W124 heater and aircon puzzle

[ian ms]

I am new to the forum and very impressed with the wealth of information. I've recently bought a 1994L 280TE 120k and I am very pleased with the car. Although the service record is bang up to date, I suspect that a non-merc-experienced mechanic has been at the heater controls. Symptoms are:
1. The air con button is working the wrong way, i.e. only operates when pushed in and lit.
2. with both temperature controls set to cold and aircon off (EC light off) hot air comes from passenger side, cool air from driver side.
3. Sensor intake fails the 'tissue paper test' (from other threads) so there does not seem to be any air flowing in.

Could the sensor be causing all these symptoms or is it a combination. Any advice out there where to start?

 

[justin1820]

fisrtly when the ec button is alight the air con is off, ec stands for economy, the business with the hot and cold air is probably the duo valve which is just like two taps located near the brake master cylinder. there is some easy tests for this

 

[ian ms]

Thanks for reply. The EC button is working the wrong way. When you press it and light the led, the a/c compressor engages and cooling starts. You can detect this by the engine idle revs dropping under load. Judging by some of the threads the duovalve is the least likely cause after heater control potentiometers, cabin sensor and sensor pump.
I tried today to get at the heater control potentiometers but backed off when the wood facia would not be parted from the control panel. After removing the knobs and the securing nuts, the facia was loose but only on one side. The other side was very tight against the temperature wheel. As it happens, this is the side that is giving the problem. I stopped before the facia cracked. I am wondering whether this area has overheated and the facia is actually stuck to the face of the panel. Are there some hidden screws I have missed which are preventing the facia from coming away? Any ideas out there? I was hoping to reverse the EC connections or at least to find out where they have been mis-connected.

 

[ian ms]

Heater panel problem solved

Problem solved.
Turned out to be a short on the PCB of the control panel which was causing all manner of strange behaviour in the heating/cooling system. As the panel was knackered anyway I thought it might be useful to the forum if I dissected it and photographed it along the way. Turns out they are a doddle to strip and re-assemble. Much easier to deal with bulbs and erratic potentiometers by this method.
I have not found the instructions for uploading graphics to the site so I have placed it on my web server.
The page can be accessed at:
http://www.gcraft.co.uk/Mercedes panel.htm

 

[NormanB]

Ian

Were you able to discover the cause of the short on the PCB of the control panel? Did you discover by eyeball or more sophisticated testing and were you able to repair. (Was it on the blue board right hand side in your pics)?

Nice pics on your link, thanks.

 

[ian ms]

Norman
Not sure what caused the short as the problem was there when I bought the car a couple of weeks ago. It was immediately apparent when I came to remove the wood trim which was free on the drivers side but firmly stuck on the passenger side. I prised it off safely (eventually!) to find that the varnish had melted on the back and stuck it to the panel. I did'nt need to test anything, the molten plastic said it all. If you are into electronics I could post you the PCB to find what caused the overheat, might be a useful reference.
Anyone else come across this problem?

 

[NormanB]

Norman
Not sure what caused the short as the problem was there when I bought the car a couple of weeks ago. It was immediately apparent when I came to remove the wood trim which was free on the drivers side but firmly stuck on the passenger side. I prised it off safely (eventually!) to find that the varnish had melted on the back and stuck it to the panel. I did'nt need to test anything, the molten plastic said it all. If you are into electronics I could post you the PCB to find what caused the overheat, might be a useful reference. Anyone else come across this problem?

Hi Ian

OK - I understand how you located the fault now

I am afraid electrickery has never been one of my strong points - so no real point sending to me, maybe others who are more expert (and that would not be difficult) would be interested.

If I have a PCB down and I prove it ain't a connector then I always have a go at repairing, on the principle that if its broke you cannot break it twice. Thats why I mentioned the MK1 eyeball - cos thats my limit I just look for iffy looking joints (dry) and flash a small iron over them (carefully) - obviously if it is near sensitive components I heat sink them. I have been successful in about half the repairs I have attempted. Funnily enough never had a Merc PCB prob (yet - touch wood). I understand MB cruise control amplifiers fail with a common theme of dry joints - I will never experience this with my car fortunately! (No cruise).

Regards

 

[ian ms]

Norman
Just a thought, should'nt a fuse have blown somewhere when this panel overheated, assuming it was caused by a short?