W124 electrical gremlins 'various'

[slowcoach1]

Hi all, for once a productive session on a quiet bank holiday weekend

I am an electrical/ electronics tech by trade so confident in following schematics and wielding a multimeter and soldering iron

This is an excellent guide resource for all manner of things re the 124 and I'm sure it has been posted before, item 24 the ETM was the bible for my faults

Mercedes Benz Model 124 - M103 Maintenance Manuals

Mine is the 2.6 six cylinder SOHC engine full of grunt and as smooth as a turbine, you can pick other varients from the front page of the link above if you have a bigger/ smaller petrol/diesel varient

Lovely old weekend chariot that is a rust free import with low miles but was suffering various electrical system faults as follows

bulb fail light illuminated even though all bulbs good
erratic temp control
no air con
erratic rev counter, random on off operation
erratic central locking regardless of which lock was used

1st port of call remove all fuses and clean contact prongs with wire wool until shiny and renew all fuses as some looked a bit tarnished

Hurruah 1st result central locking now playing the game on all locks

2nd pulled both lamp warning relay (long one within fuse box) and AC Klima relay (behind battery) and popped covers off

With aid of magnifying glass dry powdery solder joints observed in both units, so resoldered and any other dodgy looking supects given the same

Second result bulb warning lamp now fuctioning ok and tested by removing bulbs around the car to check it illuminated when a bulb is out

Still no AC although checking the sight glass showed fluid moving around under idle and start and pressure switches in the refridgerant ciruit were closed showing system was gassed up so the best fix resulted in following the ETM section of the link above to track the engine speed signal which is controlled by the ignition module as I had a hunch that this could be the culprit as it is related to the rev counter and AC operation as both use this signal to run the rev counter and contol the cut in and out via the Klima relay of the AC compressor

Third result to the side of the diagnostic socket on their own screw are the speed signal connectors from the EZL ignition unit which then goes to rev counter and elsewhere, the Klima relay included

This was again corroded and crusty looking so cleaned and shined with wire wool and rev counter stable and AC now kicking in, so a 2 for 1 fix

Lastly the easiest fix was the non control of cabin temp as this was an all or nothing affair in that heat was either on or off

Up in the overhead lamp unit is a temperature sensor that has air drawn accross it by a small fan in the dash left side behind glove box to sample the cabin air temp, dropping the lamp unit revealed the air sampling tube was dsconnected probaly by a previous ownder changing a bulb up there
fan was running as a small piece of paper held accross the end of the tube was held in place by the 'suction'

Cabin now cycles its temp up to setting then heat backs off to maintain the level

May be of use to others seeing as these old girls are getting on in age to cure any similar niggles

 

[grober]

I always take the chance when discussing older Mercedes with EZL ignition to remind folks to renew the heatsink compound between the EZL UNIT and the inner wing. Essential to dissipating unit internal heat----- if it dries out the EZL unit will eventually overheat- they can't be easily repaired if at all due to their encapsulated construction.

 

[slowcoach1]

Cheers grober next item on the list!