87'300E Defroster
 

Finally got the car inspected and on the road. I have a problem with the heat. Have checked other posts but can't find the answer.
1. The heat will come out of dash vents ok (not real hot) guage temp. only gets up to about 55C (probably thermostat)
2. I can't get any air to blow out of the defroster no matter what buttons I push (is it the climate control unit) or???
3. When I push the off button the fan doesn't turn off.
4. Alot of talk about the mono valve. where is it located I can't seem to find it to check the diaphram
Any help or insight's appreciated.

Several possibilities:
No heat: Thermostat, heater valve not opening, defective auxiliary water pump (idle and low RPM), defective control unit. clogged heat exchanger.
No defroster: defective control unit, innoperative vacuum diverters/flaps.
A dual valve failure normally result in too much heat (defaults open)
Good luck
JackD

87'300E Defroster
 

Thanks Jacked seems to be to many possibilities for my head to comprehend,LOL. Do you know what and where the monovalve is.
Thanks

The monovalve is located on the right side (passenger side) of the engine, close to the firewall
In your case I doubt the monovalve is involved in your problems.
Normally, when a monovalve fails, it causes too much heat in the cabin.
jackD

pushbutton control unit or someone wired the servos -- with no vac you should have full defrost only. However, if the fan does not respond to the buttons, it's the pushbutton unit.

Either replace it or take it apart and re-solder everything.

You can pull the connector off the monovalve and see if you get full heat all the time -- if not, you have a circulation problem of some sort with the heater core, of so, PBCU.

Verify that the aux pump is OK BEFORE replacing the PBCU -- if it is drawing too much current, you will promptly fry the new PBCU.


Peter

87'300E Defroster
 

Thanks PSFRED but I am a little slow on the lingo what is the PBCU and where is it located. I'm not really dumb just new to this mercedes stuff. Still learning
Thanks

PBCU = pushbutton control unit. The solder connections, especially the ones for the buttons and one card to card junction tend to crack with age, and cause erratic operation.

Peter

87'300E Defroster
 

Thanks Peter. Don't I feel stupid. I assume broken solder connections are common. I've seen it mentioned on other links.

Vin:

Solder isn't a very good structural material -- fatigues pretty fast. The board the pushbuttons are soldered onto flexes a bit when the buttons are pushed, worse if the pusher is somewhat vigorous about it, and the solder cracks. All sorts of strange things will happen when they don't make contact properly.

Ditto for the other boards.

Peter

87'300E Defroster
 

Well I took the board out today and used a magnifying glass. Couldn't see any broken solder joints. Used continuity tester on the contacts for the defrost button and it worked fine. I did find something else out while I was trying to figure this out. The defroster blows air when the car is not started. Just switch on. When I start the car the defroster stops blowing and I only get air out of the dash and bottom vents. Any Ideas for me to check.
Thanks

Something is wrong with the pushbutton unit, or the switchover valves are screwed up. With no vac, the system defaults to defrost, when you have vac the airflow is controlled by the ACC. If it switches when you start, the vac servos are working. Either the electo-vac valves are bad or the PBCU is shot, or the electronics (behind the glove box) are bad.

You've exhausted my capabilities, I'm afraid.

Note that the solder joints can LOOK OK -- the usuall offenders are the ones where the switches are soldered to the board, so the switches don't make good contact.

With the unit off, the fan shouldn't run at all.

Peter