W124 Fluctuating temp guage

[djwazza]

I have a fluctuating temp guage and have no idea whether this is a 'normal' w124 fault or if it has a simple solution.

Any help would be much appreciated as the regular move to the red zone is becoming unnerving. To clarify there is no cooling problem with the car.

 

[grober]

Probably just a bad connection to the sensor. Its usually a single pin sensor on the nearside of the cylinder head at the front of the engine. Get a friend to wiggle it to see if the gauge reading changes-once found clean the connections up.

 

[jaymanek]

I have the same problem with my fuel guage on the E500..

Ive played with the fuel sender to no avail. Ive read that the cause is normally a bad earth on the instrument?

 

[andy_k]

maybe the thermostat is opening too late or the thermo switch for the fans has started to fail.

I seem to remember that the auxillary fans don't kick in until the temp reaches 105 so the temp goes up, the fans switch on and the temp goes back down again

Our old 230 estate used to do that and never suffered any ill effects

 

[djwazza]

I'll check the sender unit and let you know the outcome. I don't think it can be the fans cutting in as the gauge reads 120 for a while and then drops back to the normal 80 range very suddenly; I'm guessing something electrical and magic. Thanks for the help.

 

[Smiley]

I had exactly the same fault on my 124 e300d estate when the car was about 4 years old - replacing the sensor in the head fixed it.

 

[Number_Cruncher]

I hope the OP doesn't find what I found when looking at the wiring around the temp sensor on my E300D - as is more often noted on petrol models of this age, I found the insulation on the wiring harness had degraded - especially on the wires going to the temperature sensor.

As I use my car to get to work, and I didn;t want the proble to spread and do damage to the control unit, I replaced the engine loom. It was just over £300, and took a quiet hour or two to fit in place.

Sorry to say - but the gauge moving rapidly does suggest a wiring issue might be the cause.

 

[Smiley]

TBH I think he probably will find that - and it could be the cause I guess - thing is that these cars are now getting to the age where none will escape the loom issues.
Are there cheaper part loom replacements available now?
I still think MB should do the decent thing and significantly discount the loom prices to soften the blow a bit.

 

[grober]

To be fair to Mercedes for most 15+years old cars this isn't an issue as many are in the crusher by this time. At least you can still buy one from a Merc parts department. Its unlikely anyone will make any cheap aftermarket replacements as only a very limited demand for these remains. Anyone manufacturing these now would charge similar if not more money to MB due to them catering to what would be a "bespoke market".

 

[Number_Cruncher]

Are there cheaper part loom replacements available now?

There are, but, not for the diesels - I've seen small local repair looms for the 320 petrol engines.

As it affects the hottest parts of the loom first, it *might* be possible to do a localised repair, by buying some new wire, some new connectors, new plastic shells for the multi plugs, and then doing some soldering, crimping and heat shrinking work. One wire at a time to make sure there aren't any **** ups, and doing a good mechanically and electrically sound job will take quite a while.

The danger, of course, is that as you peel back the outer insulation, looking for sound wire to solder onto, you end up unravelling half of the entire engine loom before you find any!

 

[Smiley]

To be fair to Mercedes for most 15+years old cars this isn't an issue as many are in the crusher by this time. At least you can still buy one from a Merc parts department. Its unlikely anyone will make any cheap aftermarket replacements as only a very limited demand for these remains. Anyone manufacturing these now would charge similar if not more money to MB due to them catering to what would be a "bespoke market".

Fair enough but £300-£500 depending on the model must have a fair chunk of profit for MB in it - they could sell them at cost recognising that there is a problem - without admitting liability of course.

 

[bolide]

I suspect most W124 gauge faults are in the cluster. I've seen temp gauges jump up and down and oil pressure gauges pinned as soon as the ignition goes on. Looking through the history of one car the temp senders had been replaced more than once - suggesting to me that it's not a sender fault

I've never tracked one of these faults to earth (there's a joke in there somewhere,) so can't say for sure, but I suspect a voltage regulator or other fault in the binnacle

Nick Froome
the independent Mercedes Estate specialists

 

[kth286]

has the coolant been topped up etc recently ?

could it be air locks within the circuit ?

is it the start of a bigger problem ?

 

[wilf1227]

I have a 300 124 and recenlty had the head, gasket and almost everything it seemed done!

I noticed that the temp guage fluctuates a lot more since the gasket was replaced. The head was skimmed and genuine parts used. i even changed the new thermostat to be sure.

Panic was setting in till I checked the temp the thermostat opens at and when the additional fans for the air con cut in. In particular the fans do not cut in until at least 97 degrees and anything up to 105!

There is no pressure build up and no rough running on start up so I am assuming nothing untoward with the cooling system.

Maybe I never noticed the guage moving before the work and now I seem to be looking at it all the time. From memeory though the guage did not seem to go past 80 degrees before the work was done?