Bobby53

Hello everyone,
I'm new here and like most arrive with a problem that I hope someone can advise me on.
The issue is that for a long time now I keep getting an engine management light. Using my OBDll I get the DTC UO106 'Lost communication with the glowplug control module.'. Despise being told the module was unlikely to be at fault I replaced it...and for a week or two I thought I'd fixed it. Sadly I was wrong.
The car starts every morning even when everywhere is frozen so I didn't think anything of the glow plugs but did think my original Merc battery should be replaced...I did that and after about 4 days the engine management light disappeared of its own accord so I hoped that now the new AGM battery was fully charged my electric faults maybe resolved, but today after having no DTc for 4 days it has come back again.
I think I hit a pothole (loose connection) but it may of already been on prior.
I know how to test the plugs and the ohms...also the pins between the plug black cap connection and the multi pin plug in the loom for continuity...but that's about it.
I also don't know where the power lead for the GPCM goes under the car...presumably to the starter or solenoid?

Any help greatly appreciated as it's now driving me mad!!

Thanks again Rob 👍
ps anyone near Blackpool UK know of where I'm best going other than a main dealer.
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CaliBenzDriver

Rob, you have done a lot of good for your chassis. So far you've replaced glowplug controller and AGM. This helped a bit but not completely fixed.

Are you familiar with chassis main GND strap? It's located all the way under the car near tranny and muffler. A poor connection has likely developed there causing a sizeable voltage drop under heavy load.
Try cleaning main strap and add a secondary GND up high in engine bay such as found on Honda/Toyota .

Bobby53

Thanks for that, I've just replaced all the rear brakes, handbrake shoes, pads and disc and am doing the front this week so while there I'll clean up every earth lead I can find (I only knew of a couple under the bonnet) so thanks.

After the light came on the last time I cleared it with the OBD and it's not been back on since. It's like waiting for a time bomb...is it fixed or not haha.
Thank you very much for your reply, much appreciated 👍😊
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CaliBenzDriver

Yes, all of these details are helpful to identify the source of this particular trouble.

What appears to us like randomness mas be linked to an electrical load condition that happens not too often ( full headlights + A/C fan high = high current demand = high drop = creates marginal conditions).

The main single strap located down below at salty undercarriage is like a prerequisite cleaning - OR - install a second strap high up dry where it will remain reliable.
Main strap supplies the starter 800Amp crank. It works really hard, gets hot near muffler and oxidized dark.

Bobby53

Thank you for the help, I have a question thought and that is that while the majority of the time this EML/DTc will show up on starting....however it has illuminated while driving and not under load.
Another thing I found that's strange is that I can cancel the EML (make it go out) is to do the following.
With the engine running if I turn the key back to switch off the engine...but then back quickly before the engine can stall....if I do that 3-usually 4 times (less than 10secs) the EML light will go out?
I don't know if this is a secret mb reset or if it's related to my issue.
I'm currently looking at buying a starter/alternator harness which also includes the power supply to the GPCM.

​​​​​​So to recap new GPCM, new AGM battery, no cold starting issues, smoke or noticeable noises. But a recurring DTC UO106 fault code.

Thank again for you knowledge and help

Kind regards Rob 👍

Bobby53

PS, I've just been to Mercedes, the chap there said that here in the UK we don't have the rear earth strap like you have in the USA. We just get the main one at the front.
I asked about a new starter harness that includes the plug to the GPCM and they won't sell that part (which is available on eBay from £15) on its own, only with another section of loom with other connections for £250 :-(

​​​​​​Thanks in advance for any help

Regards Rob :-)