Defrost on all the time.
 

Car 1990 300E. The defrost is running all the time. I can not turn it off. None of the switch works. It got so bad i had to remove the climate control unit. The hot air was just too much for me to drive to work everday. After the climate control unit was removed, hot air still come out from the air outlet area. But not fan forced. It is the climate control unit that is bad. Please help.

Replace the climate control fuse, it's bad. When there is no power on the unit, it defaults to defrost, outside air, full heat.

Needless to say, you need to re-install the PBCU.

If you KNOW you have power to it, the PBCU is bad, but this problem is usually a bad fuse.

Peter

I am having an almost identical problem as mb96 on my 91 300E. The defrost was working fine a few days ago. Yesterday, as I was driving and had the fan on, heat started coming out of ALL the vents. I adjusted the temp wheel all the way down to see if start blowing cold air but it did not improve. It got so hot I had to pull over the side and noticed the dashboard was hot and the little light in the recirc button was blinking. I shut the engine off and when I restarted the engine, the fan does not run anymore but I can still feel the heat from around the dashboard. Any idea as to the cause of this problem? Also, psfred mentioned about fuses. Can anyone tell me the location of the blower fuses. Thanks.

changed fuse and still the same
 

the fuse is locate is under the hood. the fuse box is on the driver side ,right next to the hinge mounted between the hood and the fender of the car. the number 7 fuse is labled for the climate control system. It also control the backup lights and a few other items. I changed it but there was no change in condition. also the air circulation light is on all the time. does this mean that the climate control unit is bad?

strange electrical problem
 

There is something i would like to add prior to this PBCU problem i m having. a few times at night while driving home from work, the lights on the dash and headlight went really bright. Brighter than 3 times normal. This would happen for about 10 to 15 seconds then it would be back to normal. What would cause a surge in brightness like that? Thanks so much. i think this could have fried my PBCU and now i m stuck with just hot air.

Check the OVP or MAS relay.

sudden bright headlights is a bad voltage regulator in the alternator, about $30 and two screws to change it. A loose positive battery cable can do this, too, though, so check that as well.

If the OVP fuse is blown or you are getting overvoltage events, it may be open all the time, so the climate control won't work. It's there to prevent just such damage (fried electronics) from voltage surges when the regulator goes bad, etc.

Peter

One way to check for faulty voltage regulator [over voltage condition] is to test battery voltage with a multimeter while hte engine is running ...

You should get max 14.5v with engine running at 2000rpm... more than that probablly means your voltage regulator is bad and overcharging the battery >> at leats that's the procedure I use to test my 911 alternator..

electrical grounds [or lack thereof] are also noted for making things weird in cars... check all your grounds..

good idea
 

for the past few years i have been swapping out old parts even though they are not bad. I remember i replaced the voltage regulator (non Bosch brand) and also the OVP about 2 years ago. I checked the fuses on the OVP this morning and they were OK. All battery cables were nice and tight. When i get home today, i will check the battery voltage. Thank you so much everyone for helping me out. I have done a lot of fixing on this car and would like to have it running well. Just recently i changed the two front brake, new radiator, new two front struts and strut mounts, new power steering pump. I cant give up now. :rolleyes:

Don't know if this will help but on my 84 300SD the monovalve was bad (one time) and one other time it was the pushbutton unit that controls the ACC.
My understanding, at least for my car, is that it defaults to heat mode as a saftey measure.