I have a 1991 300D 2.5 with the 602.962 motor. The glow plug light came on last week while driving and a few days later I have the rough idle and extra smoke for the first few minutes of a cold start. These are the typical tell tale signs of a bad GP. I tested the plug's resistance at the harness and the are all within range of each other. I did this test while the engine was cold and while the engine was hot and the resistance values did not change. Aside from pulling each plug, what else can I test.

Thanks!!

I've only seen the manual idle adjustment knob (at dash) on NON-TURBO w123's, for whatever it's worth.

CJ,

The change in engine temperature is not enough to affect the resistance of the GP significantly.

The next test, without removing the GP, would be a current test. For this you will need a 20 amp DC ammeter. First disconnect the GP cable from the GP relay. Connect one terminal of the ammeter to the + post of the battery and the other terminal to one GP. Do this for each of the GP and make note of the current readings. The GP should draw about 12 amps each, but the bad one would be the one that was different than the others.

If the previous test does not determine which GP is bad, it is time to remove all the GP and do a visual heating test. To do this you hold the GP with a locking plier on the hex area and connect the GP to the battery post: terminal of the GP to one battery post and end of locking plier to other battery post. Be careful, the GP will be very hot after testing and do not lay them on anything that will be damaged by heat.

Observe the glowing characteristics of each GP to determine if anyone of the GP have characteristics that are different than the others. If this test is inconclusive I.E. all the GP glow red hot in about the same amount of time, you GP are probably OK and you may have other problems.

If your GP are difficult to remove, you might try the above test on the GP that are easiest to remove first.

P E H

Thanks!!!!

I was able to diagnose the GP by hookung a test light up to the positive (+) side of the battery and then testing each GP at the harness. GP #1 was barely lit.

Thanks!!!

Since when did anyones sexual orientation become a topic of discussion? I could care less if you like girls or boys, dogs, cats, sheep, horses, or whatever. This is a MB board, and last time I looked my car had no sexual organs. (she is female, the *****) I may have a set of balls and like women but what the hell has that got to do with me needing help with my car? The closet has nothing to do with it. I don't give a rats ass who "comes out" or whatever. I am a raging libertarian, I think everyone has the right to live as they feel good about. Use drugs? So what. Like same sex partners? More power to ya. This is a free country, as long as you don't step on my toes I won't stomp on yours. Attacking someone for their beliefs when they differ from yours is just plain WRONG.

Guess what, the #5 GP went today so I had to fix it. Maybe I should have changed them all why I had the intake off????? Hindsight is 20/20!!!!!!