seitz

I recently bought a 1974 Corvette L82. I hop in it this morning and attempt to turn on the defrosters. To my surprise nothing happens. I play with the controls and the blower comes on when I move the switch and the turn wheel to different position. However, hardly any air blows out. When I put my hand right by the vent and the defroster I can slightly feel air. I can hear the blower motor slightly too. I let the car warm up and the air still comes out cold. Drove to work with my head out the window in 40 degree weather.

Anyone know what may be causing this? Bad blower motor, switches, relay? This is my everyday driver now. This just wont work. Any help would be appreciated.

SIXFOOTER

If you can hear the blower that pretty much eliminates the electical portion. Problem will be on the vac controlled side of things. !st, check to see if there is a hot water shutoff valve on the heater hose. Next, your going to need to dig into the vac controlls to see if the small control hoses are hooked up, then on to the duct work.

Paul L

Has the heater core been bypassed? In other words are the hoses connected to the core or to each other?

0Paul Ruggeri

Does the blower motor sound normal? I would check to see if you are getting 12 volts and a good ground to the blower motor.

7T1vette

Electrical system turns on the A/C compressor and the blower fan; vacuum system selects the airflow paths via the vacuum servo activated diverter doors in the air plenum. The main inlet air source is controlled by the "Outside Air Inlet" door which is located under the windshield 'well' at the far right side (fender area). When your car is turned OFF the diverter door on that inlet should be closed. When you turn the car and the A/C "ON", that door should open to let outside air into the plenum. Use a flashlight in that very far right side of the wiper well to see if that door is opening or not. If not, that could be your problem. You can try to shoot some PB Blaster on pivots/hinges at the servo and door to get them freed up. Otherwise, it could be a vacuum supply problem where the vacuum signal is not getting to that door. If that OAI door is opening, then you have other diverter doors in the system that are blocking the airflow when they should not be. The Chevrolet Chassis Service Manual has diagrams and logic charts for the operation of both the vacuum and the electrical systems in the "Air Conditioning" section. Trace the operation of vacuum/electrical items for all settings on your heater/A/C control head and trouble-shoot what does not work properly.

seitz

I actually hear the sucking sounds when I change between defrosters, vent, and AC. So I'm assuming the vacuum is working. However, I wasn't aware you could bypass the heater core. I will have to check that out. Is the heater core located under the dash like many modern cars? I'm not really familiar with this car yet. I switched from ricer cars, aka Turbo'd eclipse, to corvettes. I haven't even found the fuse box yet. lol.

I've noticed my temp gauge always stays near 100. It varies slightly, especially if I get on it. It will get maybe to 150 degrees. I'm wondering if my thermostat could be stuck open and not letting the water get hot enough to heat the heater core up.

seitz

I was actually just told about some type of hot water cutout that is part of the ac system. I was told it could be jammed shut. Would this still be on my car if the AC compressor has been removed?

PETKAH

The AC does not use hot water - it is a separate system. There might be one or two cutoff valves on the two hoses running to the heater box - check to make sure they are open. People use these valves to stop hot anti-freeze from going thru the heater core and thus reduce heat in summer. Caution: if you turn these on and you get liquid leaking into the passenger side floor mat then you might need a new heater core.

Peterbuilt

This is the hot water shut off valve of a 74 with A/C, it is about 30' back from the 5/8 inch hose on the hot water connection at the intake manifold. It can be stuck open or closed, if closed it will not let water flow thru the heater core. On my car when vacuum is applied the valve opens and lets hot water flow thru the heater core and you get heat inside the car.

Peterbuilt

If this is working than you have to check the right thumb wheel to see if that is working.



If it works at the right thumb wheel you have to make sure that it it also works at the heater box. PG.

73StreetRace

Do you have all speeds working on the blower ?
Sometimes the contacts of the console switch are oxidized and you only get a low speed.
At full speed, the blower should make a loud noise.
And you should hear the noise and speed of the blower changing on each position of the switch.
The switch alone is easy to remove and check...
Also, do you get more air when the thumb wheel is set to cold position or not ?

