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Hey everyone, I'm completely stumped. I've read every "no heat" thread I could find on here and I am stuck. I am only somewhat mechanically capable, so more then likely I've just missed something, but I could sure use a second opinion.
I have no heat, 1972 442 with A/C, 455. AC gets nice a cold, blower blows.
1) The two water hoses coming from the heater core are getting piping hot - I assume this means the water valve is working?
2) The Air Inlet Diaphragm and Defroster Diaphragm both open the vent doors when controls are moved. Does this mean my vacuum is good? I have a vacuum tester and have tried it on a few lines and it seems good.
3) My blower is blowing, all though, it is making a bad rattling noise.
3) The temperature control cable definitely tightens and loosens when the lever is moved.
What am I missing here, is there an electrical component that would cause no heat? I've been using this diagram to guide me but I'm out of rocks to turn over...
Sounds like the heat door is not moving to the hot setting. Follow the temp cable if you can there may be a lever at the end of that cable you can move with your hand. Good luck
Sounds like the heat door is not moving to the hot setting. Follow the temp cable if you can there may be a lever at the end of that cable you can move with your hand. Good luck
^^^THIS. You need to verify that the temperature blend door in the HVAC box is actually moving. The fact that the cable moves isn't proof, as the clip at the other end can come loose. Also note the multiple linkages that the bellcrank connects to.
Thank you guys for the reply.
I can't see it, but I can feel the cable is connected to a contraption(Bellcrank?) that moves around when the temperature lever is adjusted. Is something suppose to be connected to this?
I found this picture of a 71 heater box and am using it as a reference. Is the blend door inside the box? Is there a way to manually open it?
Last edited by Byron; March 25th, 2023 at 12:31 PM.
I have marked you picture with a blue arrow, I believe if you can either disconnect the cable from the point where it is attached to the cam, or the other end; then you could maybe try to move the cam by hand and see if you get hot air. If you can, then you know the cable is not working, if you cannot then the door is stuck or not working.
I can see where the cable in it's housing is suppose to be clamped to the box body, it is above the spring, the screw and loose part of the clamp are not there. It looks like the pivot point for the door (inside the box) is about an inch below the place where the arrow is pointing, the bottom part of the cam must actuate another door if it is moved far enough, that is what all that other stuff including the spring is for. Or it could be the other way, fully moving the cam activates the heat door, partially moving it activates the cold air. Either way the whole thing should move fairly smoothly with either your hand or the cable, the only resistance should be the spring.
Well now I'm just absolutely confused. I was just fiddling with things like you suggested, didn't unhook anything yet, had the air mode on heat, I moved the heat lever to cold and then back to heat......And now I magically have heat.
It may be that the clamp which is supposed to hold the cable housing tight is not clamped well. If the housing moves then the cable is inneffective and perhaps the clamp is loose but not completely loose, the cable housing bound up enough to cause the heat door to open. Worth some more investigation.
One heater hose should be engine temp and the other a little cooler. If both hoses are close to or the same temp as each other the heater core isn't releasing/dissipating heat and could be clogged.
One heater hose should be engine temp and the other a little cooler. If both hoses are close to or the same temp as each other the heater core isn't releasing/dissipating heat and could be clogged.
Good luck!!!
My experience with the "hand thermometer" is that is rivals the accuracy of the "butt dyno"; specifically both hoses will feel the same to the touch. And if coolant is flowing through the core, there will be some head in the HVAC box. Yes, it is possible that there are leaves or a mouse nest blocking the core, but that would show up as restricted airflow, not cold air blowing.
Did some testing this morning with my vacuum pump. I think I've got something here.
My first deduction must be wrong or misleading "1) The two water hoses coming from the heater core are getting piping hot - I assume this means the water valve is working?" They were really hot when I felt them. This morning however I seem to have different results.
I hooked my vacuum tester up to the vacuum line going to the water valve and here are the results:
- With the controls set to "Heat" and temperature set to "Warm", I have NO vacuum.
- With the controls set to "Heat" and temperature set to "Cold", I have vacuum.
I then hooked my tester to the water valve:
- Under NO vacuum, I had no heat.
