SL320 1998 Heater Problems

[MarkN]

After about 25 minutes on the road airflow from the AC heater unit stops but I can still hear the fan motor running. To date the official Mercedes distributor service personnel have had this car in 7 times with no improvement. They've replaced an airflow valve, the control panel and the pollen filter(!). Any suggestions before I go back to the VW?

 

[piestore]

a/c heater

Yes

I have had this problem before. faulty sensors on the ac evaporater cause the evaporator to freeze up and block all air flow. this is also when heater is on as the air is cooled and dried before heating. to test switch of A/c compressor if ok this is fault if not don't know.

 

[MarkN]

Thanks for the feedback! I'll run that one by the technician.

 

[Tony Musgrave]

I'll second the theory that the evaporator freezes.

My 1998 SL suffered from the same problem on longish runs, with the A/C on and low fan speed the airflow used to progressively get less until.......Nothing.

The dealer checked the problem, but by the time the technician got around to it the ice had melted,and they said they couldn't find anything wrong.To demonstrate that there was a fault, I waited until the evaporator had frozen, then drove to the dealer and left the engine running with the A/C on and insisted that the technician checked the car there and then.

Once they saw the fault for themselves they cured the problem.I think it was a sensor,but I can't remember which one.

 

[MarkN]

Definitely looks like this is the problem, I took the car for a long run (2 hrs) with the a/c off and there was no trouble with the hot air flow. This place is great and I love you all. In a matter of days you've managed to diagnose a fault that's had me tearing my hair out for almost two months.
Thanks again!