Ganey

You may just be hearing a vac. leak if the vents are not switching.
Yes, the heater can be bypassed- check heater hoses.
Sounds like a bad or no thermostat.

seitz

I called the guy I bought the car off of and he said the heater core has never been connected since he got the car. So that's definitely the reason why I'm not getting any heat. The air sources all seem to work correctly. They blow out at the floor, defrosters, and vents.

However, only two speeds seem to work on the blower. When its at the far left its off. When I flick the switch over to the first setting it lightly blows, the third setting it doesn't work, the fourth blows quite a bit more air, and the fifth position doesn't work. I'll check to see if the switch is corroded.

Where would I look for the connectors for the heater core under the hood. I'm assuming I'll have to buy new rubber hoses since they've been bypassed.

7T1vette

If the heater core had "never been connected" prior to your purchase, you should be wary about connecting it now. Chances are that the core is a 'leaker'...which is probably why it was disconnected. If you want to hook it up, at least test the core for leakage before running water to it. Seal off one fitting on the heater core (2 tubes at the firewall/engine compartment, right side) and connect a vacuum pump/gauge to the other. Pump down the heater core to get a modest level of vacuum on it then let it sit & watch for needle movement. If (after a few seconds to stabilize) the needle drops vacuum...or you can't build any vacuum at all..., you have a leak in the heater core; if it does not, connect it back up and see how it works. It may also be that, with the heater connected, the interior of the cabin was too hot for the previous owner. There had to be a reason for the core being disconnected; no one would go to that trouble just for the 'heck' of it.

Paul L

The core spouts point downward, passenger side, near firewall. I assume the car has/had A/C.



This is a simple bypass, one of several ways to do it. One heater hose is used to connect the spout on the intake manifold to the waterpump. The second hose is discarded. Most likely your cat will have a similar arrangement. I used this in 2008 for several weeks to drive the car after the core blew. After those weeks of procrastination I screwed up enough courage to tackle the core replacement.

seitz

When you say the right side, do you mean drivers side or passenger side? I was looking on the passenger side and there appears to be a canister of some kind with hoses coming out. It right by the firewall. The hoses are cut though. Is this some kind of charcoal canister or the leads to my heater core?

Ganey

Driver side is left. Pass. is right. Heater on is right.
Paul's first pic. is under car pass. side.

Paul L

As Ganey has said, driver's side is left, passenger is right. This is a standard in the automotive industry taken basically from the driver's seat perspective.

The charcoal canister is on the driver's (left) side. There is no canister on the passenger side. Can you take a pic? I really do not know what you are describing!!

*Edit* - I'll try this another way. Does that area look like this? If you have spouts there it is a non A/C car.

mandingo214

Along these same lines, the HVAC system was not working on my '74, so I take the covers off around the heater control and notice that the coupler with all the vacuum lines attached is not hooked to the control "manifold". Normally I would think that it might have just slipped off after 35 years, but the bolt was still in the side of the control box.

I hooked it back up, and it seems to be working. Defrost works great, but vent and heat are not blowing very hard. The AC is another issue.

My questions are:
1) Is disconnecting the vacuum lines from the control a standard fix for some problem (like should I soon be expecting antifreeze to be coming out of the vents)? Based on the bolt still being in the control, it seems it had to be intentionally removed.

2) Is it an obvious problem when the defrost blows strong but the other outlets do not, or is it just chasing down vacuum lines and lubing doors?

Thanks.

7T1vette

All these systems should have been checked out before you bought the car...especially since the car would be your main mode of transportation. The A/C-heater-vent system is a complex hodge-podge of parts that interrelate the vacuum, electrical, and mechanical parts of your car. It is not something that can be checked out simply...unless you are a specialist in that area. There is only one sure way to get this fixed: diagnose what works and what doesn't...then determine why it doesn't work correctly...then disassemble to repair those faults [and any other worn/old parts that make sense to fix while you are in there]. You might be able to "Band-Aid" a fix temporarily; but a 'permanent' solution is not likely to be found with a piece-meal repair effort.