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

Auxiliary water pump may also be at fault.

__________________
Regards,
Aaron

I have seen microcracks on the circuit board inside the climate control unit cause this ptoblem. Specifically on the temp setting control knob potentiometer. If you are handy they can be fixed.

__________________
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

This morning started out okay. Some heat for the first 20 minutes then it was barely warm. What's strange is that the engine was reading half a needle width below 80C when there was some heat and a needle width above 80C (normal) when I lost heat. It usually goes just past 80C and stays there. This is the first time I notice an interim steady state. I mean it went straight to the lower setpoint and hung there for a few mintues of freeway cruising before creeping to the normal temp.

Maybe a faulty thermostat is causing the engine to run cooler until the car sees freeway load. But it doesn't explain why the heat goes when it gets to operating temperature.

BTW, ambient temp as indicated by the car's thermometer has been in the high 40s F thus far.

Dave, I know a good guy for soldering. Are these microcracks obvious to someone who understands electronics? Are there specific traces to check?

Sixto
91 300SE ... brr
81 300SD

I had similar problems with my wifes 300SE. Was chasing it down, including changing Monovalve with no improvement, when the water pump went south. Since replacing the pump heat has been fine. I'm suspicous given the number of miles on yours that you might be heading the same way.
Hope this helps.
Steve

__________________
Steve Short
'91 300SE sold
'92 190E 2.3 sold
'99 S420
'06 Mini Cooper S
'08 Triumph Rocket III
'02 CLK430

Old thread, but I am wondering if this was ever resolved. I have a similar issue.

W126 sudden heat loss

In short I have no heat at higher RPM, including sitting in the driveway. I have no viscus fan, so there is no air movement when sitting still. I just replaced the t-stat, which was pretty new, and there has been no change.

My last two thoughts are blown head gasket with exhaust leaking into coolant or the water pump. Although I would think with the water pump going bad the engine would run hotter, not cooler.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car