I wouldn’t even be asking this, but the CD unit on my computer crapped out & I can't access my service CD

This seems pretty basic, however there is conflicting information in the archives.

The question is regarding the proper functioning of the glow plug relay on a 1984 123 chassis.
1. Are the glow plugs supposed to remain energized *during* cranking?
2. Are the glow plugs supposed to remain energized for a short period of time once the engine first fires?

When I attach a test light to one of the glow plug leads the test light will remain lit (for a little while) after the dashboard indicator goes out, as it is supposed to do. However while cranking the starter motor, the test light goes out. Once the engine fires, the test light still remains out. So the only time the plugs are getting voltage is during the pre-glow phase. Is this the correct way that the glow plugs are designed to function, or is it time for a new relay?

Once again, thanks for the help everyone!

RTH

This is how the relay works. Are you having any starting problems, if so, exactly what's going on.

Good luck,

That doesn't seem correct, but it might be. Since the glow plugs (GP) cool quickly, if the engine didn't start immediately, it would never start because the GP would not be glowing. I would think the GP would stay on the 30 to 35 seconds even if the engine started because they help a cold engine run smother. I know the old series (loop) GP stayed on during cranking even though the dash mounted indicator went out. It was shunted to compensate for the drop in battery voltage during the time the starter was cranking the engine.

When I get a chance I will try my 300SD to see if the GP stay on during and after starting. Can't do it now because it raining here.

P E H

P E H: Thank you for checking the way that your car functions – I can hardly wait to here your results.

Larry, I’m not quite clear on your response – do you mean that the way my relay is behaving IS how they are supposed to function? (Mine is supplying voltage to the glow plugs ONLY during pre-start, and NOT during cranking, and NOT after the engine first fires up.) And yes, I am having starting problems:

1.When the engine is cold (30-40 degrees F.) it will fire up after just a few compression strokes, but run rough & misfire for about 20 seconds. 2.When the engine is hot, but has sat for more than 10-15 minutes, it takes a few seconds of cranking & once started, it blows a substantial cloud of smoke from unburned (or partially burned) fuel until it ‘clears its throat’. (a white/gray cloud that does *not* dissipate - so it is not steam, thank goodness.) Being that it is not mosquito season, I’d really like to stop fumigating the neighborhood.

I recently installed new injectors that I had double-checked for pattern & opening pressures. I also replaced the injection pump delivery valve seals recently, & there is definitely no air getting into the system from other leaks. I replaced all the glow plugs a few months ago, so they are fresh – they are Bosch brand plugs, so inferior plugs should not be the issue here.

I hope to get a new CD player for my computer in the next few days, so hopefully I won’t have to bother you guys much longer – but in the mean time, I really do appreciate your help!

RTH

I have two questions for you RTH:


What kind of test light are you using? Maybe the light only picks up voltages/currents from a minimum volt/amp, when you crank and run, the voltage/current drops below it's sensing range.

Have you checked your grounds? Could be a loose ground somewhere.

__________________
83 300TDT BROWNBENZ
04 Sprinter

Yellowmerc:

I am using a small, low wattage 12V bulb, just to make the process simpler. The results can be duplicated with a volt/ohm meter, but a bulb is much easier to use in this case. All grounds are good & solid – this is not about limited amperage to the glowplugs, it is about the proper timing of their function. Thanks anyways.

RTH

RTH,

I checked the function of the GP controller today by connecting a voltmeter to GP #1. First I turned the key to the on position and left it there. The voltmeter read 10.5 volts and the GP stayed on for 75 seconds. The temperature was about 35F. Then I turned the switch off and again turned it to the run position for about 15 seconds. Next I turned it to the start position and the engine started and the glow plugs went off as soon as the engine started. I then shut engine off and had someone else try starting the engine while I pushed on the shut off lever to prevent the engine from starting. GP stayed on while engine cranked but went off as soon as engine started.

So it appears that the GP stay on before and while cranking the engine but are disconnected as soon as the enging starts. I don't know what signals the GP relay that the engine is running so it turns off the GP. Maybe someone else knows and will tell us.

P E H

Thank you very much P.E.H for going through that extra effort! (A good example of how nice & helpful some people can be here on this forum.) I wrote this little summary right before logging on & seeing your reply:


I feel that is always good to finish off a thread so that people searching the archives in the future may get some use out of it.

According to the somewhat butchered German to English translation & the wiring diagrams in the service CD, the following is supposed to be the proper glow plug relay function:

The glow plugs are supposed to turn on in the pre-glow stage (duh). They also remain on for a short while after the dashboard indicator light goes out (again, duh). The length of time for each stage is temperature dependant, determined via either a temperature sensor located on the engine (for earlier cars) or, on the later cars, via a temperature sensor located internally in the relay. This function is activated by voltage to terminal 15 when the key is in position 2.

The glow plug relay is supposed to continue supplying voltage to the plugs *while* the engine is cranking. This function is activated by voltage supplied by terminal 50 when the key is in position 3 (cranking).

Once the engine fires & the key is released back to position 2, there is no longer any voltage supplied to the glow plugs. They do not remain on to help keep the engine firing smoothly, as some people have assumed/posted.

So… if you're not getting voltage at terminal 50 (or the relay is not responding to the voltage at terminal 50) while trying to crank the car, the glow plugs can cool off a bit if you need to crank the engine for more than a couple seconds.

Hope this helps someone in the future.

RTH

RTH,

I'm glad that our results agree, yours by the book and mine by experimentation.

It would make make sense to me if the GP stayed on after the engine was running but the MB engineers should know more about it than I do. Possibly it puts a strain on the GP that shortens their life if they are subject to the heat and pressure of a running engine when the GP are red hot.

I remember reading about another make Diesel car (I can't remember what make) that the GP came on every time the engine was at idle for lower emissions. However it seems to me that if that engine was at idle too long, the battery would go dead.

P E H