The buttons on my Tempmatic pushbutton unit (1987 2.3-16v) have no effect on the climate control. No matter what button is pushed, even the "O" (off) button, the fan stays on. Also, the other buttons have no effect in rerouting air to different vents (including the defrost setting). Note the following tidbits of info:

the pushbutton unit is new;
the switchover valve unit is new;
the feedback potentiometer and associated valve (on top of heater box beneath dashboard) are new;
I hear a clicking in the blower control switch (controls blower speed) as the blower turns off, then turns right back on again;

What could the problem be? A bad blower control switch? A bad blower motor resistor group? Something more or less obscure?

John

[This message has been edited by revbond (edited 02-22-2000).]

The sequence of on-off clicking of the blower switch is a way of diagnosis. The most common problem is the aux water pump.
Also any of the temp sensors that short out will cause this problem. If you time the clicking & have a W201 cd you can see what the car is trying to tell you.

thanks for the reply!

do you know, off-hand, the section of the 201 CD which describes this diagnosis?

john

Look in the climate control section, there is a diagnostic section that should help. Look at the 1987 insert for the answeres to your question.

I'd bet on the feedback potentiometer being faulty. Check it and see. (You need to look in the 87 supplement.)

------------------
Benzmac:
1981 280GE SWB
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

do you think that the new feedback potentiometer would also be faulty? would the fault lie in the connection plug?

John

If you have the diagnostic LED connected, you can count the blinks. Otherwise, count the time interval in seconds the blower is running (when the blower is cycling).

10 seconds (5 LED blinks) = outside temp sensor
20 seconds (10 LED blinks)= inside temp sensor
30 seconds (15 LED blinks)= feedback potentiometer
40 seconds (20 LED blinks)= evap temp sensor
50 seconds (25 LED blinks) = coolant temp guage sensor

This is a secondhand info. It may not apply to your car. I have never worked on the system before. That is all I know. Please comment if anyone has more info.


David

Well, just my luck, the AC cycles at 45 seconds. Go figure. So I gave up and took it to an independent mechanic, who spent two hours on it, and also gave up. Or, rather, did not feel that he could solve the problem within the 3hrs allotted, but would have to follow every wire to determine the source of the problem. So I am taking it to another independent and, after that, if the problem is still not diagnosed ... well ... I guess I will just have to roll it off a cliff.

Any other suggestions?

John

Well, this problem has been resolved, sort of. After giving up, I took the car to two independent shops. The first, a two person operation (one of whom an elder german fellow), tried all the usual tests (temp sensors, push button units, etc) and could not find the problem. He felt that he could continue only if I gave him license to spend unlimited time till the problem was identified (essentially checking every wire and every connection). I would have been ok with this since, really, what choice did I have, and it is clear that the answer was not obvious. He, oddly enough didn't feel so comfortable about it. By the way, he didn't charge me for the time since he didn't solve anything. I told him I would get back to him.

I decided to try another independent shop with about 10 techs (the local mercedes dealer shop would not take it with the dash removed. They would prefer to have the dash installed, so they could charge me to remove it again). These guys, again, did the usual tests, tried numerous different pushbutton units and, of course, were dumbfounded. Until one had a recollection: this same problem had occured, twice before, on 190s. The solution? A push button unit from a more recent model year.

In went push button unit from model year 89, and out went the problem. I don't understand why. They don't understand why (other than the 89 unit has a slightly different pin configuration). All the flaps work. The fan shuts off when I push the "o" button. The defrost button sends warm air at full fan speed to the windshield. This is all a miracle to me, since none of these things has worked properly since I purchased the car (5 yrs ago).

The AC compressor still does not engage, which they tell me is due to a bad compressor speed sensor switch. Replacing the switch requires evacuating the system of freon, removing the compressor, etc. I can live without this 'til summer, I guess. Yes, it did occur to me that maybe the bad speed sensor switch was causing the problem with the system to begin with. But hey, what can you do?

John