Blinking Glow Plug Light - PeachParts Mercedes-Benz Forum

dsvw · #1 02-09-2024, 04:42 PM

Hi, I have a 1980 240D with a manual 4-speed.

Car is running great, but after it starts, the glow plug light blinks 11 times and then stops until the next time I start the car.

The car starts immediately in the mornings when it is in the 20s. So there is absolutely no problems starting the car.

The fuse strip is intact, I also took it out and cleaned the contacts. I know new Bosch glow plugs were put in the car about 4 years/15K miles ago.

I am going to take the leads off of the glow plugs and clean them up to see if that helps any, before I remove and test the plugs.

Any other ideas of what could be causing this? Thanks.

JHZR2 · #2 02-10-2024, 08:36 AM

Pull the harness at the relay and use a multimeter in ohms setting to measure the resistance of each GP. I suspect though it’s starting well, that one is bad.

A 240d engine is rough enough at cold idle that you may not notice the difference. And if compression is good (sounds like it is), it will still start easy.

dsvw · #3 02-10-2024, 10:29 PM

Thanks. What should the ohms / resistance, read for each plug?

Also, I take it that I can test each glow plug while it is in the car and still connected to each wire lead without having to remove each plug, as long as the main harness is disconnected?

JHZR2 · #4 02-11-2024, 09:17 AM

Quote:

Originally Posted by dsvw

Thanks. What should the ohms / resistance, read for each plug?

Also, I take it that I can test each glow plug while it is in the car and still connected to each wire lead without having to remove each plug, as long as the main harness is disconnected?

Yes, you leave the glow plugs wired up. That’s why you can use the harness at the relay instead of having to fish around at the engine block with your meter. Also ensures you know you don’t have a bad connection.

IIRC good plugs are around 0.6-0.8 Ohm. You’re really looking at differences between them to flag where there may be an issue and investigate further.

dsvw · #5 02-11-2024, 10:03 AM

OK. I'll have a look. I guess you can hook ground to any ground and then probe the positive meter lead into the end of the harness?

dsvw · #6 02-11-2024, 01:57 PM

OK.

I probed numbers 1,2,3,5 and they all measured .8 ohms

I thought I would need to probe 1,2,3,4, because this is a 4 cylinder, but 4 is empty with nothing to probe. It seems like 5 is for the #4 glow plug?

#4 and #7 were empty with no metal sleeve.

#6 and #8 had a metal sleeve but did not measure .8 I am thinking those two go to something else and not the 4 glow plugs.

It looks like my glow plugs are good as are the connections.....Is this correct?

dsvw · #7 02-11-2024, 04:12 PM

Also...I am getting 12 volts to the fuse, on both sides. I also pulled the fuse out and looked at it closely and it is free of cracks and corrosion.

I am also getting 12 volts to each pin that services each glow plug.
There are 2 pins in there that I am getting around 5-6 volts but they are in line with the ones that are not measuring in the ohm range for the glow plugs. I think those 2 pins #6 and #8, service something else because there are 6 pins in there and only 4 plugs.

I can hear the relay clicking on and off.

I just did a cold start and the car started with just a bump of the key.
But.....my glow plug light is no longer coming on. I did forget and leave the key in the start position for around 20 minutes while I was troubleshooting. Do you think that has anything to do with the glow plug light not coming on?

dsvw · #8 02-11-2024, 08:46 PM

Fixed....for now!

I pulled out both of the silver relays under the black plastic box on the left side front fender. I cleaned the contacts for both of them and it seems to have done the trick.

I am glad I didn't pull out the Glow Plugs and started with the easy stuff first.

JHZR2 · #9 02-11-2024, 10:04 PM

Quote:

Originally Posted by dsvw

Also...I am getting 12 volts to the fuse, on both sides. I also pulled the fuse out and looked at it closely and it is free of cracks and corrosion.

I am also getting 12 volts to each pin that services each glow plug.
There are 2 pins in there that I am getting around 5-6 volts but they are in line with the ones that are not measuring in the ohm range for the glow plugs. I think those 2 pins #6 and #8, service something else because there are 6 pins in there and only 4 plugs.

I can hear the relay clicking on and off.

