPS, unplug the GP wiring harness plug from the GP relay, with an Ohmmeter check each plug for resistance. The good GPs should read about one ohm, a bad one will read much higher depending what caused the GP to malfunction. Replacing the bad GPs should make the GP system operate properly again.

Darrell - You were right on point. Number 6 glowplug read open. All the rest read about an ohm. I had forgotten that my old 240D had a similar problem which was easily checked at the glowpug connector pins. Its tough to get old. Many thanks - this forum is a great resource.

[Jim Grillot]

When the glow plug light does not illuminate, the GP relay is telling you that there is one or more bad glow plugs.

Resistance is not a reliable way of checking glow plug condition. The better way is current draw. Cause for rejection is less than 8 amps or more than 15 amps.

[Marshall Booth]

Jim, You are correct that resistance is not the perfect glow plug test, but over almost 40 years I can only think of one or two times when a resistance check didn't pick up a bad plug. That's maybe a 1 or 2% failure. So MAYBE one or two times out of 100, WHEN a plug is bad, the resistance check won't pick out the bad plug. Considering how easy a resistance check is - I'd do a resistance checjk FIRST and then if the bad plug(s) isn't revealed, THEN DO A CURRENT CHECK.

Marshall