PeachParts Mercedes-Benz Forum - View Single Post - New to me 1980 300CD

Jeremy5848 · #2 07-28-2006, 08:26 PM

The simplest test is to get in the car, shut the door, wait for the timer on the inside light to time out. Then turn the light on manually using the switch. Now insert your key and turn it to the "preglow" position. The overhead light should dim a little when the key goes on because the current drawn by the preglow system pulls the battery voltage down. You can even monitor the battery with a voltmeter if you want to get fancy. If you listen carefully, you may be able to hear a "click" as the relay closes. One possible reason for not hearing the "click" is that the relay may be stuck in the closed position.

After a few seconds (depending on outside temp), the glow plug light will go out. Don't start the car, just keep watching the overhead light. The light tells you that the engine is warm enough to start but the glow plugs actually stay on for a total of about 30 seconds. When the glow plug relay times out, the overhead light will brighten a bit, the battery voltage will come up a little, and you may be able to hear a "clack" from the relay. Now you can start the engine or turn off the key switch.

If you wait a few minutes and don't get any of that, then the glow plug relay is indeed staying on all the time. If you don't want to replace that expensive part just now, you can always jury-rig a switch that allows you to turn off the pre-glow relay/timer once the engine has started.

Incidentally, the failed pre-glow relay may have caused the demise of your alternator: having to generate that extra current for who knows how long would have put quite a strain on the alternator.

Jeremy