[77 cruiser]

Driving at hiway speed I turned the AC to max (the AC isn't hooked up BTW belt is off) so the only thing it does is shut off the coolant to the heater. About a mile after I did this the temp was closing in on 210 turned the AC off & the temp went back to 185.

Tried it again to make sure it wasn't a fluke mile later temp went back up & then started to cycle up & down.

Is there another problem hiding? Normally runs 185- 190, stock pump 2 row Northern alum rad., stock clutch fan.

 

[70GS455]

Heater core was acting as an auxiallary radiator?

 

[77 cruiser]

Nah, the heater wasn't on so no airflow through it.

 

[Bryan59EC]

The heater acts as a bypass
Perhaps the bypass hole in your water pump or block is plugged up (assuming sbc here), and if the engine is not COMPLETELY up to temperature, when you turn on the AC to close off the valve----no bypass.

I found this out the hard way on my BB---blocked the bypass---the temps would go to 230--then rapidly drop to 160---climb to 225----drop again.
This would keep up untill the coolant in the engine had thermally equalized.

I eventually put the bypass hose back on

 

[77 cruiser]

It's a SB, I don't have the bypass plugged, but I was kinda thinking along that line.
The thermostat has 2 holes drilled in it also. I'm thinking the flow is try to hold the thermostat closed without the added bypass from the heater loop.
Make sense?

 

[77 cruiser]

Tried it again but with the engine cold, gauge didn't move till I put the control back to normal then the temp jumped up to about where it should be for the amount of time it had been running.
For some reason when the heater flow stops, the flow stops in the thermostat area too. Weird, can't be normal?

 

[LRW69]

Perhaps the thermostat isn't opening all the way.

 

[Q-ship]

Newer SBC like GM 330 hp crate engine don't have the bypass like the earlier SBC did. Are you running a block from a late model?

 

[novaderrik]

the AC condenser does kick a bunch of heat into the air flowing thru the radiator.
maybe the radiator and/or fan aren't up to the task of keeping the car cool with the AC turned on, or maybe the fins on the condenser or radiator are bent over or clogged and air can't get thru.

 

[77 cruiser]

Perhaps the thermostat isn't opening all the way.

It's possible, but when it was cold it seemed liked flow was stagnant gauge didn't move.

 

[77 cruiser]

Newer SBC like GM 330 hp crate engine don't have the bypass like the earlier SBC did. Are you running a block from a late model?

Older block.

the AC condenser does kick a bunch of heat into the air flowing thru the radiator.
maybe the radiator and/or fan aren't up to the task of keeping the car cool with the AC turned on, or maybe the fins on the condenser or radiator are bent over or clogged and air can't get thru.

No belt on the AC pump.

 

[Dean]

Turning the AC to max won't help it cool any better if the belt is off.

Just a thought but maybe the coolant temperature isn't actually going up at all, maybe the gauge is somehow getting a ground through the A/C.

 

[77 cruiser]

Just a thought but maybe the coolant temperature isn't actually going up at all, maybe the gauge is somehow getting a ground through the A/C.

Anything is possible Dean.

I looked in the rad when doing this & there was circulation, with the motor cold, so I know I've got holes drilled in the thermostat, 2 3/16 if I remember right.

 

[charbilly2001]

Max a/c recirculates the air in the cabin and normal a/c brings in outside air. But then you said that max a/c shuts off the heater flow. I find fault with that statement.


I think you meant that it shuts airflow through the heater core not coolant flow. In my 72 Chevelle coolant is constantly flowing thru the heater core. I have no shutoff valve.


( actually if the truth be told I don't use my heater at all any more. I am in southern California where the sun shunes 24/7. I don't need a heater. ) But for the sake of accuracy I just wanted to point a couple of things out.


So far as I know the only thing that changes, with the selector switch between max and norm was mode door position. Only the "source" of air flow through the system changes.

Not that that solves your problem.

 

[77 cruiser]

On mine it also shut off the flow to the heater core via a vacuum operated valve. Maybe the earlier vehicles didn't have that feature.