slowcorrado

Like the title says: It is a heat blasting in the interior issue, but I promise I've searched and taken care of all the typical stuff 1st!

Issue:

Drive car, blower temp at lowest setting, it will blow ambient temp air in about half the time. The other half is MAX heat. I know my clips are OK, I know the heater valve is working and the vacuum lines are intact (tested with mighty-vac) The interior sensor seems to be outputting an appropriate resistance and the little fan there does work. I replaced the control-head once as it never worked before that, so I'm guessing this head is OK.

Any other suggestions? Does anyone have a diagnostic flowchart for the control-head? Schematic? Its amazing that it will be fine, then 5 minutes of insane heat, then fine again. I can drive the car like a grandma, then tear it up and it makes no difference.

NoRush993/951

An outlier idea is the temp sensor in the engine compartment which is under the blower cover on the passenger side. That affects the mix of fresh air entering the cabin depending on the air temp. Though I have to say I have had your situation more than once and it was either a failing interior temp sensor or the head control unit. Also check your vacuum hoses to make sure they are connected snug. The one by the battery entering the cabin can loosen up due to age.

black944 turbo

Check the servo that the clip connects to, mounted on the heater box. My servo only worked about half the time. I was told it was probably the head unit, however I found out it was the servo motor not working. I took it out and took it apart and was able to clean it. It has been working fine since.

Be very careful when disassembling it. The metal portion at the top houses the magnets for the motor and has to have the tabs bent out to disassemble. On mine the wiper arms were very dirty and it was causing a bad connection.

slowcorrado

Both awesome suggestions! I wasn't aware of a sensor on the exterior side of the firewall.

Also, disassembling/cleaning the servo sounds like a reasonable step since I've tried so much else.

Will post back with results asap. Thanks a ton.

Jfrahm

Heater valves can appear to be working but still pass coolant when "closed" which will roast you. If it's an older stock-style valve I suggest replacing it with a more robust design. Audi 5000, Ford Escort fitment should have compatible valves with a stronger design.

slowcorrado

Today's update:

Pulled up the rain tray and found the outside temp sender. Checked it with the multimeter and it seemed to be working properly. (10K ohms at ambient temp, decreasing quickly as heated up) I cleaned the rain tray area and then verified that all motors/flaps moved properly again. With the car off, key on, I could slide each flap lever and see their corresponding servo move. The blend servo also properly moved with the rotation of the temp ****.

Took the car out for a drive and it SEEMED to be OK for 1/2 the trip. At my normal turn around area I put the heat up and it worked, then down and it went down, but as I took off (briskly) I could hear the blend door moving and VOILA! Full heat. After that, no movement of the temp **** would move the blend servo (yes, you can reach down while driving and feel the linkage) :-)

Strange part: With the temp **** at its lowest setting (and heat still blasting) I was able to get the blend servo to move when I would floor the car at low RPM, it would immediately return to hot when I let off the gas. Now, I know that sounds 100% like a vacuum issue, but the blend servo is electrical?! I must be missing some key knowledge here.

mikeyoman

The solenoid controlling the heater water valve is vacuum operated - maybe this is not operating correctly and allowing hot water into the unit when cold is dialled up ?

AlphaOmegaPower

Op what year is your car?

AlphaOmegaPower

I would suggest this strongly to rule it out.

slowcorrado

1986

Checked the heater valve and it seems to be working well. I think I'm going to get the mighty-vac back out and go over all the lines and actuators again. I never did the ones up under the cowl cover.

black944 turbo

There is also a vacuume actuator on the passenger side of the dash underneath that controls the heater. It is a common problem as well.

slowcorrado

Thanks! Will check tomorrow. Tonight, I checked the 2 actuators that are in the engine bay/cowl. Both tested out perfectly. Sadly, I'm not getting much time on the 951 as I'm doing a Corrado VR6 restoration and it is a basket case. LOL!

Will post back with results.

slowcorrado

I didn't see an actuator down there, there are 2 solenoids, both test out OK both physically and electrically.

I was able to completely rule out a vacuum issue tonight. Using the image below I was able to test every numbered item and both vacuum actuators on the cowl-side box.



This leads me to an electrical issue...

I've tested:

Exterior temp sensor - OK
Interior temp sensor + little fan - OK
Power to temp controls - OK
Ground to controls - OK
Upper vent flap motor and controls - OK
Lower vent flap motor and controls - OK
Blower motor - OK on all 4 settings
Recirc button - OK, solenoids/actuators move properly
AC button - works, exterior fans blow but no AC as I'm pretty sure its not properly charged
Defrost button - OK

Inside the box blend temp sensor - Not tested yet
Mix flap motor and temp **** - only works RANDOMLY It doesn't feel like the **** has an issue

Well, that's it for tonight. It sounds more and more like my control head is hosed, but I'm thinking maybe the blend stepper motor or wiring is suspect. I'd just like to know what the proper logic is for hot or cool air is. Example: Outside temp sensor sees 80 deg, inside temp sees 80 deg, temp set to 75 degrees = use blend temp sender's reading to turn off all heat? How do all 3 temp senders affect the control head. *hmm*

slowcorrado

Tonight's update:

I pulled the blend flap control motor and disassembled/checked it. After cleaning the control traces and wipers I reinstalled and aligned it based on the shop manual.

Every time I turn the temp control ****, the motor responds properly now. Thinking it was fixed I went out for a drive and sadly I was wrong. Now about 90% of the time its OK. The other 10% its full heat still. When it goes full heat, the motor will move the blend flap.

This says that one of 2 things is happening:

Either the motor is screwed up internally and moving because of a faulty board, or the HVAC control head is sending it the signal to move.

I'm leaning towards a signal from the control head. This means I still need to learn what inputs could be incorrect and causing it to go nuts. It sounds like just a loose wire now!

NoRush993/951

Request to borrow somebody's control unit and try it out. With a little luck, somebody on the board is close to you and will help out.