240D's glow plug light is only UP for about 2 seconds. - PeachParts Mercedes-Benz Forum

ericnguyen · #1 04-28-2003, 05:00 PM

Dear friends:

My 1981 240D (manual transmission) is extremely hard to start.
I already did valve adjustments, replaced and flushed spin-on mail filter/inline pre-filter, changed engine oil/power steering but it's still extremely hard to start it with the ignition key. Only a push-start can start it up.

Now I suspect that the glow plug relay does not work properly. I did test all 4 glow plugs and they seemed to be OK (resistance ~ 0.5 - 0.8 Ohms). However, when I turned the ignition key to the #2 position, the glow plug light was only up for about 2 seconds before it was off again. The ambient temperature was about 60-65 Fahrenheit. Is this a symptom of a bad glow plug relay? Do you have any other suggestions (besides a low compression/worn piston rings) ?

BTW, I wonder whether a NEW diesel engine can just start right away when the ignition key is in START position, without needing
any pre-glow time when the ambient temperature is about 60 F
or more?

Thanks in advance for all your precious help.

Best regards,

Eric

psfred · #2 04-28-2003, 08:31 PM

Don't know which system you have -- probably the newer parallel one with no temp sensor.

If that is the case, you have a bad relay. Glow plugs should run about 15 sec or so, maybe more, at 60 F cold engine, and no, it usually won't start. Prechamber diesels are hard to start cold anyway, simply won't fire very well stone cold. You have to crank the .....out of them.

Peter

ericnguyen · #3 04-28-2003, 08:52 PM

Dear psfred,

Yes, my 1981 240D does have parallel pencil-type glow plugs.

BTW, all Mercedes diesel cars since 1980 have parallel GP system.

Eric

P.E.Haiges · #4 04-28-2003, 11:06 PM

Eric,

No wonder you are having trouble, trying to start your engine with an "ignition key". Do you wind it up or what? You should try to start it by using the starter motor. LOL

Ok enough joking.

The dash light may not be any indication of how long the GP are actually energized. I suggest you time how long the GP are actually turned on by the GP relay. An easy way to do this is to observe the cabin light. It will dim noticeably when the GP are energized. The GP should turn on for about 40 seconds each time or until the starter is energized. If the GP are only on for a short time, the relay might be bad and that may be why you are having starting problems.

Pre-glow of 20 seconds should be sufficient at 60F.

P E H

vwbuge · #5 04-29-2003, 03:28 AM

I would try to replace the glow plugs before the relay.

P.E.Haiges · #6 04-29-2003, 07:03 AM

Vwbuge,

Eric said all the GO check out OK so why would he replace them? Even with new GP, if the relay isn't turning the GP on long enough, the problem won't be solved.

P E H

#7 04-29-2003, 07:10 AM

I would suggest that you take a voltage reading at the glow plugs immediately after turning on the key. Once you KNOW that there is 12 volts at each of the plugs, THEN replace the plugs.

Taking resistance readings of the plugs do not tell the whole story. I have had plugs that had good resistance readings but would not get the job done.

If there is 12 volts or so at each plug, replace the plugs. If there is NOT, then work toward the glow plug relay.

Any yes, a fresh engine or even an old engine with good compression and everything right will start right up in warm weather. Sixty degrees would require a few seconds of glowing.

Good luck,

P.E.Haiges · #8 04-29-2003, 07:39 AM

A better test than a voltage test would be to measure the current drawn by each GP. This is more difficult because the wires have removed unless you have an inductive ammeter that you just place the probe around the individual GP wire.

If all the GP are drawing about the same current, about 15 amps, the GP are OK and do not need to be replaced.

I still think the problem might be how long the relay is turning on the GP and that should be investigated before you replace any GP.

P E H

rivermasternc · #9 04-29-2003, 08:22 AM

Amp draw as recommended by P.E.Haiges is the MB method for debugging the plugs. I ran into this similar problem over the winter where one of my plugs did not show fault by the resistance method.

Rather than changing all the plugs, and not willing to spend $200 at the time for the correct ammeter, I removed each plug, one at a time and tested them by hooking them to a battery (used jumpercables for this). Turned out to be #3 plug. All well now.