In the last half hour of my trip home tonight in my 91 300SE, I lost cabin heat. The dial setting was fine until that last half hour. Then it became too warm. I tweaked it a couple of degrees cooler. Then it became too cool. It seemed like there wasn't an adjustment small enough to maintain a comfortable steady state temperature. In the end it went on a cold swing and it's been there since.

The registers and blower speed respond predictably to the various mode and temperature settings. AC compressor comes on when it should. There is no hint of warm air at all.

The archives suggest that the monovalve is the culprit although the common failure mode is stuck open, not closed.

Questions:

1) Is the constant power to the monovalve 12V or something else?

2) Is the monovalve protected by the AC fuse or another fuse?

3) Will an 81 SD monovalve fit a 91 SE if only for testing purposes?

I've had this car for the last 15K miles of its 144K miles. The AC's been great (GM cold when necessary) except for demanding a new AC fuse each fall I've had the car.

Thanks,
Sixto
91 300SE
81 300SD