GP White 928 S4

I have been on a quest all Spring/Summer to get the Climate Control system and A/C fully working on my 1988 S4. I have done a ton of work and feel close to success but still not 100% there. Would appreciate some help on where to go next.

Here's what's new/replaced so far:

-All four leaky vacuum actuator diaphragms replaced with new. Confirmed all flaps opening/closing as intended and tested vacuum successfully
-Vacuum solenoids replaced with new old stock BMW E23 7 Series shared part interchange and tested working
-New blower fan
-HVAC Head unit relay replaced with known beefier replacement
-New Kuehl A/C Compressor, Hoses, O-Rings, expansion valve, and fresh R134/oil
-Interior temperature sensor replaced
-New A/C blower resistor pack with cage
-Tested heater control valve working with vacuum and replaced a few years ago with new

After the new A/C kit was installed by my mechanic a couple weeks ago all this work got me great working A/C - BUT only at Max Cool 65 Degree setting (and recirc on). Bumping the temperature just a single degree up resulted in heat coming out of the vents.

Reading several other threads lead me to my external temperature sensor mounted in the left front fender as a potential cause of this behavior. The alternator hose and all wiring was intact and visually it looked ok but I ordered a new one anyway. I installed the new external temperature sensor today and it resulted in no change in climate control behavior. Still getting heat at anything other than 65 degrees at the vents.

I feel like I'm running out of things to replace and test. Where would you go next with these symptoms?

The Forgotten On

The solder joints in the factory connector for the external temp sensor are notorious for failing or poor contact.

I would inspect it closely and if necessary replace the connector, Kroon sells it as a kit if I remember correctly. If you want no problems forever, just use a 2 pin weatherpack connector.

Otherwise the molex connectors behind the center console can have poor connections which can also cause this issue. It is what did it in my 81 anyway...

StratfordShark

You’ve already done most of the work!

Sounds as if all the vacuum side is sorted, and problem with electrical control ‘string’ comprising external sensor, internal sensor, temp slider on hvac head.

You’ve checked external sensor, but not the internal one behind the grill on dash. If this drifts high as it ages, the system will go to full hot as soon as moved off cold override. Mine did this and I replaced with new sensor from Porsche (I think it came with the fan but need to check my records).

The other suggestion is problem with mixer control motor.

As first step remove one side of centre console, identify connector from internal sensor, disconnect and measure resistance. Report back what you find and what was temp when you measured.

Darklands

Nice suggestion with the aged inward sensor. My 84 is doing this!