michael2e

Hey guys, just trying to sort out atypical "blowing hot" issue. No luck with search so any experts on these HVACs appreciated. My issue is whenever I press the A/C button I'm getting hot air through the vents. Here's the workup so far:

1. R134 conversion, fully charged
2. compressor working normal
3 All vacuum hoses replaced fairly recently. hose to heater control valve (HCV) looks good holds good vacuum
4. Engine pulls 13-14 mm Hg vacuum when cold, warmed up 19-20 mm HG
5. Heater clips under dash look intact seem to be working right
6. temperature control switch working normally - vent blows cool when dial to blue and changes over to warm/hot with rotation of switch to red.
7. heater control valve in engine bay seems to function normally when A/C not in use - i.e. cold vent setting closes heater control valve and heat vent setting opens it up.
8. turning on A/C (pushing the A/C button) causes the HCV to OPEN. I'm pretty sure thats not supposed to happen.

So with the A/C off if the temperature control switch is turned all the way counter clockwise to blue the HCV is closed and the vents blow cool. As soon as I hit the A/C button the HCV opens up and the system blows hot. I don't recall what the solenoid under the passenger foot well was doing when the system worked properly but is there any way to differentiate whether one of the solenoids versus the Temp Control Unit or the A/C switch itself is bad causing the HCV to open? Thanks

michael2e

Just thinking things over - since the heater control valve is working properly when A/C is NOT turned on (opens when temp switch rotated to hot and closes [under vacuum] when rotated to cold) wouldn't that suggest that the solenoids are working normally and rule out vacuum leaks in the system? Upon depressing the A/C switch to turn on the system my HCV immediately opens (thus allowing hot coolant into the heater core) so there must be some erroneous input (or lack thereof) to the solenoid that controls the HCV. Can anybody verify for me that the HCV remains closed when A/C turned on. Thx

ramius665

Do you have access to a spare HVAC head unit? Based on your description and the explanation of functions in the factory service manual, there doesn't appear to be any mechanical fault which would create your symptoms.

michael2e

Thanks for your input Aaron - unfortunately I don't have a spare HVAC to try. I'm surprised no one's had a similar problem? Anyway I think your implication is right that it might be an electrical problem - perhaps from the A/C switch or the temp control unit itself. I hate to take it in as you know the mechanics will waste time (and $$) pulling out the pressure gauges, checking over the compressor and all of that. Just not sure where to look next

michael2e

bump; time to dig into this again. Same problem unresolved. Anyone with experience like this? Thx

Scott H

You say you replaced the vacuum lines, do you mean the ones in the engine bay or the ones under the dash? It's been ~9 years but I seem to remember having a similar problem and I think I tracked it to this area. Test stuff with a Mityvac and double check that all the hoses are routed correctly. Could also be that one of the solenoids (#9, the one on the left in the diagram would be for the heater, the right one is for recirc./fresh air doors) went bad.

Charlie

The HCV stays closed when the AC switch is on and the temp setting is on cold.

Charlie

Check and make sure the two vacuum lines on the firewall going to the hvac are correct. Not sure what would happen if they were backwards.

michael2e

Thanks for the input guys. I should say that the HCV appears to work perfectly normal. It closes when it should and opens when it should. Except when I actuate the A/C switch. Then the HCV opens instead of staying closed. Me thinks that the vacuum circuit all the way through to the solenoid therefore must be intact since it does function properly without the A/C? Could there be a faulty signal from the switch or lack thereof that doesn't actuate the solenoid as it should? If those switches weren't so expensive I'd just try swapping it out. I appreciate the problem solving ideas

Scott H

You could try bypassing the solenoids and testing function that way, or bench test the solenoid and make sure that when you give it 12v it opens. Also use a multimeter to test the connector and make sure you're getting 12v when you should.

michael2e

SOLVED: Replaced HVAC unit with a new one and the system now works perfectly. I will further test to determine wether the A/C switch itself was bad or the HVAC unit and will post. I'm no expert at schematics but this one put together by Schibsted a fellow lister shows that the HCV is controlled directly by an Integrated Circuit IC8 in the HVAC. If the HCV functions normally as mine does then that suggests that the entire circuit distal to the IC including the vacuum system and solenoid is therefore fully operational. The problem is the signal to IC8. Heres a link to his more detailed description
https://rennlist.com/forums/924-931-...chematics.html