I just did a cold start and the car started with just a bump of the key.
But.....my glow plug light is no longer coming on. I did forget and leave the key in the start position for around 20 minutes while I was troubleshooting. Do you think that has anything to do with the glow plug light not coming on?

You’ll need to look at an FSM wiring diagram for what those are.

https://www.tonk.ca/models/w123/w123CD2/Program/Engine/615/15-510.pdf

dsvw · #10 02-12-2024, 12:54 PM

Well, the blinking light came back on. Blinks maybe a dozen times for about 10 seconds. Everything checks out electrically and the plug resistance checks out fine. I just went out to do a cold start and it fired right up with just a bump of the key.

The only other thing I can think of is maybe the port that the glow plug goes into is carbon fouled and needs to be reamed out?
Any thoughts?

evranch · #11 02-13-2024, 01:07 AM

It won't have anything to do with carbon fouling. The glow plug relay monitors the current drawn by the individual glow plugs. If one or more don't draw enough current, then it flashes the light.

If cleaning things in the circuit helps, then that means something is marginal somewhere. Get someone else to turn the key, and measure the actual voltage delivered during pre-glow to the heads of the glow plugs, as this is often more revealing than measuring resistance. If the voltage is sagging much below the battery voltage, there is a bad connection somewhere. If it's low for all of them, this bad connection is in the relay or upstream of it. However they will still get hot, and the engine will still start if it's not a particularly cold day.

You can test each glow plug for actual loaded performance in the same way as you did with the ohmmeter, unplug the harness from the relay, then use a decent sized wire to jumper the plugs to the battery in sequence. You should see a spark when you connect and disconnect the wire, and all the sparks should be the same. Don't use a regular ammeter as they will draw too much current for the 10A shunt in a regular meter.

dsvw · #12 02-13-2024, 09:19 PM

"Get someone else to turn the key, and measure the actual voltage delivered during pre-glow to the heads of the glow plugs, as this is often more revealing than measuring resistance. If the voltage is sagging much below the battery voltage, there is a bad connection somewhere."

I am getting a little less than 12V on each glow plug when I glow the plugs. That is using a meter and physically checking each plug while they are in the block.

Each glow plug measures .8 ohms.

The glow plug light comes on just fine. After the car starts, and it starts with just a slight bump of the key, with temperatures in the high 20s to low
30s, the glow plug light will blink about 25 times and then turn off.

I guess I will check each plug for loaded performance next. The spark is what I should look for across each plug???? I don't use a meter??

I am thinking I should take each connector off of each plug and clean it?

I can't see how it would be an electrical issue if everything is measuring ok, I can hear the relay kick on and off. All plugs ohm equally, I am getting 12V to the fuse and the pins in the relay as well as 12V to each plug when I glow the plugs.

Other than removing the plug connectors to clean them I can't think of what else it could be.

Any thoughts?

evranch · #13 02-13-2024, 10:47 PM

I'm not certain on the current drawn by these specific glow plugs, but most glows draw more than 10A when cold. That will blow the fuse in most common meters, and most people don't have an external shunt available.

However you can use this product if you happen to have one around. They are handy for all sorts of moderate power DC testing and surprisingly accurate for their low cost.

But if you don't have something like that, the size of the spark is something people have been doing for years. More current = more spark, and a plug that measures fine for cold resistance may expose connection issues as it heats up, resulting in a weak spark for a weak glow plug.

However with the readings you've shared, it's starting to look like a failure of the monitoring system in your glow plug relay. As long as the car starts well, it's more of a cosmetic issue at this point. I'm guessing it's fully functional - I currently have 1 dead glow plug on my 300D and any time it's much below freezing that cylinder misfires and puffs white smoke until it warms up.

As long as the glows heat up and then switch off, that's all you need out of it. Glow plugs are a simple system. In fact my Mercedes is the only diesel I own for which you don't just hold in a glow plug switch for 30 seconds, then crank.

dsvw · #14 02-13-2024, 10:53 PM

Thank you.

hercules · #15 02-13-2024, 11:03 PM

The best and most accurate test is testing under load. A amp test will reveal what a
voltage or resistance test can miss. A clamp around meter would be needed for best
results. Note starting amps and how much fall after a few seconds, all must be with in
2 amps in same amount of time, if any are different, high or low can cause a problem.