Defrost Turns the AC on WTH?!?!

[RodsGreen10]

Truck is a 76 factory air.
Put new 385 stroker in with Sanden AC compressor 1 wire deal.
Also put new heater control on dash in.

When I slide over to heater it's fine no AC but very little heat at all and motor is at temp. But when I slide it on over to defrost the blend door dumps you can hear it and the AC clutch engages and blows cold air. WTH guys?? Any have any idea point me in the right direction. If you need more info ask and I will tell you.

[80Sierra3500]

Some cars and trucks run the AC compressor during defrost/defog to dry the air and then heat it so it evaporates window fog. There's probably something going on with the blend doors to make it not be warm. You may have a coolant flow restriction in the heater core since you mentioned it never gets really hot. Probably when you get the core heating properly the defrost air will be warm enough to be of any use.

[RodsGreen10]

Originally Posted by 80Sierra3500

Some cars and trucks run the AC compressor during defrost/defog to dry the air and then heat it so it evaporates window fog. There's probably something going on with the blend doors to make it not be warm. You may have a coolant flow restriction in the heater core since you mentioned it never gets really hot. Probably when you get the core heating properly the defrost air will be warm enough to be of any use.

Ok. So with the defrost on the AC clutch could possibly engage. Cause like I said it engages and feels like the AC is blowing through the defrost. The AC dryer even react the same as if I had the AC on. I just don't remember it doing that before. And it never felt like the AC was blowing when set to defrost. All the other setting work but the defrost feels just like the AC is blowing out the window vents. And I agree with a possible clogged heater core it's the original one as far as I know.
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[80Sierra3500]

Yep, I'd check for heater core issues before I did anything else. There are videos on U tube about flushing heater cores. If you go hooking it to a garden hose remember cooling systems run at 15 PSI or so and the garden hose can be 50+ PSI.

[RodsGreen10]

Originally Posted by 80Sierra3500

Yep, I'd check for heater core issues before I did anything else. There are videos on U tube about flushing heater cores. If you go hooking it to a garden hose remember cooling systems run at 15 PSI or so and the garden hose can be 50+ PSI.

Thank you sir. I may try some of that Prestone Extreme Flush stuff first and that does not work then I will try the hose flush. And I do not want to bust the heater core that is a pain to have to change out. Everything is new under the hood but my heater core a lesson learned for sure. Should have just changed that when everything was torn down and out of truck.
Thanks
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[RodsGreen10]

Now that you say that I know my heater core is clogged. Cause the return line running to rad only dribbles coolant back in the rad. I was checking this waiting on someone to post. So yep you are correct clogged heater core.
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[RodsGreen10]

Ok flushed heater core. It was not clogged up. So I guess time to check blend door.
Also I forgot I have a new 3 core rad and a high flow 180 TSTAT. If that is going to make a difference. Any other ideas besides blend door?

[RodsGreen10]

Forgot tested it with just the heat setting not defrost and its not getting real hot like I just have the vents running warm. Anything anyone?

[80Sierra3500]

If the core is ok then the only other thing that comes to mind is the temperature blend door being out of adjustment or not moving properly.

[RodsGreen10]

Location? Behind/Under glove box? Could it be the new heater/AC switch I installed? Everything is is reacting proper interms of the sounds and the locations the air is blowing. And if it is the temp blend door what is the correct setting and how do you set it? And thanks.
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[RodsGreen10]

Ok so I see the t location all are hooked up but 1. It was hanging under the glove box. Here is the t. In the pic which is rotated the one on the left run to the vacuum diaphragm under the cowl screen. The one on the bottom look to run to the switch. And the one on top look to run to the passenger side but is not hooked up. What does it hook up to?

[RodsGreen10]

This is the one running off the t to the passenger side. It is in the pic above. What does this go to. I feel that this is my problem. I do not see anything like a vacuum source to hook this up.

[RodsGreen10]

Can someone walk out and check on your truck and see what this hooks up on?
My truck is a 76 also.

[Chaplain]

When I removed my factory AC I recall a connection like that going to the vent in the footwell.

[Pine Bark Auto]

Referring back to posts #6 and #7, If you know the core is not clogged but you have only a dribble returning to the radiator, could there be a poorly functioning hot water shut-off ? I am not sure if your model even has one. Chip.

[RodsGreen10]

Originally Posted by Chaplain

When I remove my factory AC I recall a connection like that going to the vent in the footwell.

Well that blows my thought out of the water then.

[RodsGreen10]

Originally Posted by Pine Bark Auto

Referring back to posts #6 and #7, If you know the core is not clogged but you have only a dribble returning to the radiator, could there be a poorly functioning hot water shut-off ? I am not sure if your model even has one. Chip.

I took that valve out of the system when I first got the truck. It worked flawlessly without it. Now I have all new under the hood motor rad electric fans. I have everything hook up the way it was before. Only thing I didn't touch was the AC area on the fire wall.
This has me stumped.

[RodsGreen10]

Anything under the hood I could check.

[RodsGreen10]

Frustrating

[Dead Parrot]

The hose in post #12 probably goes to the recirculate door that is behind that round thing you can just start to see in the pic. At least the hose in my 75 looks just like that one.

[Chaplain]

Sorry I couldn't help any more than that. Don't mean to frustrate you even more but maybe tracing vacuum lines from the control panel through the firewall would help considering you did an engine swap.

[Pine Bark Auto]

Since you have removed the shut-off valve, have you determined what might be restricting the flow to the radiator? Maybe I misunderstood your earlier post, but I thought you meant you only had a dribble returning to the radiator from the heater core. If that is the case, you will not get much heat in the cab. If you have corrected the coolant flow problem, I would turn my attention to the functions of the doors in the heater/AC unit under the dash. A good manual will help a lot in tracing vacuum lines and determining door positions during the different modes of operation. Chip.

[RodsGreen10]

Originally Posted by Pine Bark Auto

Since you have removed the shut-off valve, have you determined what might be restricting the flow to the radiator? Maybe I misunderstood your earlier post, but I thought you meant you only had a dribble returning to the radiator from the heater core. If that is the case, you will not get much heat in the cab. If you have corrected the coolant flow problem, I would turn my attention to the functions of the doors in the heater/AC unit under the dash. A good manual will help a lot in tracing vacuum lines and determining door positions during the different modes of operation. Chip.

You didn't misunderstand me that is correct a dribble. Flushed the core forward and reverse with garden hose no restrictions at all water flowed just like it was coming out of the hose. Flushed for 10 min both ways. I have a manual and have checked the vacuum lines the only one that was unhooked was the on in the pic above. But I do not see anything about door positions. Just makes me mad that everything is the same minis new motor new rad and dual electric fans and new heater/AC control. Truck came with a 350 and I put a 385 stroker race motor in. I appreciate everyone's help. Just frustrated at this point.

[Chaplain]

New heater/AC control...

[RodsGreen10]

Originally Posted by Chaplain

New heater/AC control...

Meaning? Everything works AC vents low high air flow locations setting. just no heat.