Help diagnosing glow plugs - PeachParts Mercedes-Benz Forum

turbo97se · #1 10-16-2010, 03:19 PM

Wasn't sure if I should continue my old thread or start a new one .. .thought it would be better to start new one.

I checked my glow plugs today using the tutorial on Dieselgiant. I checked resistance of glow plugs at the relay and also on the glow plugs themselves. They all read about 0.2 Ohms which seems low but it isn't zero and definitely not open. I tried 2 different meters and did a short circuit test for both (ie touching leads together to see what lead resistance is ... so the number 0.2 is compensated for).

Next I tried turning the ignition on (using remote start), it does not attempt to start the engine until the cluster glow plug light goes out ... I shut it off before it tries to crank. Anyway, I can get 12V at each of the pins in the relay box with the plug off.

The other more significant thing I noticed is that my glow plug light (in the cluster) never seems to stay on for more than 5 seconds. I am wondering if this is a glow plug or timer issue.

I am also having super lag issues. I am going to hook up a vacuum/boost gauge to try to figure that out too! Grrr ... starting to annoy me. I did quite a lot of reading up on lag issues like cleaning out ALDA banjo bolt of gunk ... only to find the OM603 doesn't have a banjo. Also read a lot about how to set valves ... only to find OM603 has hydraulic lifters .. guess I need to be smarter about how I search! Thought most of the diesels would be the same! Bad assumption on my part!

Hopefully some more experienced guys can help here.

Thanks!

micalk · #2 10-16-2010, 04:28 PM

You need to check the voltage at each of the glow plugs with the plug installed on the relay. When that plug is off, there's no current flowing, so no voltage drops. You should see battery voltage as long as there's any kind of connection except a short to ground. When you have current flowing, then any resistance in the circuit will drop voltage.

First, tell us what symptoms you are experiencing.

Second, partial shorts in the glow plugs could account for the low resistance. Your best bet would be to pull each one and test it outside the engine block. Put 12V and ground across it and see if it glows red. If you have any doubt about a plug, replace it. If they all have the same resistance, and you have any doubts, replace them all.

As far as your lights are concerned, the GP relay factors in the outside temp and determines the length of time the dash light illuminates. Doesn't mean that the relay shuts off, just the light. Listen for a loud click and that should be the relay.

turbo97se · #3 10-16-2010, 06:27 PM

Thanks for the reply ... sorry symptoms I listed in another thread. The car is very hard starting in winter. It starts but then dies pretty quickly even if I cycle the key a few times to warm the glow plugs. When I try to start it, it seems almost like it is flooded and I can't get it to start for a VERY long time. My battery would die long before I could get it started - It's a BRAND NEW battery ... well since last winter.

When I put the plug on and measure the voltage at the glow plug relay, the voltage is 10V (approximately) for all the glow plugs. The voltage seems to stay at 10V for a long time, does not seem to click off after any period of time which is odd. I can hear a relay click off inside the car but in the engine bay, the lines stay hot at 10V. Is that normal?

Thanks!

babymog · #4 10-16-2010, 08:13 PM

The light in your '87 is controlled by a relay, independent of the glow-plug relay, in the same box however.

The light and its relay is supposed to approximate the time necessary to glow for a good clean start, based on the temperature of the relay box under the hood, a healthy engine, and properly operating glow-plugs.

The glow plugs and their relay on the other hand, will glow for (IIRC) 45seconds, regardless of any other factors, unless you turn the key to start where the glow-plug cycle is then cancelled.

The voltage at the plugs is going to be a bit lower than the battery voltage, 10v sounds a bit low, but I've never measured mine. Someone else with a 603 might chime in here.

If there are no high-resistance plugs, I'd try a bit longer glow cycle, try the full 45seconds (wait until the dome-light goes brigher, or if it's quiet in your car until you hear the relay drop out). See how it starts then, hopefully it is just a bad light-timing circuit, I've had two relays fail that way and it is a bit inconvenient to have the glow-light turn off early, but not a show-stopper.

layback40 · #5 10-16-2010, 09:09 PM

The only way to be sure that GP's are good is to remove them and test them. They need to glow bright red. All the resistance testing in the world can not guarantee they are good. You will also get an indication of how much carbon build up you have in the prechamber.

turbo97se · #6 10-16-2010, 09:40 PM

Ok Thanks for all the replies. I think there is nothing for it .. I have to pull the glow plugs and look at them. They are a real pain to get to ... I'll just pull the intake manifold and clean that while I am at it. I am having serious starting issues even when I cycle the key 6 to 8 times waiting for the glow plug light to go off each time. I might just go ahead and replace them anyway ... one less variable to deal with. I guess I need to get a reamer too!

