No Heat
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No Heat
I have a 78 w/L82. Very stock. First winter with car, AC worked in summer so expect it still does. 2 months ago I went to test the temp slider to heat and it snapped. New HVAC controls and cable. Today, drive to work and turn on for heat. No heat from defroster or other settings. Fan works fine. What should I be testing to find the issue.
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I have a 78 w/L82. Very stock. First winter with car, AC worked in summer so expect it still does. 2 months ago I went to test the temp slider to heat and it snapped. New HVAC controls and cable. Today, drive to work and turn on for heat. No heat from defroster or other settings. Fan works fine. What should I be testing to find the issue.
Many of these cars have the heater core bypassed because it's a F'N ...... PITA to change !!!
Le Mans Master
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I believe that the AC cars have a heater control valve that, when open, send hot water through the heater core in the cabin. If that vacuum valve is not working or is not drawing vacuum, the valve will be closed and no heat. You may also check to see if the previous owner removed the thermostat. Stay warm and good luck. Jerry
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If you have the shutoff valve and it's working you will then need to check the vacuum lines goingto the temp controls in the dash. Make sure they are working and connected properly. Then there is also a heater control valve up under the dash above the heater box, that you will find when you are checking the vacuum lines. 
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I agree. Back when I actually had a heater, I cut the white poly line just a few inches from where it comes out of the firewall. Installed a section of rubber vacuum line straight to the water shut off valve. Worked fine. 1/8 inch ID vacuum line is available at all auto parts stores. Easy fix.
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New HVAC controls.??
what was that?
the selector is a common vacuum failure at the switch.
here is a thread for all years. Aftermarket switches seem to have been problematic.
make sure temp slider releases heater shut off valve and away from switch.
also check coolant level and feel the hoses
https://www.corvetteforum.com/forums...-switch-2.html
what was that?
the selector is a common vacuum failure at the switch.
here is a thread for all years. Aftermarket switches seem to have been problematic.
make sure temp slider releases heater shut off valve and away from switch.
also check coolant level and feel the hoses
https://www.corvetteforum.com/forums...-switch-2.html
Last edited by interpon; Jan 31, 2026 at 12:29 PM.
Melting Slicks
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From: Fleming Island, FL
If you have the shutoff valve and it's working you will then need to check the vacuum lines goingto the temp controls in the dash. Make sure they are working and connected properly. Then there is also a heater control valve up under the dash above the heater box, that you will find when you are checking the vacuum lines.
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OP stated that he has no heat on defrost. By this I am assuming that he is getting air flow out of the defroster vent. If that is so. Then it's not a vacuum selector issue. Something I have fixed twice over the years on my car. Now the vacuum switch on top of the heater box needs to function when the hot/cold lever is swung all the way to Hot. The door needs to be completely closed to activate the switch. No hot water if the lever is left in the middle. It's pretty easy to check. Put lever all the way to hot. And upper lever on HTR. Check for vacuum at the white line in the engine compartment. Slide lower lever to left off of full hot. Again check for vacuum at white line. Obviously you need to have engine running to check this. You should have vacuum on hot and no vacuum off of hot.
Now, if the vacuum selector switch fails. The heat will only blow on the floor and will not switch up to defrost. But I believe that's not his issue based on the description.
Now, if the vacuum selector switch fails. The heat will only blow on the floor and will not switch up to defrost. But I believe that's not his issue based on the description.
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Well that's one good reason to NEVER live in Minnesota !! -24 is expected there, 13.............. & -5 the other day with the chill on Long Island is NOT !!!
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Waiting on a new water valve as it’s stuck closed. Meanwhile testing the system and noticed when the fan is running wind is blowing up and out the cowl. Air comes out all the vent points in the dash but also out the cowl. Anyone ever notice this?
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If so yours is the 3rd that I know of...
Smashed in almost the exact same spot...
From a 1979 & my 1977 in the photo below...
