Glow plug light off, doesn't feel like they're working. 87 300D - PeachParts Mercedes-Benz Forum

Crazy_Nate · #1 11-24-2010, 08:32 AM

I've been having this issue very intermittently. It has me a little stumped

Here's what's happening:

Insert key
Turn to glow position (all typical lights on, except glow light).
Attempt to crank (almost no response, no stumbling, etc).

Here's what usually happens:

Insert key.
Glow.
Crank.
Car starts (might stumble for a second or two if it's around 40F).

I think I may have a glow plug out because it sometimes stumbles slightly for a few seconds when I start on the the colder mornings, however, this current problem is like the glow plugs aren't even working.

Is this a relay issue? Control issue?

Unfortunately, my DMM is at work...is there anything I can visually check? Corrosion, cracked fuse, etc? Any ideas as to why it'd behave like this?

Crazy_Nate · #2 11-24-2010, 08:53 AM

Ok, a little more information. The glow cycle timing (in terms of length), indicated by the glow light, has always seemed to be a little short (by at least a couple of seconds). Typically if it's warm out, it might be off within 3 seconds, cold, off within 6 (rough guesses).

I've usually ignored the light itself and given it a couple more seconds beyond what they light has indicated... Does this point at the relay?

I'm gonna take a look at the strip fuse a little more closely, but it looks like it's more or less in one piece.

vstech · #3 11-24-2010, 09:02 AM

the light is usually just an indicator of outside temp, it's got very little to do with the glow plugs itself. if the light is not coming on at all, you have to test all your plugs and fix the bad ones.
if you leave the key in glow position for 40 seconds, does the car start smoothly?

Crazy_Nate · #4 11-24-2010, 10:18 AM

Thanks for the reply John.

I don't think that it's a glow plug issue. The cabin light didn't dim when it happens. I finally got the car started (glow plug light came on, started pretty easily, slight stumbling).

When it happens (no glow plug light), I can crank and crank and crank, and it doesn't feel like there's a single bit of response (no firing, stumbling, etc). I can try replacing the strip fuse, but I might not be able to experience whether or not that fixes anything until it randomly happens again (which is usually exactly when I can't get a ride to work or to pick up parts). I'm looking at just replacing the relay and keeping a strip fuse on hand.

I can check the plugs now that I'm at work. I'm basically pulling the glow plug wiring harness from its connection at the relay and checking the resistance between the individual pins and the nearest ground point (engine block?). Sound right? I'll post up the resistances when I check it.

Jim H · #5 11-25-2010, 10:08 AM

Quote:

Originally Posted by Crazy_Nate

I don't think that it's a glow plug issue. The cabin light didn't dim when it happens. I finally got the car started (glow plug light came on, started pretty easily, slight stumbling).

This is the first of two significant observations.

First, since the GPs draw a lot of current, the overhead cabin light should dim. When the the GP relay did not pull in to power the GPs, the lights did not dim.

Quote:

I finally got the car started (glow plug light came on, started pretty easily, slight stumbling).

This observation is more significant and raises suspicion of an intermittent 'bad' connection in the power wiring to the relay itself, tougher to track down than a complete failure, but not impossible. Let's suspect a loose, intermittent connection somewhere between the 'ignition' switch and the relay.

Having a helper for this part is, well, helpful! While you may be able to hear the relay under the hood 'click' while sitting inside the cabin when you turn the key, it is easier if you are able to stand near the front with the hood open. You can hear better and you may be able to feel the relay click by placing a finger on it. Cycle the key on and off a several times, listen for the click and observe the cabin lights each time. No click and no dim lights in the cabin = no relay action. Sometimes click and dim means a sometimes bad connection.

Turning the key to ON applies voltage to the GP relay, and this voltage stays on as long as the key is ON. You can test this by using a test lamp or a voltmeter to probe the correct pin socket after you unplug it from the relay. When you turn the key you should see the test lamp light, or read 12V on the voltmeter. Again, having a helper to turn the key on and off in the car is easier, but you can do this solo if you place the test light or meter where you can see it from the driver's seat.

If you never get no test light or meter indication, the relay circuit is broken. If you 'sometimes get an indication, there may be a loose connection in the wiring or a bad switch contact. Try 'jiggling' the key when the light or meter is not reading, and if the light then comes on you can suspect the contact in the 'ignition switch' is going bad.

Crazy_Nate · #6 11-24-2010, 02:09 PM

Glow plug resistance values:
G1: 0.3 Ohm
G2: 0.4 Ohm
G3: 0.4 Ohm
G4: 0.3 Ohm
G5: 0.4 Ohm
G6: 0.4 Ohm

Measured from glowplug wiring harness pins to the engine block.

I picked up a new relay (not sure of which brand, it is identical to the Stribel one on fastlane). I'll update this thread if the issue resurfaces.

#7 11-24-2010, 02:15 PM

I've never had much luck relying on ohmmeter tests for glow plug analysis. With the pin type, even one that shows good on the Ohmmeter, can have a burned shell or other malady.

Ten years or more ago, they were so cheap I just replaced them when there was any doubt. With the later engines they are more difficult to get at and they are expen$ive in todays parts world.

I'm not sure about the 124 cars, but on the 123's with pin type plugs, number one plug served as the sample for the glow plug relay mechanism. The current measured throught hat one indicated if the system needed to continue to glow or not.

Hope this helps.

