1970 C10 A/C not blowing cold after 60 MPH - The 1947 - Present Chevrolet & GMC Truck Message Board Network

bille1ear · 10-31-2024, 12:13 PM

My 70 C10 cools great until I'm going about 60 then the air goes to about room temperature.
It has been upgraded to modern Freon, compressor, condenser, and valves.
Everything else is still stock.
Any ideas?

weq92f · 10-31-2024, 01:22 PM

.

I would start by determining what the pump is doing when the vent air is room temp @60MPH. Is the clutch turning the pump? Maybe rig a 12v wire off the compressor clutch positive and run that to a bulb inside the cab so you know when it's being demanded on.

I know that my A/C will sometimes freeze up at speed when outside humidity is high. That is, the evaporator will freeze externally and the air flow becomes less and less out the vent...still "cool" but barely flowing.

My guess is either you have a valve freezing stopping the flow of refrigerant into the evaporator OR the RPM at speed is causing too much pressure and your safety switch shuts it down (with perhaps a restriction). Overcharge can be a problem here too.

If your system goes room temp and then a few minutes later starts cooling down again, that would be pressure related methinks. An experiment would be turn off the compressor while vent=room temp @60MPH and keep driving for say 10-15 minutes then turn it back on...any cool air?

Hth,

-Kevin

**MikeB** · 10-31-2024, 05:53 PM

An old heater valve not closing all the way caused this to happen on my 69 C10 with a late 1990s Vintage Air system. The tech support guy said it worked OK at low rpm, but when the revs went up, the water pump output pressure and volume increased.

Also, you want to make sure the heater valve is connected to the output of the water pump, which is usually the intake manifold port.

**HO455** · 10-31-2024, 06:38 PM

It's possible the drive belts for the compressor are slipping at rpm and not squealing enough to hear.

bille1ear · 11-01-2024, 10:26 AM

Thanks for all the great ideas.
I have a manual valve on the heater hose that is off. It didn't take long for me to realize the heater valve was leaking in the Texas heat. So I doubt that's my problem.
The cool air comes back quickly when I drop the speed so I don't think freezing is my problem.
I was thinking overcharge when I posted. My gauge set is from the Freon 12 days and the hoses are shot, so I need to get one of those quick charge kits and see if it's in the red.
Will post back when I have an answer.

Thanks again...

10-31-2024, 12:13 PM	#1
bille1ear Registered User Join Date: Sep 2020 Location: houston tx Posts: 2	1970 C10 A/C not blowing cold after 60 MPH My 70 C10 cools great until I'm going about 60 then the air goes to about room temperature. It has been upgraded to modern Freon, compressor, condenser, and valves. Everything else is still stock. Any ideas?

10-31-2024, 01:22 PM	#2
weq92f Registered User Join Date: Oct 2016 Location: Dallas Texas Posts: 3,223	Re: 1970 C10 A/C not blowing cold after 60 MPH . I would start by determining what the pump is doing when the vent air is room temp @60MPH. Is the clutch turning the pump? Maybe rig a 12v wire off the compressor clutch positive and run that to a bulb inside the cab so you know when it's being demanded on. I know that my A/C will sometimes freeze up at speed when outside humidity is high. That is, the evaporator will freeze externally and the air flow becomes less and less out the vent...still "cool" but barely flowing. My guess is either you have a valve freezing stopping the flow of refrigerant into the evaporator OR the RPM at speed is causing too much pressure and your safety switch shuts it down (with perhaps a restriction). Overcharge can be a problem here too. If your system goes room temp and then a few minutes later starts cooling down again, that would be pressure related methinks. An experiment would be turn off the compressor while vent=room temp @60MPH and keep driving for say 10-15 minutes then turn it back on...any cool air? Hth, -Kevin __________________ 67 C10 fleet fuel injected '70 402, 700r4, 3.73 posi 07 335 sport turbo 6sp 94 Trans Am GT LT1 6sp posi -- sold after 22yrs 99 540 sport V8 6sp -- sold 73 240z L24 4sp -- given to friend 68 C10 step 350/350 3.73 open -- sold

10-31-2024, 05:53 PM	#3
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,597	Re: 1970 C10 A/C not blowing cold after 60 MPH An old heater valve not closing all the way caused this to happen on my 69 C10 with a late 1990s Vintage Air system. The tech support guy said it worked OK at low rpm, but when the revs went up, the water pump output pressure and volume increased. Also, you want to make sure the heater valve is connected to the output of the water pump, which is usually the intake manifold port. __________________ Mike 1969 C10 LWB -- owned for 35 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, recent AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!

10-31-2024, 06:38 PM	#4
HO455 Post Whore Join Date: Feb 2016 Location: Portland Oregon Posts: 11,366	Re: 1970 C10 A/C not blowing cold after 60 MPH It's possible the drive belts for the compressor are slipping at rpm and not squealing enough to hear. __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377

11-01-2024, 10:26 AM	#5
bille1ear Registered User Join Date: Sep 2020 Location: houston tx Posts: 2	Re: 1970 C10 A/C not blowing cold after 60 MPH Thanks for all the great ideas. I have a manual valve on the heater hose that is off. It didn't take long for me to realize the heater valve was leaking in the Texas heat. So I doubt that's my problem. The cool air comes back quickly when I drop the speed so I don't think freezing is my problem. I was thinking overcharge when I posted. My gauge set is from the Freon 12 days and the hoses are shot, so I need to get one of those quick charge kits and see if it's in the red. Will post back when I have an answer. Thanks again...

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