Greetings group - I need some help with my 2003 C270cdi diesel. The
engine turns over well but just won't fire or start. The only thing I
changed in the engine recently was the air filter, and it has run well
since then. Local ambient temperature is around 4 deg celsius. All
fuses are fine, incl the 50-amp for the glow-plug circuit ( I
presume). There is electrical continuity through the glow plugs, The
car has covered 80,000 miles and the diagnostics says 'no
malfunctions'. The glow plug light comes on and goes out as per
The only thing I have been able to find that is puzzling me it there
is electrical continuity between the supply to each glow plug and
earth, and no volts at the supply in the starting cycle.
All advice welcome!!
Sounds like glow plug relay is not working. Double check for power at the
glow plug relay. Just follow the thick wire from the glow plug fuse and you
will find the relay. Make sure you got 12V or higher there.
But if the engine turns well and the temp is 4 degrees Celsius, the car
should start without any help from glow plugs, it is a CDI.
Could it be that you have unplugged some of the sensors, like camshaft
position sensor or something? Should be a coinsidence if the crankshaft
sensor had failed just when you were working elsewhere in the car. Not sure
if the instrument cluster would report these sensors, should let the dealer
read the codes.
Thanks Tiger and Anon. I doubt that any sensors were unplugged
accidentally or otherwise. I have since tried a shot of diesel easy-
start in the air intake and the engine fired on that. That has made me
suspect the glow plugs primarily. I tested the middle three (most
easily accessed) with an ohmmeter, and resistances were around 1.6 ohm
which seems to be within tolerances per the data on this group.
Nevertheless, I'll change them all tomorrow. At the mileage on the
car, they are going to start giving trouble sooner or later anyway.
I tried tracing the wire to the glow plug relay as Tiger suggested,
but it is impossible to do that without disassembling the fuseboard to
see underneath, and at this stage I don't want to disturb too many
complex electrical gizmos if I can help it, in case I worsen the
All further comms very welcome, I will report after I have fitted the
glow plugs, I now have the battery on a charger as it was beginning to
slow down due to all the cranking.
Dealer is a last dire resort - 28 miles away and well-earned poor
reputation for servicing, I never found them to be any way
satisfactory with previous MB's I owned, and I go back as far as a
1959 190D ponton, 1968 280SE, 1978 220D/8 etc!
Generally speaking. Glow plug relay is located on driver side wheel well
behind the headlight location. You should see a black box with a cover on
the bottom... Look up the part on online store so you get an idea what it
I think your problem is at the glow plug relay... not the glow plug. Even
with one or two bad plug, the car should start up roughly.
Thanks again to everybody who is contributing to this - you seem to
have a really helpful and friendly group here. The update is - last
night I changed all five glow plugs. All measured on the bench, all
between 1 and 2 ohms resistance. I and 5 are fiddly to get out, 2, 3
and 4 are easy. None as difficult as a peugeot 405 I had a few years
ago where two plugs were behind the injection pump.
Anyhow, with five new plugs installed, and the battery trickle charged
overnight the car started easily this morning at ambient temperature
of about 6 Celsius. Then the show started. The computer reported loss
of the engine stability programme, and loss of the ASP (traction
control). The little yellow engine light came on and stayed on. The
blue headlight lamp came on a few times, even though the lights were
off. I had visions of a big wad of money going to my local dealer
after all to resolve these issues. However, after a few miles of
driving, the fault displays disappeared and it reverted to 'no
malfunctions', only this time with the engine running for a change.
Once the car was switched off and restarted, the yellow engine light
stayed off, so as of now all seems to be well. I have driven about
seventy miles in it today, and it seems fine again, so I am hoping the
problem lay with the glow plugs somehow.
I'll be back with more questions no doubt, for now, thanks again.
John in Ireland
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