1987 300TDT (W124) Glow Plug Relay Problem?

[swogee]

My 1987 300TDT has a problem in that it smokes and idles rough during cold starts. Occasionally the engine will not start on the initial start. It seems to emit a white smoke that smells of unburnt fuel. When the engine is warm, it starts without a problem.

After reading a bunch of posts in the Shop Forum, I thought the problem might be a bad glow plug or bad glow plugs. I measured the resistance of all of the glow plugs at the glow plug relay connector and they all measured between 0.6 and 0.7 ohms which indicates they are all good (no shorts or opens). I did find one of the previous owners had soldered across a crack in the 80A fuse in the relay to fix it, so I replaced the fuse. The cold starts did not improve.

I then measured the current to each of the glow plugs in the start position. The current would start out around 18-20 amps initially, and just before the relay shut off it would be down to 13 amps. I think the intial current is supposed to be closer to 30A according to the MB shop manual, but my meter was not fast enough to catch it. I also timed how long the glow plug lamp stayed on and it was about 1 second. The ambient temp outside was about 51 degrees F, which is about 10.5 degrees C. According to the MB shop manual the glow plug light should stay lit for 3 to 5 seconds at 10 degrees C. I also measured the time the glow plug relay stayed closed after switching on the ignition and it was 6.4 seconds. I'm not sure how long the G.P. relay is supposed to stay closed after the ignition switch is turned on, but I thought it was 30 seconds or so. I then tried to see if a longer wait after the the G.P. lamp went out would improve a cold start. The engine started easier with the longer glow time and smoke was reduce substantially.

Based on the data I gathered above, I think the G.P. relay is going bad. Does anyone have any other ideas on what else might be causing the problems? I want to make sure the G.P. relay is actually bad before replacing it since it costs $132. The previous owner also mentioned the glow plugs had been replaced in 2003 so I don't think they are bad.

Sorry for the long post, but I wanted to make sure all of the info. was present.

Any ideas or thoughts are welcome. Thanks in advance.

-Steve

[swogee]

Bump.

Any thoughts or input would be welcome!

Thanks

-Steve

[AlexTheSeal]

Yes, the relays go bad over time. No, there's no other likely cause once you have checked the GPs themselves out.

The 603 NEEDS a good long glow time when cold regardless of the ambient temperature. That extra cylinder makes life hard for the starter! You don't need the dash light to tell you when to start the car... just wait until you hear the relay click off. Either that or shell out the bucks for a new relay. There's really no other solution.

[swogee]

AlexTheSeal,

Thanks for taking the time to respond. I appreciate it. A new G.P. relay it is then. The fact that the relay clicks off after six seconds indicates that it is dying I suppose.

Thanks again.

-Steve

[madsen]

I would not assume that the glow plugs are good given the test you did they can test good and still not heat up. Also age does not necessarially mean they are good or bad . I would take them out and check them visually before I spent the 132.
JOHN M

[dlssmith]

Ignore the glow plug lamp indicator on the dash and listen for the relay. It will click after about 45 seconds, shutting off the plugs, if it's working ok. Then start the engine.

Mine won't start very well in cold temps if I follow the glow lamp indicator and start it when the lamp shuts off. I almost always glow for 30 seconds or more.

[Jeremy5848]

Ah so! How did I miss that? Thanks, Brian.

[drkam6]

1987 W124 250D, European model, OM602 5-cyl engine.

My mechanic made the GP Relay fully manual, like an old W114/W115 /8.
He installed a switch under the dash to start the heating of the glow plugs manually. While he indicated to hold the switch for about 10 seconds, I found out that the actual holding length should be about 18 seconds on temperatures of +5 / -10 C. The car starts fine. Then alternatively, holding the switch in intervals of 2 to 3 seconds clears out all the smoking (if any). This is how I start my car, and so far it's been without problems. No more checking out if the GP Relay is bad or still good, no electronic-depending control, no more spending $132 on a new GP Relay.

This method also acts as a "theft deterrent". The car will not start at all if not activating the glow plugs via the switch. Only drawback is that in an emergency, the car will not quickly start as it has to follow the glow plug warming routine, but I believe that it's the same issue whether the glow plug control is manual or electronic. However, if the engine has just been shut off and it's been a couple of minutes after this, the car will still start without using the switch.

From your posts it seems that the actual glow plug activity is actually 30 seconds (not the 18 seconds that I count holding the switch at first start up). The actual activity time was something I wanted to know, so I thank you all for this information.

[Diesel911]

Originally Posted by drkam6

1987 W124 250D, European model, OM602 5-cyl engine.

My mechanic made the GP Relay fully manual, like an old W114/W115 /8.
He installed a switch under the dash to start the heating of the glow plugs manually. While he indicated to hold the switch for about 10 seconds, I found out that the actual holding length should be about 18 seconds on temperatures of +5 / -10 C. The car starts fine. Then alternatively, holding the switch in intervals of 2 to 3 seconds clears out all the smoking (if any). This is how I start my car, and so far it's been without problems. No more checking out if the GP Relay is bad or still good, no electronic-depending control, no more spending $132 on a new GP Relay.

This method also acts as a "theft deterrent". The car will not start at all if not activating the glow plugs via the switch. Only drawback is that in an emergency, the car will not quickly start as it has to follow the glow plug warming routine, but I believe that it's the same issue whether the glow plug control is manual or electronic. However, if the engine has just been shut off and it's been a couple of minutes after this, the car will still start without using the switch.

From your posts it seems that the actual glow plug activity is actually 30 seconds (not the 18 seconds that I count holding the switch at first start up). The actual activity time was something I wanted to know, so I thank you all for this information.

Simplicity is good. However, there is another feature that the glow plug relay does. The relay is wired into the cranking circuit so that that while you are cranking the glow plugs are on.

[Air&Road]

Another thing to be aware of with a manual switched glow plug system with pin type glow plugs: if you glow too long, you can toast a pin type plug. The series plugs were much more tolerant of overheating, but the element in a pin plug can make enough heat to burn through the shell.

The colder it is when glowing, the longer to hold the switch, but experiment and glow it on the quick side at first until you get a feel for it.

In the eighties and 90’s, glow plugs were about a $1.25 each. Those days are long gone so move forward carefully.

Good luck.