Just repaired AC/Vent/Defrost Switch but no heat?
 

Hi All,

Thanks for the help on the replacement of all of the vacuum lines so that everything is working as original.
Everything on that panel is working but I am not getting heat.
When I had the dash apart the cable did move the flap at the heater core and I put my hand in and felt it.
I just had the engine running to 180 and all heater hoses are hot even the in and out from the heater core but when I move the bottom lever over to the heat side the cold air stops but I only am getting outside air temp coming through the lower duct or the defroster. It is not hot air. If I move it to blue or cold the a/c does come on. I can hear the flap working when I move the lower lever. There are no leaks from the heater core.
I know that I don't need heat now but I want it working correctly.

Thanks
Jack Miller
1977 C3

Let me get this right.
You have hot coolant flowing through the heater core.
When moving the hot-cold lever the door does go to hot position.
But with your fan on NO hot air is coming out any of your ducts ?

Try turning your [add-on] heater shut-off valve to ON. (just a thought)

The other possibility is that the heater core is totally plugged up so that no water is flowing through it. Feel the heater inlet pipes on the firewall to see if hot water is even getting to the core.

On the 77 there is a vacuum valve mounted on top of the heater box. Did you check that valve for function? It supplys vac. to the heater control valve in the engine compartment, to open or close

Correct. I am stumped too.

Jack

Yes,

I have a new hose on that too and it was working before I put the dash back on.

Jack

Both heater core hoses are getting hot, so anti-freeze is passing through

Jack

Achem's Razor...whenever there is a cunundrum, the answer is usually the simplest explanation. You 'obviously' have a mouse nest built in the heater housing and the nest material is blocking the heat. :leaving:

But the defrost has the option of cold or hot air too. I can't get hot air out of that duct either.

Jack

what is this flap your hearing when you move the temp selctor from hot-cold.

the only thing that is attached to the temp selector is a solid adjustable cable that goes to the heatercore vacuum switch on the top of the air box.

also are you getting any air moving at all can you hear your blower motor even running?

I've got the dash out of my 77 right now working on the air flow and vacuum issues. looking it over I don't see how its possible for the air to not be coming out some where. with the diverter gates all in the neutral position the air will come out the floor vents (heat). The temp selector (bottom slider) will control the temp, hot or cold.

ellk,10 there is a diverter door attached to the temp selector cable but it also actuates the vacuum switch.

can you hear the blower running?

Just went back and read your first post, you are getting air flow just not hot air.

Can you get pics of were the heater hoses are connected for us to see?

Looking over the stuff I have apart on my car the air has to flow either over the heater core or around it with the operation of the temp door. With you verifying the operation of the temp door the air has to be flowing through the heater core.


Neal :thumbs:

If I read your problem correctly, you stated that you are not getting "heat". You didn't indicate that you weren't getting any airflow. If no flow, your air inlet damper/door (under the right-side cowl) is probably not opening. That would prevent any fresh air from entering the system. Put your control to "Max A/C" and see if you get air from those ducts...that is 'recirulated' air from inside the cabin.

Motor is blowing just fine on all speeds.
Thanks
Jack

I am getting airflow from every vent. When I turn on Defroster air comes out of the top and whatever speed I set it to. When I turn on A/C (Max or Reg) or Vent air comes out of all of the air ducts that it is supposed to.
When I move the bottom lever to red (or hot) I am not getting heat out of the defroster of lower ducts as I am supposed to. Yes, the door opens and closes when I move the lower lever. I had a brand new 74, 76 & 77 back in the day when I went into a dealer and ordered what I wanted so I know how these are supposed to work. I just need help repairing it.

Thanks
Jack
77 C3

Hey man,

Thanks but I know how it is supposed to work and it works correctly, the air speed switch works on every stop too but when I move the lower slide to red or heat I should get hot air from the defroster, a/c vent, or lower duct when I move that lower lever and I am not.
Both in and out water hoses are both hot going to and from my heater core.
I also tested every vacuum door with a mighty vac hand held vacuum tool and every one opened and closed as they should a few times each.
I have done many cars in my time but this car is different.
Before I redid all vacuum hoses I had no air to defroster or center and now I have air coming out from what ever I move the switch to.
But no heat, I want it set up for the winter as it will be here before we know it.

