Glow plug woes

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ChrisEdu

MB Enthusiast
Joined
Sep 5, 2005
Messages
8,326
Location
By the southern sea
Car
E320CDi Estate - gone E320CDi Saloon - written off CLS 350CDi Coupé
Having had the temporary lighting of the coil indicator on the IC, I duly went about replacing all 6 glow plugs in the hope that this would rectify the issue. Alas, no! I was still getting the light come on and a code telling me that the plug on cylinder four had an open circuit.

So, next, I replaced the relay module, with another new Beru unit. Fingers crossed, in I got, turned the key and bam, same thing 0 on came the light.

Run another diagnostic - this time it reported both an open circuit on cylinder 4 AND a short to ground on cylinder four!

SO, I popped the bonnet, got the tools out and removed the plug from cylinder four (assuming that it is the front cylinder on the off-side?).

Having removed it, I noticed that it was actually warped! It had a slight, but perceptible waviness along the part inside the cylinder!

Thinking that that wasn't a good sign, I replaced it with an old one from the six that I had replaced. Still the same error codes coming up!

Please, could anyone help me with this conundrum? Do I start thinking about trying to replace the wiring harness for the glow plugs? Do I disassemble the plug cap and see if there's anything untoward in there?

I'm really starting to get quite frustrated with it. What I had hoped would be a nice simple job is rapidly turning into a right ball ache!
 
Are you getting power to plug 4? Should be easy to test with a multimeter
 
Are you getting power to plug 4? Should be easy to test with a multimeter
I would think so, otherwise I can't think how the new plug had warped!? Do you have idiot proof instructions to test whether it's getting power. I do have a multimeter, but with the frequent migraines, medication, etc... my brain often struggles!
 
You just need to test for voltage between the connector and earth.
 
Youll only see live whilst the glow plug timer runs.
 
Having had the temporary lighting of the coil indicator on the IC, I duly went about replacing all 6 glow plugs in the hope that this would rectify the issue. Alas, no! I was still getting the light come on and a code telling me that the plug on cylinder four had an open circuit.

So, next, I replaced the relay module, with another new Beru unit. Fingers crossed, in I got, turned the key and bam, same thing 0 on came the light.

Run another diagnostic - this time it reported both an open circuit on cylinder 4 AND a short to ground on cylinder four!

SO, I popped the bonnet, got the tools out and removed the plug from cylinder four (assuming that it is the front cylinder on the off-side?).

Having removed it, I noticed that it was actually warped! It had a slight, but perceptible waviness along the part inside the cylinder!

Thinking that that wasn't a good sign, I replaced it with an old one from the six that I had replaced. Still the same error codes coming up!

Please, could anyone help me with this conundrum? Do I start thinking about trying to replace the wiring harness for the glow plugs? Do I disassemble the plug cap and see if there's anything untoward in there?

I'm really starting to get quite frustrated with it. What I had hoped would be a nice simple job is rapidly turning into a right ball ache!
If you have an OM642 have a look at this very informative video.
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Thanks - I'll take a look when my head has calmed down - mega-migraine today!
 
Make up a solution of acetone and Auto gearbox fluid. Mix well and dribble in with a syringe. Run the engine for a week. Before trying to remove the plugs ensure the engine is hot. As previous very gently torque tighten and then loosen. If no sign of movement soak again. Take time I had to soak one for 3 weeks. Once you get movement undo and tighten 1/4 turn until it comes out.
 
Make up a solution of acetone and Auto gearbox fluid. Mix well and dribble in with a syringe. Run the engine for a week. Before trying to remove the plugs ensure the engine is hot. As previous very gently torque tighten and then loosen. If no sign of movement soak again. Take time I had to soak one for 3 weeks. Once you get movement undo and tighten 1/4 turn until it comes out.
The plugs all came out easily enough when replacing the old ones.
 
Update, as we've had a day without rain!

I ran an Ohm test on the plug in cylinder 4, the one returning a fault code, and got a reading of approx 1 - so the plug appears to be fine.
Next, I ran a test on the wiring from the plug for cylinder 4 back to the glow plug relay module - again all okay!
So, considering the relay is new, the plugs have been replaced, albeit #4 is an old one after the new one warped somehow, and the wiring checks out, I'm at a loss as to what to try next.

Anymore pearls of wisdom from you folks before I have to admit defeat and get ready for more open wallet surgery!?
 
Maybe swap out #4 plug with another and see if the fault follows. Sometimes a failed plug can short out a new relay so it could be the relay again. Maybe a check on a Star machine would be better to confirm.
 
Could be worth checking the wiring between GP relay and GP for resistance as it may be failing under load because of a partial break. Cannot think what beyond that it could be unless the GP Relay has a fault. Did you go through the full test procedure in the video
 
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Well, it's a new day, as the song goes!

Having had no luck with the problem, and thinking I was going to have to go back over everything step-by-step, I got in the car today to drive to Hove, actually!

Expecting to see the coil light come back on, I braced myself for that bitter sensation of Mercedes ownership. However, nothing!I looked again. Nope, it's still not on! I drove the car, parked, switched off the engine. Start back up again, and still nothing! Having had several re-starts of the car today, not once did the light come back on. So, either the magical Mercedes fairies, or my chickens, have repaired the car overnight, or whatever was causing the light to come on has, somehow, spontaneously corrected itself and all the work I had done changing plugs, relay, checking and re-checking has finally paid off!

Oh, the joys of these cars! :D
 
The light should come on after you turn the key and then go out.
 

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