- Under vacuum, I had heat. My tester had a slow leak and had to pump it every 30 seconds, but that could be due to my bad tester hoses and mcgyvered hose connections.
It appears that I have a valve that allows water to flow when vacuum is applied but that control unit doesn't operate this way?
I've seen discussions saying how the valves work under vacuum changed in "71" but I've also seen someone say that it was after "72" they changed. My valve just from appearances looks to be an aftermarket valve, could it be a previous owner put the wrong valve on, if so, do I just need to find the correct valve?
https://classicoldsmobile.com/forums...cutlass-64687/ Black Gold: "The dividing line between old and new heater water control valves was between the 1970/1971 model years. 1970 and earlier (at least back to 1968) used a valve which flowed coolant when vacuum was applied. 1971 and later blocked the flow when vacuum was applied. The 1970 Chassis Service Manual states that vacuum is sent to the valve (to open it) dependant only on the position of the temperature lever. As soon as you move the lever off of "cold", it opens the valve. Doesn't matter which mode (heat, vent, A/C, Max A/C) you're in. I don't know if later years operated the same."
I have the same valve on the shelf for my car.
It is stamped with:
TYPE H25
RANCO
COLO, USA
The backside has an ink label:
H25-124
The valve is normally open and takes vacuum to close.
Just today I was working on the car, changing the heater core and I'll install the new heater valve tomorrow. I connected a vacuum gauge to the heater valve vacuum hose on my car and it has no vacuum with the temp slider on WARM and vacuum with the temp slider on COOL, so this valve matches my car's vacuum operation.
Hey Fun71, I had previously read that thread but discounted it when I thought my valve wasn't the culprit. Thanks for linking to that.
I'm confused as to what your saying though. If "my car and it has no vacuum with the temp slider on WARM and vacuum with the temp slider on COOL" but"The valve is normally open and takes vacuum to close." Wouldn't this valve be the wrong valve since its only open under vacuum and our cars have no vacuum when set to warm? Doesn't open = heat?
I completely read that wrong. Fun71 is spot on here.
My experience with the "hand thermometer" is that is rivals the accuracy of the "butt dyno"; specifically both hoses will feel the same to the touch. And if coolant is flowing through the core, there will be some head in the HVAC box. Yes, it is possible that there are leaves or a mouse nest blocking the core, but that would show up as restricted airflow, not cold air blowing.
Joe,
I agree with the "hand thermometer" rivaling the accuracy of the "butt dyno" and that is why I rely on an infrared point and shoot thermometer as a diagnostic tool. I do not see the relevance of your reference to a hand thermometer in the post.
As far as the heater core being blocked I'm referring to an internal restriction; but, i wouldn't rule out something in the heater box/plenum restricting airflow either.
If a heater core is getting rid of heat the supply line will be a higher temperature than the discharge line.
Fun71, you are awesome man. You must be tired of answering these HVAC questions. I think I have read 4 or 5 threads on the subject and you seem to be in every one. I just found this thread and you solved my issue in it. https://classicoldsmobile.com/forums...-valve-145499/
I must have a Four Season 74601 but I need a 74602.
THANK YOU FUN!!! And thank you everyone for the help!!!
74602 is shown as fitting pretty much all full size GMs from 1971-1985, which means that it is a normally open valve that closes when vacuum is applied. All A-body cars through the 1972 model year use a heater valve that is normally closed and opens when vacuum is applied.
Here's an easy test. Disconnect the vacuum line to the heater valve and plug it. Now see if you get heat. If you do, that's the wrong valve.
I can only ASSUME (based upon your part number 74601) you have the correct valve. I did not lookup your part number 74601.
I can only ASSUME (based upon your part number 74601) you have the correct valve. I did not lookup your part number 74601.
I think I just figured this out. I do have the wrong part number 74601. I need the 74602 which is open under no vacuum. A previous owner must have installed a 74601 as that's what seems to be the correct part for the 72's, but for whatever reason my car and apparently Fun71's have the vacuums reversed.
I have heat!!! Just in time for the ridiculously hot North Carolina Summers....
Thank you everyone for the replies and the help! I could not have figured this out on my own. This forum has been great and the 10+ year old threads that I read through taught me a lot! Thank you!!
March 25th, 2023, 10:00 AM
