E300 Diesel glow plug problem (W124 1995)

[TonyVianoAmbiente]

My 1995 E300 Diesel auto has been laid up for a while because I have been using another car. I decided to MOT it last week and found it very difficult to start.

I think the glow plugs are not working. The glow plug light on the dash doesn't come on when I turn on the ignition, but it does come on randomly when the car has warmed up and I am driving it.

Once the engine starts, it runs smoothly, but it will only start when the outside temperature exceeds about 5C, and only with a lot of cranking - as much as 1 or 2 minutes. It's killing the battery, so I need to sort it out. At least it passed the MOT. ;-)

I do have a Haynes manual for the W124 but this particular variant isn't covered, and the Haynes manual is poor in any case.

I vaguely recall reading on this forum about a control box or relay under the bonnet at the driver's side, or possibly a fuse. Any pointers please?

 

[Dieselman]

At a guess the glow relay has gone faulty. It's mounted on the n/s inner wing by the washer bottle.

 

[Number_Cruncher]

>>the glow relay has gone faulty

Before buying a new relay, also check the engine wiring harness. For example, the glow plug relay relies on a signal from the coolant temperature sensor to determine how long to glow for. If the wires to the coolant temperature sensor are shorted, the glow plug relay will think the engine is (very!) hot, and needs no glow cycle.

On my 1995 E300D, the wiring had degraded the most in the area of this coolant temperature sensor.

 

[spock500]

Its easy to get at or at least it was on my C250D.

Have a look on the offside front wing nr the washer bottle, once found it comes apart with a couple of screws.

You can activate the ignition and watch the magnetic coils pull the contacts together.

 

[Olivier]

that's my old relay. You need to look at the fuse inside, might be the trouble, but you canno change the fuse.

 

[TonyVianoAmbiente]

Thanks for all the replies - very useful, especially the photos.

This relay (MB Part No. 015 545 30 32) is extremely expensive at well over £100 and I would be wasting a lot of money if it isn't at fault. So before I splash the cash, I would be very grateful for any fault-finding advice.

I'm not technically skilled so basic advice would be particularly welcome!

 

[Number_Cruncher]

>>I would be very grateful for any fault-finding advice.

The signal wire for the coolant temperature sensor is terminal 6 (white). I don't know what the proper resistance values are, but, I would hope to see something like 1K or 2K Ohms between the disconnected terminal 6 and earth when the car is cold. If you find a very low resistance, say, less than 100 Ohms when cold, it's probably worth investigating further.

If you get someone else to turn the key, have a listen to the relay - it closes with quite a clunk. Another tell-tale way to see what's happening is to leave the door ajar, and watch the inerior lamp when the relay clunks. You should see the lamp dim, and then go bright again when the relay contacts open again.

 

[TonyVianoAmbiente]

The signal wire for the coolant temperature sensor is terminal 6 (white). I don't know what the proper resistance values are, but, I would hope to see something like 1K or 2K Ohms between the disconnected terminal 6 and earth when the car is cold. If you find a very low resistance, say, less than 100 Ohms when cold, it's probably worth investigating further.

If you get someone else to turn the key, have a listen to the relay - it closes with quite a clunk. Another tell-tale way to see what's happening is to leave the door ajar, and watch the inerior lamp when the relay clunks. You should see the lamp dim, and then go bright again when the relay contacts open again.

That's great, thanks. :bannana:

I've also managed to locate two used relays, one at £50 and the other at about £65. A new one is more than twice the price, which is why I wanted to do some checks before shelling out the cash.

 

[Mercedes cozy]

Some of these relays have a fuse screwed on the relay.

Check it as i found one last week, it was cracked.

If you have a multimeter you can check to see if you have any voltage going to the glowplugs. Put the lead into the plug that fits into the glow plug and the other multimeter lead to earth anywhere on the car.

If you have voltage then it can be faulty glow plugs as well.

 

[TonyVianoAmbiente]

E300 Diesel glow plug problem (W124 1995) UPDATE

OK, I have fitted a replacement relay from a scrapyard (£34 including postage) and the glow plugs now work. Only problem is, the glow plug warning light still doesn't appear when you turn the ignition on, and comes on randomly while driving.

I know the glow plugs are working because the car starts after 2 seconds of cranking rather than 2 minutes. ;-)

Any suggestions?

 

[Mercedes cozy]

it means that one or more glow plug is gone faulty. change it and it will start better.