Ac clutch engages on defrost and not dash - First Generation Pontiac Firebird (1967 - 1969) - Firebird Nation

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Ac clutch engages on defrost and not dash


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#1 Birdstein

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Posted 07 June 2020 - 12:34 PM

I have a 1968 pontiac firebird. The ac works good, but only when selector switch is turned to defrost. I want it to blow cold air when selector switch is on dash. I put on a EFI system and maybe crossed some wires somewhere. Maybe heater control switch. I have a horton Heater control valve 46115vuh. there are two male spade terminals. If I have the wrong one grounded could this cause my problem.



#2 sea dog

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Posted 07 June 2020 - 02:04 PM

AC control unit uses vacuum to control what passage ac goes to. The most common problem is a vacuum line or lines are leaking or have become disconnected from control unit.

 

Also the control unit can break down.

 

If you'll go to 1st gen tech section. Then hit master index at top, new page will open with 69 Pontiac service manual. It should have vacuum diagram of ac control unit. Plus other info on ac system.



#3 Birdstein

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Posted 07 June 2020 - 05:23 PM

Thank you I downloaded the diagrams. I will go thru them. I looked for a vacuum line coming thru the firewall to connect to engine somewhere. Have not found one yet.  I will get under dash on passenger side a look for something. It probably is a vacuum. 



#4 fourthirteen

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Posted 07 June 2020 - 06:26 PM

Your subject line says “Ac clutch engages on defrost and not dash”...does this mean the compressor clutch doesn’t engage when the dash vents are selected? If so then it’s an electrical problem. I don’t know the answer but I think that you need to be looking at electrical issues and not vacuum issues as vacuum is used to actuate the duct dampers, not to actuate the AC compressor clutch...that’s electrical.

#5 Birdstein

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Posted 08 June 2020 - 05:58 PM

Yes clutch engages on defrost and blows cold air to defrost vents, but when I have selector switch to vent the compressor disengages. Of course other selectors are on Hi and cold.  Do you think I have power and ground switched on horton Heater control valve 46115vuh.  What would happen if I switched the wires on that solenoid. And I cannot find any vacuum lines on my ac control unit. I am not sure my unit is factory. DO all units have vacuum lines coming off of them.    Thanks for the help. I really appreciate it.



#6 sea dog

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Posted 09 June 2020 - 04:45 AM

GM is a big believer in vacuum control. But on many aftermatket units, it's either electric or cable control

 

When you say you switch to vent, are you switching ac control to the vent setting? If yes, this turns off compressor and only fan runs. Ac compressor runs in ac, mak, & bi-level modes.



#7 Birdstein

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Posted 09 June 2020 - 08:25 AM

R561167.JPG

 

 

 

 

 

This is unit I have. has electronic not vacuum hose.   I will post picture of my heater control system later, but top lever says fan; left is off and right is HI, Middle lever is cold left hot right and bottom lever is vent left defrost right. 



#8 fourthirteen

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Posted 09 June 2020 - 10:10 AM

So its an aftermarket system? Are you using the non-A/C factory controls that look like this? If so then how do you turn the A/C on...is there an aftermarket A/C button that you push? If no aftermarket button then whoever installed it must have decided to use the Defrost setting to trigger the A/C compressor clutch, although Im not sure how you would do that.
I still dont know the answer but some things are becoming more clear now.
Well get there.

Attached Files


Edited by fourthirteen, 09 June 2020 - 10:19 AM.


#9 Birdstein

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Posted 09 June 2020 - 10:13 AM

Don't know if this information helps, but when the middle lever is full cold I have 12v to heater control valve wire. When I begin to move it towards heat I lose 12 volts to heater control valve wire. If wires are hooked up right I assume it opens the valve and sends water to heater core. When that happens does it send a signal to AC compressor to do something?



#10 Birdstein

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Posted 09 June 2020 - 12:44 PM

This is a picture of my control panel

 

 

 

 


Edited by Birdstein, 09 June 2020 - 12:45 PM.


#11 Birdstein

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Posted 09 June 2020 - 12:51 PM


Edited by Birdstein, 09 June 2020 - 01:03 PM.


#12 sea dog

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Posted 09 June 2020 - 12:55 PM

Ah yes, the invisible control unit



#13 Birdstein

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Posted 09 June 2020 - 01:08 PM

Attached File  heater.jpg   121.39KB   0 downloads Sorry can't get it to attach and show up


Edited by Birdstein, 09 June 2020 - 01:12 PM.


#14 fourthirteen

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Posted 09 June 2020 - 01:38 PM

www.vintageair.com has the installation and operating instructions for your unit, as well as wiring diagrams. It says compressor should engage when temp lever is pushed to full cold position, and you should be able to direct the cold air to any of the three locations (dash, floor, or defrost). Heat is actuated when temp lever pushed full hot. So I’d think that you should have 12v to heater valve in “Hot” and 12v to compressor relay in “Cold”. I don’t understand why compressor only runs in Def position.
May want go through the lever calibration exercise they describe.
I’d post link but these are downloads I’m looking at from their website.

#15 Birdstein

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Posted 09 June 2020 - 03:04 PM

Thank you found calibration instructions. https://www.vintagea..._pdf/473047.pdf    Will give it a try and respond back.



#16 Birdstein

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Posted 10 June 2020 - 04:53 PM

:rockindude: Thank you very much. That calibration was the key. First it didn't work, but something different happened, so instead of having only one lever in beginning when calibrating, I had all three to the left then two back to the right for calibration. Cold and hot air when and where it is supposed to be. Again Thank you.



#17 sea dog

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Posted 10 June 2020 - 05:30 PM

Even us old timers learn something new every day at the firebird nation.



#18 fourthirteen

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Posted 10 June 2020 - 06:29 PM

Great! Thanks for the update.




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