Glow plug light flickering on and off when driving? - PeachParts Mercedes-Benz Forum

chetwesley · #1 03-31-2008, 10:02 AM

I looked through the archives but didn't see this one exactly.

This morning I started my car, it started easily, though I did have the block heater plugged in. I let the car warm up for a few minutes before I left. As I was pulling out of the driveway, I saw the glow plug light was still on and would occasionally flicker off. I pulled off and looked at the fuse in the black box, but it appeared ok visually anyway.

As I drove it stayed on with the occasional flicker until I got on the highway and it eventually went off and stayed off for the rest of the drive.

It is misty this morning here, about 40 degrees.

Any ideas what to look for?

xdjio · #2 03-31-2008, 03:15 PM

What happens if you don't start the car but just turn the key to position 2? Does the GP light come on and stay on and then turn off (ie. normal behaviour) or does it cycle on and off?

I ask because I was recently diagnosing my own GP troubles (on my 300D, not SD) but my factory manual does indicate the GP malfunction warning modes for BOTH the SD and the D. I seem to recall that if there is a GP system issue on a 300SD it will cycle the GP light off and on to indicate a fault.

I don't have the FSM with me right now but I can take a closer look later this evening.

chetwesley · #3 03-31-2008, 04:50 PM

It just came on and went off as usual when glowing before starting.

The weird thing is that it is not blinking regularly, it is like it is mostly on, but then flickered off a couple of times randomly, and then would come back on, and eventually it finally just went off again after driving a few minutes. It was flickering as a light would if there was a bad connection to it's power source.

xdjio · #4 03-31-2008, 05:02 PM

Aha, well then you probably ought to ignore my warning-system theory. As I recall when the SD GP relay warns you of a fault, it blinks the GP light in a regular fashion for 30 seconds.

The FSM has a very detailed procedure for checking the operation of the relay and the lamp. I know that on mine (admitted, not an SD) the relay indicates which wire from its harness goes to the lamp, so you may want to start there.

I know on the D the GP relay is located a ways behind the driver's headlamp on the fender wall. I think (but I could be wrong) on the SD it is located up on the driver's side firewall. As for the wiring harness on an SD GP relay, I am not sure, but I would start by checking continuity to the lamp.

Can you glow and start up normally without your block heater? How cold does it get there?

compress ignite · #5 03-31-2008, 07:25 PM

Chet,

Picture is of your Glow relay,may be on the driver's side "wheel hump"

The strip fuse can look good,but have a hairline crack that affects the
relay...Take it out and test with a Multi-meter.

Oh and...
Ist dieses Bild urheberrechtlich geschützt?

chetwesley · #6 03-31-2008, 08:20 PM

yea i checked the fuse visually, but i'll check it with a meter too

xdjio · #7 03-31-2008, 09:01 PM

Hi Chet,

Assuming you have the same relay as me (ie. the one pictured) (and you do, since you're post-1980, so my apologies for ambiguity about that earlier on) here's what my manual says for diagnosing the system:

1) Pull the 4-point couple from the relay. Turn the key to position 2 and bridge jack 1 and jack 3 on the coupler. The preglow lamp should illuminate. If it doesn't, you could have an issue with the bulb. Possibly with the actual like to the bulb.

2) Check the connection from jack 3 on the 4-point couple to the bulb itself. This one could be tough since you'll have to make the connection yourself.

One of the lines coming from the 4-point coupler is labelled "LA" (as I recall) and this is the actual connection to the lamp. Probably the right place to start... ?

Assuming you can get the lamp to light reliably (or: replicate your symptoms in the manner above) then you've probably got the issue isolated to a flaky lamp connection which it sounds like you're supposing anyway.

I guess if you wanted you might try also cracking open the relay itself and having a look. I just fixed my non-functioning glow system this way yesterday.

SW · #8 03-31-2008, 09:09 PM

Is your key tumbler loose? Maybe the ket is cycling on and off.

Mustang_man298 · #9 03-31-2008, 11:04 PM

You know, just an odd hunch here, you said the car fired off normally, which is indicative that the glows are working properly, and that it was an erratic flickering, not a pulsing flash. So, I have to ask, since it was morning, were your headlights on? I am suspicious you might have a bad ground at the cluster somehow causing a backfeeding from the backlights thru that lamp. You most likely still had them turned off when pregolwing before the engine was started so it still would act normally then. Try it again with the same conditions you had this morning, then try turning off the headlights and see if the light goes to normal. Also see if bringing the engine off idle as you would normally driving changes anything (makes the altermator put out more power & can act to offset a bad connection or ground problem)

chetwesley · #10 04-01-2008, 10:38 AM

Thanks for the advice guys

Hmmm... well the problem did not repeat itself when I left work yesterday, nor when I drove to work this morning. It might be kind of hard to diagnose until it happens again, if it ever does.

I think I did have my lights on, or at least my parking lights because it was foggy/misty.