HVAC Follies

[911/Q45]

I've been chasing an HVAC problem for a while now. The system apparently thinks the interior needs more cooling than required after a hot start. Even on relatively cool days the A/C will blast cold air for a few minutes, then realize it's error and resume normal operation. Everything comes out of the appropriate outlets and the fan blows at the correct speed, at least for what it thinks is going on. It works fine on a cold start. The self test indicates all is OK. I've replaced the ambient sensor, the in vehicle sensor and the engine temp sensor near the thermostat that feeds the ECU. According to the FSM that leaves the intake sensor, the sunload sensor, the water temp sensor and the auto amp control box. Sensors are relatively cheap, the control box is very expensive. Is it any use replacing more sensors, or is my likely culprit the auto amp/control panel or perhaps a third draft choice?

[Q45tech]

Why would the sensors go bad, they are just thermistors and their resistance can be read and verified in FSM.

Start the system in vent that way it will draw interior air past the sensors and possibly remove some of the heat soak........should help isolate problem [or normality].

[911/Q45]

We've had the car for 14 years and it just started doing this in the last year, so I don't believe I'm confusing normal action with malfunction. I'll pull the glove box and check the sensors resistance, but is it possible that they could only cause trouble at a certain temperature and would test fine at others, since the system functions properly under most conditions? Or does that lead to the control box being the problem?

[Q45tech]

Who knows, we always let the HVAC ecu swap be the final arbitrator.

The FSM gives you great charts to compare temp to sensor resistance..........137 pages of things to test.

[911/Q45]

Anyone have a 90-93 Q down for something else that would rent me their functional control panel for a swap test?

[911/Q45]

Finally solved my HVAC problem. Many thanks to the moderator that lent me his control box. Unfortunately his box had the same problem as mine, so I took a deep breath and ordered a new box from Joe($500). Fortunately that turned out to be the fix. I would guess that many first gen Qs have bad control units, so if you don't think your's is working right, it probably isn't. I checked inside and found no evidence of heat damage or cracked solder joints. The display #s worked, it just couldn't perform it's function of ignoring a heat soaked ambient temp sensor.

[Q45tech]

Changed the software program multiple times 89-92 to address issues.

Didn't think any one might still have 89-90 even 91 due to display failure issues........the down side of getting it repaired......old software .......well actually firmware since it is burned in and not reloadable.

[Q451990]

Didn't think any one might still have 89-90 even 91 due to display failure issues...

Strangely enough my early production Q with 199K has its original controller. Still works great - actually it's being borrowed by Q2 so that I can send it's controller off for repair due to display failure. It actually wouldn't turn the compressor on during one display failure too... that was the last straw

Heath