Thanks
Jack
77 C3

My 'last gasp' suggestion is to check the [stock] heater shut-off valve and see if it is permanently fixed on OFF. You can use your Mighty-Vac and hook it to the 'signal' line going to that valve. Then put vacuum on it, then open to 'vent' and see if it the heater operates with one of those conditions.

Hi,

I checked all of the vacuum valves when the car was cold with the Mighty-vac and they all worked fine.
I even moved the lower lever and the heat door opened and closed.
I am really stumped.

Jack

The diaphragm on the water shut-off valve may have held vacuum...but how do you know that the valve wasn't stuck in the OFF position. The diaphragm is rubber and would stretch with vacuum, even if the valve didn't move. Only way to know for sure [I think] is to check it for flow when opened [and no-flow when closed].

The door moved back and forth and air was traveling through the big lower squarish hole when I had all 3 front vents off.

Thanks
Jack

Hi,

I took the passengers side and center dash out again.
I found the vacuum switch on top of the air box and it was leaking air constantly so I just bought another.
I did notice that when I moved the cable that it just touches the top of the spring button when cold and comes off when moved to hot.
Can you explain how that vacuum switch is supposed to work? Is the spring button supposed to expand or contract?
I also bought the vacuum switch that connects the heater hose that goes to the heater core just in case. This way all heater parts are new.

Thanks
Jack

what happened
 

What happened; you guys just dropped off on this. I have the exact same problem on my 81. Everything on the system works but it will not blow hot air. Air flows thru the proper vents but its never hot.
I removed the glove box and watch the temperature control switch move and open the door. When the cable moves the switch to the cold position, it does depress the vacuum switch like it is suppose to. I'm wondering if my correction is my problem. My car wasn't blowing A/C air out of the vent registries on the dash. I found a vacuum leak on my intake manifold and connected the line and my A/c started blowing out of the registry again. I notice that the originator of the post here said that he just connected his A/C and defrost system. Could it be something there.

Heat
 

Hi,

For now I managed to get the heat and defrost working and I know that it is the replacement switch that I bought that is now allowing the vacuum for the A/C to work.
For heat on the 77 I had to replace 2 switches. One on top of the vent behind the passengers side dash, with 2 vacuum lines, with a spring loaded switch, that when you move the bottom cold- hot lever it pushes that switch on and off. One of the vacuum lines goes to a control in the engine compartment between the hot water (anti-freeze) to the heater core. It makes the hot water flow or not. If you move the lever for cold to hot the when it is on hot feel both hoses on each side of the control on the passengers side. The one going to it will be hot as soon as the motor warms up. If the switch is working, the side going to the core will be hot too. If not do what I did, replace both as they are not too much money.
I will fix the A/C in the spring. It is the new plug in behind the main switch with he 5 vacuum lines. I am happy to have cool & Hot air out of my lower ducts and defrost in the winter. I do not need A/C in this blizzard that we just got.
I have found that some of the new re produced parts really suck. I had to make a few good parts from a new and old part.
Good Luck
Jack
77 C3

I found that on the new part, the spring was too heavy for the lever and cable to push the white cap in to make the vacuum work. I had to use the original spring, which was lighter and old cap on the new body and internal nail.

Thanks
Jack

I think i know the switch
 

I think i know the switch you guys are talking about. It resides at the end of the stroke when you move the temperature cable to cold. there is a little tab that pushes the switch in when the cable moves all the way to cold. I thought that may be my problem but wonder about this: - isn't the car design so that if you move the cable between hot and cold, you are suppose to receive a mixture of cold air and hot air (warm)? I don't see how you could ever get warm air if the only way to get the hot air to flow is to depress the switch with the temp cable being in the absolute cold position. However, I did manually hold the switch in just to see if the air would blow hot and it didn't. I guess my problem is something else!

Maybe your thermostat is stuck open.

nah
 

I don't think its the thermostat. If it was, there would be some hint of warming. My air is as cold as a/c air. I think its my water shut off valve under the hood. I don't think its opening to allow hot water in. I was going to change it a couple of days ago but my Opti Spark died and took precedence. Once I get it running again, I will change the water valve and let you guys know how it turned out!

Water Temp Control Valve
 

That was it! There is a water temperature control valve (or I have heard others say Heater Control Valve) under the hood on the passenger fender well. Heater hose goes to it before it enters the car. I change that and I'm as warm as can be!

take the blower resistor out and u can see if the temp door is opening and closing...