1997 850 GLT, Heat air temp fluctuates when in manual mode

bob850 · Post by **bob850** » 14 Oct 2015, 07:15

I posted this over on VS about a year ago, but was never able to get a resolution, thought I'd try here.

1997 850 GLT with automatic climate control.

When running the heat in manual mode in winter, the heat will be working fine, then randomly, the air coming out of the vents will turn cold. It seems to stay cold for a couple minutes, then the air gets hot again. No real pattern that I can discern. The temperature difference is dramatic (I notice the cold air on my feet). Also, it occurs in the air on both sides (and through whichever vents are engaged at the time).

There are no codes or flashing light on the climate control unit.

Any ideas? The heater core was recently replaced with a new volvo OE unit, and the coolant is regularly flushed. I thought it might be a flap motor going crazy, but I think that would only effect the heat on one side (correct me if I'm wrong). I drive it daily, and coolant level is fine, shouldn't be any air pockets.

Also, the engine temperature gauge does not move during the process (stays put at 3 o'clock like normal). I guess I should also mention that all of the coolant hoses, including heater hoses, turbo coolant hoses, radiator hoses, reservoir hoses, and the radiator itself have also been replaced (didn't seem to affect the problem - I changed them due to age as a preventative maintenance when the radiator was replaced).

Post by **kahl** » 14 Oct 2015, 09:37

I had this same issue on a 96 850. I switched out the HVAC a control module and my problem went away.

Post by **jreed** » 15 Oct 2015, 05:11

I guess you have already checked that the damper motor is solidly connector to the damper shaft and that the shaft is not split... this is a common failure point from what I've read here.