Heater Troubleshooting - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**landy** · 06-09-2013, 11:21 AM

I've got a '72 with 350 and factory A/C. The A/C system is uncharged, but the compressor does come on. I've verified that all the vacuum operated valves open and close, and all the damper doors close per the manual instructions.

There's hot air always blowing out the ducts (with the fan on). I'd like to be able to get outside fresh air at ambient temperature blowing into the cab. The lever on top of the core box in the engine compartment moves with the control lever, so that would seem to indicate that the door inside the box is moving.

I think the manual indicates that the vacuum controlled hot water shut off valve should close with the A/C on, which would mean that hot water is always circulating through the heater core. I have vacuum to this valve, but I don't think it's closing. It seems that in order to get outside, uncooled air, it would be better to have this valve at closed or mostly closed except when heat is needed. Am I understanding how this valve is supposed to work correctly? On other older vehicles, there was a valve on the heater line that one could turn off to stop the flow of heated water through the heater core.

Any thoughts on this problem?

Thanks!

tinsoldier · 06-09-2013, 12:08 PM

I went ahead and converted the vacuum valve to a manual one , that way I was able to shut off the coolant . in the winter I can turn out back on. You can buy the 5/8 valve at any flap . Mine is a 71 so not sure if you have the same vacuum operated valve sitting on top of the inner fender.

**landy** · 06-09-2013, 11:50 PM

Mine is mounted on the passenger-side inner fender too. Yeah, it does seem that having a valve in the line to shut off the flow of coolant through the heater core would greatly help in keeping it cooler. I think I'll do that as well.

Not sure what you meant by "... buy the 5/8 valve at any flap"?

**landy** · 06-11-2013, 11:49 PM

Bump for any more info.

Thanks!

48richard · 06-12-2013, 12:48 AM

The vacuum controlled water valve only closes when the control is set to "Inside". When you are using outside air the heater is still getting hot water. For the summer I have connected my heater valve directly to the vacuum reservoir, so whenever the engine is running the water valve is closed. Makes a big difference to AC performance.

**landy** · 06-12-2013, 02:05 PM

That's a good idea. Have you noticed any temperature difference with the A/C off and just pulling outside air (with the hot water valve closed)?

06-09-2013, 11:21 AM	#1
landy Senior Member Join Date: Jun 2000 Location: alb, nm Posts: 413	Heater Troubleshooting I've got a '72 with 350 and factory A/C. The A/C system is uncharged, but the compressor does come on. I've verified that all the vacuum operated valves open and close, and all the damper doors close per the manual instructions. There's hot air always blowing out the ducts (with the fan on). I'd like to be able to get outside fresh air at ambient temperature blowing into the cab. The lever on top of the core box in the engine compartment moves with the control lever, so that would seem to indicate that the door inside the box is moving. I think the manual indicates that the vacuum controlled hot water shut off valve should close with the A/C on, which would mean that hot water is always circulating through the heater core. I have vacuum to this valve, but I don't think it's closing. It seems that in order to get outside, uncooled air, it would be better to have this valve at closed or mostly closed except when heat is needed. Am I understanding how this valve is supposed to work correctly? On other older vehicles, there was a valve on the heater line that one could turn off to stop the flow of heated water through the heater core. Any thoughts on this problem? Thanks!

06-09-2013, 12:08 PM	#2
tinsoldier Registered User Join Date: Mar 2013 Location: Peachtree Corners, GA Posts: 84	Re: Heater Troubleshooting I went ahead and converted the vacuum valve to a manual one , that way I was able to shut off the coolant . in the winter I can turn out back on. You can buy the 5/8 valve at any flap . Mine is a 71 so not sure if you have the same vacuum operated valve sitting on top of the inner fender.

06-09-2013, 11:50 PM	#3
landy Senior Member Join Date: Jun 2000 Location: alb, nm Posts: 413	Re: Heater Troubleshooting Mine is mounted on the passenger-side inner fender too. Yeah, it does seem that having a valve in the line to shut off the flow of coolant through the heater core would greatly help in keeping it cooler. I think I'll do that as well. Not sure what you meant by "... buy the 5/8 valve at any flap"?

06-11-2013, 11:49 PM	#4
landy Senior Member Join Date: Jun 2000 Location: alb, nm Posts: 413	Re: Heater Troubleshooting Bump for any more info. Thanks!

06-12-2013, 12:48 AM	#5
48richard Registered User Join Date: Oct 2009 Location: Phoenix, AZ Posts: 475	Re: Heater Troubleshooting The vacuum controlled water valve only closes when the control is set to "Inside". When you are using outside air the heater is still getting hot water. For the summer I have connected my heater valve directly to the vacuum reservoir, so whenever the engine is running the water valve is closed. Makes a big difference to AC performance. __________________ 1971 C10 Longbed, 350/TH350

06-12-2013, 02:05 PM	#6
landy Senior Member Join Date: Jun 2000 Location: alb, nm Posts: 413	Re: Heater Troubleshooting That's a good idea. Have you noticed any temperature difference with the A/C off and just pulling outside air (with the hot water valve closed)?



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