Glow Plug Warning Light Problem?

[Thistle]

Hi folks.
I’m seeking a little guidance from the experts here, about a problem with my car’s glow plug system.
The car is an S320CDi 2009 build.
A few months ago the glow plug warning light remained illuminated and I traced the problem to an inoperative glow plug. Plug replaced, the light went out as it is supposed to after warming on start-up.

Now, the problem is back with a twist. The glow plug light comes on at cold start-up briefly goes off then comes on again. It also occasionally comes on in the middle of longer journeys.

Resistance across each glow plug is about 0.5 Ohms, and each one is seeing 5.5v from the relay control unit at cold start-up.
Am I correct in thinking that the glow plug control unit is causing the problem or should I look elsewhere? The control unit is part number A642 153 20 79 and made by Beru with their code 0 522 140 202.

Thanks for reading.
Thistle

 

[merc85]

Just had this with my e320 cdi (s211) and a change of the Glow plug relay sorted it.

 

[Zulu 10]

I've just to replace my glow plug control unit on my OM642 engined CLS, but the fault was hard, rather than intermittent, and was confirmed by Star.

There are at least two super-cessions relating to this part number, and the latest iteration is a significantly different shape to the version you've got.

As you're getting the correct 5V feed to each glow plug, and you're getting the fault at times when the glow plugs are not in use, then it may be that there is an intermittent failure to communicate rather than the more common output stage failure.

You're only going to be certain by either (1) spending £120 on a control unit, or; (2) getting it plugged into Star to see what the fault log shows.

 

[t-dawg1]

+1 for glow plug relay,I had same issue on my OM642 and cured with new relay. New design is a different shape to old one and is made by BERU

 

[Thistle]

Thanks for the input. I'll pick up a new controller module this weekend and report back if it fixes the problem.

 

[Thistle]

The follow-up. I fitted a Glow Plug Control Unit A6429005801, bought from the local MB dealer and the problem has now gone. It took about 10 mins to swap the units. As pointed out above, the unit has been through several iterations of model number; also, there’s plenty of them for sale (and already sold) on Ebay. I wonder if this is a known problem that the revised units are designed to address.

 

[merc85]

Glad you got it sorted easily

 

[diverzeusy]

thanks for posting follow up, i have been looking all over for new part number as my old one has failed.

 

[E280wilson]

Yes it's a known problem. I did mine a few weeks ago. On a some of the old units you could take them apart and repair the wire that had burned out. I tried this with mine only to find the circuit board coated in resin so I just bought a new one for £125 inc.