93 300d glow plug issue
 

Lately my glow plugs haven't been working right. The light doesn't come on when I turn the key on but after the car cranks up the light comes on and stays on for a little while. It doesn't seem like the glow plugs are coming on during initial starting because it starts slow if he doesn't try to start it all on cold mornings. Any ideas what might be causing that issue

Your relay self-test feature is telling you that one or more glow plugs have failed.

You can do a quick check with a meter at the glow plug relay. Black box on the forward edge of the left front wheel well. Be careful under the cover, the big screw head and strip fuse have direct battery voltage on them so watch out you don't short that out.

There is a 4-pin (control) and 6-pin (to glow plug) harness. Unplug the 6-pin plug and check resistance of each glow plug to the car body. Meter pin 1 to frame, pin 2 to frame, etc. You have a 5 cylinder engine so pin 6 is empty.

If you have one or more plugs where the resistance is > 1 ohm then there's your problem.

Glow plug change on an OM602 is not too difficult. I would go ahead and change all 5 while you are in there.

The intake manifold has to come off unless you are a contortionist. So be sure to also order the intake manifold gasket, and cross over pipe gaskets as well (o-ring on one end, flat gasket on the other).

Thanks for the fast response! I'll get to checking!

>1 and <50.

Not only can they short, they can also open.

I believe the behaviour of your light indicates at least 1 GP that is shorted and no power is going to the GPs. You should also try to find a GP that has 0 ohms which would be the short. Also check for .7 or greater ohms but anything above 2 ohms should also be considered no good. If its the first time you will be doing this allow some time and also clean out the intake since it will all be off anyways.