I've searched the forum and haven't found this exact problem - -

My 1997 850T5 wagon has developed mystifying climate control behavior: The AC blows cold(ish) for a while and then arbitrarily starts blowing warm (Heated) air when I get in the car, and it's hot inside. The blower cycles up to high speed initially and cool air exits. After a few minutes, the blower cycles down in speed and progressively warmer air is delivered. It's no bueno where I live in Dallas, TX.

I've checked for error codes with an OBDII and found none. I've checked the four temperature sensors in the cabin (passenger and driver cabin air in the cabin roof, passenger and driver air mix in the dashboard) and all measure ok. Checked the various servos (removed and operated on the bench) and they all operate as expected. The sun sensor, when blocked with something opaque causes a distinct reaction in the air flow and temperature.

I can't find any trace of an outside temperature sensor (the one that is in the air intake where the cabin filter is fitted in earlier cars). If the 97s have one, it has moved.

Low side pressure is normal - 80 or so psi static and >25psi with the compressor operating. Low pressure and high pressure switches appear to operate normally (I replaced the high side switch to be sure).

On Auto the system has a mind of it's own and starts tepid and gets warmer. If the driver side temp is cranked to full cold, coldish air is delivered for a while. With the driver side full cold, if the passenger control is turned to full hot, the air delivered on that side is HOT. Ditto the driver side when the reverse is demanded. The defroster, foot well, dash vent switch works as expected, distributing the air flow where it says it will.

I am wondering if there is indeed an OAT sensor (Perhaps in the driver's mirror like in the S70s) that is the culprit. Everything suggests that the EEC computer is doing what is expected of it - it just seems to think that cooling demand is lower than than it actually is.

The compressor cycles normally - no clutch problem (the car only has 115K miles), I have replaced the orifice tube, thinking it may be clogged - no joy. The system has been pumped down and recharged with a new drier installed.

I'm going nuts. Any suggestions?

AARD

 

Let's keep it simple (sounds like you have ECC) so let's constrain our discussion to manual mode (no auto).

When cabin console starts blowing warm air, is the compressor engaged? Is the aux fan (in front of rad and condenser) blowing? If the aux fan never comes on, there's your problem.

Compressor could also have issues, though low side pressure going down compressor on would indicate not. Really too bad these systems don't have high side port.

Have you taken it to an A/C pro just to get an opinion if nothing else?

 

Thanks for the response gdog.

Yes, it's ECC. The problem does manifest itself in manual operation: Fan on full, Temps set to max cold. The aux fan is operating. Yes, my European car mechanic friend has fiddled with it - he used an OBDII to reset the calibration of the servos, thinking that might help.

I'll double check to be sure that the compressor is/isn't running when the cooling "stops". I confess I haven't confirmed that specifically. That might suggest another possibility that I have discounted: That the evaporator is freezing up. The line into the evaporator (between the orifice tube and firewall) is cold and sweats profusely, but does not frost up. I think the compressor is running then, but not sure.

The system seems to act like it thinks the outside temperature is much lower than it really is (like the outside temp is well below normal room temp) and the system is initially trying to compensate for greenhouse warming of the cabin followed by maintaining the comfort zone by supplying tepid air. That's why I was looking for the OAT sensor. A bad (resistive) connection to that sensor would usually cause it to report a lower than actual temperature.

In any event, lacking discovery of a defective OAT sensor, it would seem the next step is replacing the ECC control module in the dash - an expense I'd rather avoid if it's not the problem.

Thanks again,

Aard

 

The problem has been solved, though it's a bit embarrassing to admit the cause - gdog put us on the trail with the mention of the auxiliary fan. Bottom line: The condenser/radiator needed cleaning of bugs, dirt and related detritus.

Diagnosing this failure mode was complicated by Texas summer heat and what turned out to be a slightly undercharged state. In some circumstances, the low pressure switch was opening and in others, the high pressure/high temperature switch was opening. It was the high pressure fault that caused the consternation. The system would cool for a while and then stop cooling and actually spout warm air until the car was parked with the engine off for a relatively long period. Other times, the system would cool for several minutes and then blow warm for several more minutes before appearing to stabilize on "semi-cool/warm".

A high side service port would have allowed immediate diagnosis. Boo Hoo! In all probability, a seasoned service tech who worked on Volvos regularly would have diagnosed this immediately. As it was, it took us weeks of fiddling and checking including partial disassembly of several areas of the cabin interior.

We can laugh now - we're cool and comfortable again. We hope knowledge of our experience can short circuit the diagnosis and repair cycle for others who might experience this befuddling set of symptoms.

Aard