Glow plug light comes on after start but goes off when accelarating - PeachParts Mercedes-Benz Forum

MB300D81 · #1 07-14-2011, 08:29 AM

Hello fine people,

On my 1991 W124 300D...

Today I went to change the oil and filter as usual.

I took it off the hoist and started it as normal...

The glow plug cycle light went on as usual and went off, I started the engine but a few seconds later the light came back on.

The relay has been replaced less than a year ago "old one stopped working".

The unusual thing is that the light goes off when I am accelerating /driving over 50 kms/h.

I checked the battary as it was low on charge. Replaced it with a fully charged one but I still get the light back on after the engine starts and the light goes off when driving as mentioned above.

Using DVM all 6 pins read 7.5V when the light comes on the second time.

The relay sounds a click when the light comes back on.

What could be the cause please

Regards,

markg612 · #2 07-14-2011, 08:44 AM

I've had two fail under MB 1 yr parts warrantee. If you still have the receipt and you and bought it from MB, it's still under warrantee.

MB300D81 · #3 07-17-2011, 06:54 AM

Thanks Mark,

So you reckon it is the 100% the relay?
I was wondering why the GP light goes off while accelerating? Do you or any member have an explaination please?

markg612 · #4 07-17-2011, 09:39 AM

The relay has some minor intelligence and it senses resistance and current draw if I remember correctly in each plug among other things. During acceleration, the plug will heat up from the energy of combustion and increase the resistance measurement. The sensing circuitry may be returning a "false positive"of plug condition or actually detecting a fault in one of the plugs.

One other thing first, You mentioned you checked voltage to each plug with a DVM--check resistance, cold they should be in.4~.5 ohms, warm .6~.7 ohms.

Pull the larger 6 pin plug from the top of the relay and ohm out each plug--takes ten minutes.

The light on after start is a "check glow plug integrity" light so to speak.

I've included the condensed versions of chapter 15 from the 602-603 shop manual. The last page talks about fault detections.

Ensure you plugs are 100%, and if they are it's the relay.

MB300D81 · #5 07-19-2011, 10:16 AM

Thanks again Mark,

I gave the issue some rest until I get a day off work "in couple of days".

Even though I need the car to get to work, I am using the train for now as I need to spend time with my kids after work rather than the car.

In my day off I will go through the steps you recommended as far as the plugs. Then I will take it from there.

I will report back soon

markg612 · #6 07-19-2011, 10:18 AM

Good luck!

We're all counting on you. :-)

MB300D81 · #7 07-30-2011, 08:07 PM

Ok, After giving it a good tests. I found that I was running on only two good glow plugs out of the six.

No wonder the relay was showing the GP light on the dash.

I ordered a set of GPs "Bosch" and took on the job.
Managed to replace all six without taking the inlet manifold out.
I would not have done it with out all the socket joints and extensions I have in my tool box.

Took around three hours. But it was worth it.
Now the engine starts at first kick and money is well saved.

Thanks to you all for the write up and advises. Much appreciated.

Still have to do the job for my wife's W210. But this I know I will have to take the inlet manifold out.

Regards

tangofox007 · #8 08-01-2011, 09:50 AM

Quote:

Originally Posted by markg612

The relay has some minor intelligence and it senses resistance and current draw if I remember correctly in each plug among other things. During acceleration, the plug will heat up from the energy of combustion and increase the resistance measurement. The sensing circuitry may be returning a "false positive"of plug condition or actually detecting a fault in one of the plugs.

The fault detection function should not be measuring anything once the preglow sequence has been completed. Also, the lamp indicates a fault by not illuminating.

Quote:

Originally Posted by MB300D81

Ok, After giving it a good tests. I found that I was running on only two good glow plugs out of the six.

No wonder the relay was showing the GP light on the dash.

Why was the relay "showing the GP light on the dash?"

markg612 · #9 08-01-2011, 10:18 AM

Quote:

Originally Posted by tangofox007

The fault detection function should not be measuring anything once the preglow sequence has been completed. Also, the lamp indicates a fault by not illuminating.

Why was the relay "showing the GP light on the dash?"

Because, unlike your incorrect statement above, the relay is always looking at resistance in the glow plugs. With a bad glow plug, I've had the GP light cycle on and off while driving, many times. Change the gp, solve the problem.

tangofox007 · #10 08-01-2011, 11:24 AM

Quote:

Originally Posted by markg612

Because, unlike your incorrect statement above, the relay is always looking at resistance in the glow plugs. With a bad glow plug, I've had the GP light cycle on and off while driving, many times. Change the gp, solve the problem.

My mistake. I read too much into the OP's screen name. Then realized the subject vehicle is not a '81 300D!