I am also having some issues with either vacuum pump or brake booster. When I hook up the boost/vacuum gauge to tee off the brake booster port, it pulls around 18-20 Inches Hg, Each time I press the brake pedal, the vacuum drops by 1-2 inches. After 10 presses, the vacuum is all gone. The brakes still feel solid but it takes a few seconds before the vacuum gets back up to 18 Inches Hg ... any thoughts on this? I piggybacked this question on another thread that looked similar but not sure it would catch anyone's attention. Anyone have any thoughts on this one? Gee a few issues to fix!

scottmcphee · #7 10-16-2010, 09:53 PM

So you're on the right track for the GP's

Somebody said when they're out, look at them... by connecting jumper cables to a GP hanging in mid air to the battery. The thing should get BEYOND red hot, beyond orange, and into bright yellow. That's a good GP anything else file it in the circular bin.

If vacuum is the issue, start another thread.

babymog · #8 10-16-2010, 10:32 PM

Did you try the 45 second glow yet? If not, please stop wasting forum bandwidth.

You've posted that you try to start it after the light goes off, and that the light goes off at no more than 5 seconds, you're not glowing long enough for a cold engine.

Try this, and report back.

Also, reading your post it is difficult to discern whether you understand the pre-glow, seems like you might also be cranking your car immediately and not waiting for the glow light to go off (3rd paragraph in first post). Please clarify.

Key to position 2, glow-light on, wait 30seconds, turn to start position / crank, see if it starts normally. Report results.

If this changes the starting behaviour of your engine, then we have something to troubleshoot. If you want to pull parts and see the pretty glow, have at it.

turbo97se · #9 10-16-2010, 10:58 PM

I did not understand before about this. I did after reading you post. In the past I had thought that when the glow light went out that it was done with the glowplugs and so as I had mentioned, I cycled the key 6-8 times (essentially giving 30-40 seconds of glow plug warm up). I have indeed tried leaving it for longer but it just seems like when it does start, it's still cold. It shudders until warm almost like it is missing (not firing on all cylinders).

Quote:

Originally Posted by babymog

Did you try the 45 second glow yet? If not, please stop wasting forum bandwidth.

You've posted that you try to start it after the light goes off, and that the light goes off at no more than 5 seconds, you're not glowing long enough for a cold engine.

Try this, and report back.

Also, reading your post it is difficult to discern whether you understand the pre-glow, seems like you might also be cranking your car immediately and not waiting for the glow light to go off (3rd paragraph in first post). Please clarify.

Key to position 2, glow-light on, wait 30seconds, turn to start position / crank, see if it starts normally. Report results.

If this changes the starting behaviour of your engine, then we have something to troubleshoot. If you want to pull parts and see the pretty glow, have at it.

babymog · #10 10-17-2010, 08:49 PM

Quote:

Originally Posted by turbo97se

I did not understand before about this. I did after reading you post. In the past I had thought that when the glow light went out that it was done with the glowplugs and so as I had mentioned, I cycled the key 6-8 times (essentially giving 30-40 seconds of glow plug warm up). I have indeed tried leaving it for longer but it just seems like when it does start, it's still cold. It shudders until warm almost like it is missing (not firing on all cylinders).

I understand that your cumulative glow time might be 30-40 seconds, but that is significantly different from a 30-40 second glow, which is why I suggested it.

Good luck, any change in symptoms please post.

turbo97se · #11 10-18-2010, 11:17 PM

Had a couple of unusually warm days the past few days so nothing to report earlier. Today got a little colder so I tried just waiting for the full 40 -45 seconds until I could hear the relay click, it does indeed make a difference. Thank you for scolding me not to waste bandwidth or I would have missed the difference between the glow plug light and actual glow plug relay time! LOL

. Clearly there is something wrong with the glow plug indicator timer for the bulb in the cluster, but at least the glow plugs stay on! As the days get colder, I will know for sure!

Quote:

Originally Posted by babymog

I understand that your cumulative glow time might be 30-40 seconds, but that is significantly different from a 30-40 second glow, which is why I suggested it.

Good luck, any change in symptoms please post.

micalk · #12 10-19-2010, 08:42 AM

In another similar thread, someone suggested checking the temp sensor that is the input to the GP relay. You might give that a try and see if it extends the GP dash light.

Diesel911 · #13 10-16-2010, 11:10 PM

If you remove your Glow Plugs you cleaning the Carbon out of the Glow Plug Holes will allow your Glow Plugs to last longer.

The make a Glow Plug Reamer for that but there are other methods discussed in the DIY section.

DIY Repair Links
http://www.peachparts.com/shopforum/diy-links-parts-category/

http://www.peachparts.com/Wikka/DoItYourSelf

sixto · #14 10-17-2010, 12:20 AM

The 603 doesn't have a banjo bolt but you should check and clean the fitting on the intake manifold above the ALDA from where the ALDA gets a manifold pressure signal. It's easy to break it if you overtighten it putting it back in place.

Sixto
87 300D

vstech · #15 10-19-2010, 08:51 AM

OK, 45 seconds makes a difference...
so, the engine starts smoothly when you wait the entire 45 seconds and start?
or does it start OK, but stumble for a bit after starting?
the light is just a reminder for drivers to use the glow plugs. don't worry about it.
as long as it IS coming on, it's fine.
on REALLY cold days, you may have to let it glow the entire 45 and click, then turn it off, and glow for another 45 to get the PC warm enough for COLD fuel to light.