Crazy_Nate · #8 11-24-2010, 02:54 PM

I believe the previous owner had the plugs replaced (meaning, they're about 2 years old and < 12k miles). I'm not certain about their life, but it very well could be that one or more needs replacing because it's not functioning up to spec. If I continue to have issue with slight stumbling when the weather gets colder, I'll definitely replace them all. They are a bit of a pain to get to, but I need to take my intake off anyways (it appears to be leaking goo from the interface at the cylinder head).

The car starts...probably 95-97% of the time (I've only had the problem 2x in the last couple weeks, and it's a recent development). From the not starting symptoms (cranking without any response from the engine), it feels exactly the same as if I were to stick the key in and immediate crank a cold engine over.

It simply seems too uncanny for all the glow plugs to simply start working or stop working at random.

Thanks for the input

katja · #9 11-24-2010, 03:56 PM

My parents' Peugeot 505 diesel had that issue from time to time before we sold it, and I never figured out the cause. You would just have to turn the key on & off and back on until the GP light lit, otherwise, the GP's wouldn't go on at all (and engine would crank endlessly and not start). I think it was the relay starting to fail, but it could have also been the ignition switch wearing out and not activating the GP position. Or (less likely) possibly an intermittent connection somewhere in the wiring.

These would be some things to check...I doubt it's an individual plug in this case. We got lucky that the only time we couldn't get the GP light to go on at all after multiple tries was in the garage at home, and it worked fine again the next day.

micalk · #10 11-25-2010, 08:11 AM

Did you try removing the GP relay fuse ? Sometime they develop a hairline crack that's hard to see but falls part when pulled.

Wodnek · #11 11-25-2010, 10:47 AM

Quote:

Originally Posted by Crazy_Nate

If I continue to have issue with slight stumbling when the weather gets colder, I'll definitely replace them all. They are a bit of a pain to get to, but I need to take my intake off anyways (it appears to be leaking goo from the interface at the cylinder head).

Thanks for the input

Slight stumbling when its cold seems to be the norm. Both of my 87s do it. On the sedan I replaced the relay and the plugs because my glow light wasn't comming on. I still have the slight stumbling when really cold.

Oh, and the light? A burned out bulb, so i have an extra relay now.

JPUTT · #12 11-25-2010, 11:03 AM

I think you just have a bad glow plug. My '87 had the light go out during the summer but it started without any problem until cold weather this fall. It progressively got harder to start and eventually it would take 20-30 seconds to start then stumble terribly for 10 or 15 seconds.

I ordered the glow plugs (never changed over the 100k miles I have owned the car), unplugged the wiring harness from the relay box, (the fuse was fine), removed the intake manifold (order yourself a new gasket also), and replaced the plugs making sure the wiring was intact and fixing any bare wire that could potentially cause a short.

I found that the #1 plug was damaged and the shaft of it would spin when you tried to remove the wiring nut. After replacing them it now starts easily and the light works as normal once again. I have noticed a much improved start in cold weather.

John in NC

Crazy_Nate · #13 11-25-2010, 11:00 AM

Next time I'll definitely be checking for the relay to click. I've also brought the multimeter home, so I can have an easier time troubleshooting.

If the relay doesn't fix the issue, I'll be looking at the ignition switch in very close detail. Last summer I had an issue with no response from turning the key (no lights, no clicks, no cranking). It was very, very seldom - perhaps 3 times overall.

Are there separate wires for the ignition switch based on what position the key is at (accessories, glow, crank)?

I don't really want to confuse the issue with my prior summertime issues. That was no-response from the ignition switch. This is a definite response from the ignition switch in all three positions, just nothing from the glow plugs.

I may be finding that I have two issues that complement each other but are separate, independent processes. Is rewiring the ignition switch an invasive procedure? I can replace the switch itself (and maybe take apart the switch as a curiosity), but, if it's a problem with the wiring harness, I'd rather just get it over with. Intermittent problems really bug me

Crazy_Nate · #14 12-08-2010, 08:02 AM

Ok, still haven't changed the relay yet. I figured that the problem might reappear and I could swap them and see if it changed. It hasn't reappeared...yet...

However, the glowplug light is coming on after starting the vehicle, usually after driving for a minute, sometimes longer or shorter. I tried wiggling the key / ignition placement, but it is hit or miss if the light goes off (could be just the internal timing) and I can't hear the relay clicking if the glow plugs are still on after the predetermined timeout period.

Does this sound like an issue with just the relay, or could something else contribute?

Would the temp sensor in the head cause a problem like this? What tells the circuitry that the car is running and not to keep the glow plugs on? There isn't an afterglow system on this car.

oldiesel · #15 12-09-2010, 09:25 PM

On my 91 2.5 602 engine i have had exactly that problem a couple of times.No glow light until a bit after the engine starts and then the light comes on for 30 sec or so,and both times it has been one bad glow plug.I was able to determine which one by testing with The DMM,your test numbers in post 5 look normal,try that test again.If they still all seem normal one sure test is to remove them and apply 12 v with jumper cables to see which ones dont get as hot as the others but thats alot of work just to find one bad one.If you have access to an ammeter that can read 30 or so amps you can test the current draw to each plug one at a time by connecting 12V + to each socket in the plug that plugs into the relay one or more will probably not draw much current those will be the bad ones. They can be replaced without removing the manifold if you have a 10mm universal 1/4" drive and a 12mm deep 1/4 dr and a 1/4 u-joint and or wobble extension and some patience.
the nuts that hold the wire to the plug are captive in the wire end connector so they dont fall and get lost.Some anti-seize on the threads of the new plugs is a good idea too.
Hope this helps Don