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True_Blue 10-02-2011 07:25 PM

73 cdv heater issue
Hi I am trying to work out what is wrong with the heating on my 73 coupe deville. The heater worked when I first got it a couple of years ago however it stopped working shortly thereafter and now I'm trying to fix.

HW out of block is present, the heater core isn't blocked however, there is always vacuum to the Heater Water Valve whichever temp and mode setting, therefore it never opens i.e. no hot water to heater core.

Vacuum: If I vent the vacuum intlet to this valve, the heat comes on. Am I correct in thinking this is therefore unlikley to be a low vacuum problem and more likely to be an electrical/ sensor issue?

Engine temp sensor: I am not sure of the logic to this however here is what happens: the blower will not operate when the temp sensor is disconnected and the temp dial set on heat i.e. open circuit. When the enginer temp sensor is connected, the blower will operate (blowing cold air) whether the engine is above or below 120 degrees. Is this correct operation?

An other ideas and what would maintain vacuum on the heater water valve?


Schurkey 10-02-2011 08:21 PM

You need to figure out why the heater control on the dash won't shut off the vacuum to the heater valve.

Temp sensor on the head prevents the fan from blowing cold air when the heater control on the dash is set to "heat", and the engine hasn't warmed up enough. (GM could do that in the '60's, but can't manage to figure out how to do it on my 2003 Trailblazer.) Sounds like the temp sensor has failed--always has continuity instead of opening the circuit when cold.

CadillacKid 10-04-2011 09:05 AM

IF the heater was working and nothing was done to the system, the most likely cause for the problem you correctly diagnosed (continuous vacuum to the hot water valve, thus keeping it closed) would be that one of the two multi port vacuum valve has "gone south". You need to use the vacuum diagrams in the shop manual and track down the culprit. I would suggest looking at the vacuum valve on the programmer, the black plastic covered device below the glove box on the passanger side of the car (inside).
If the system operates in AC correctly with the fan going from high through low and back to high as you rotate the dial , slowly from 85 degrees to 65 degrees (take about 30 seconds to do this with the car fully warmed up and the system on "Auto") then it is safe to say the programmer itself is functional.

True_Blue 10-25-2011 08:11 PM

thanks all - after running through the electrical side, I think it's down to the vacuum system which doesn't seem to align with the vacuum diagram in the shop manual. The white vacuum line out of the bulkhead is split between the heater water valve and a vacuum storage tank. No sign of the green vacuum line through the bulkhead.
Question - I'm assuming the shop manuals faithfully followed the implementation on the cars (and vice versa), right?

CadillacKid 10-26-2011 08:22 AM

First of all, follow the vacuum line from its source to it's destination. There is always the possibility that the line was changed at some time in the past, or the line was mis directed. The answer is that sometimes the factory manuual uses diagrams that are "generic", but they are usually correct.

andytrig32 10-27-2011 01:09 PM

So any status on the project?

True_Blue 11-01-2011 10:05 AM

not yet. Didn't get a chance to look at it over the weekend with Halloween and the snow. I'll see if I can get time tonight.

True_Blue 11-10-2011 03:42 PM

Here's the update. The vacuum lines in the engine bay have been altered. I traced the white vacuum line from the programmer and found it hanging loose in the engine bay with a screw corking it. The heater water valve was being fed from a tee from the vacuum reservoir that sits centrally on the bulkhead. I checked inside the cabin and I suspect this has been altered as well but I have still to trace them i.e. newish tape binding the vacuum lines together as well as (I suspect) an additional tee.
Anyhow, reconnected the white vacuum line to the water heater valve and got rid of the tee to the reservoir and hey presto, the heater works.
This car has been in Florida most of it's life and I suspect the heater was disabled at some stage - I am wondering if there was heat leakage via the mixer door that led to this alteration. Something to think about.

Stage 2 - investigation of the internal vacuum lines begins tonight.

cadracer 01-26-2012 01:22 AM

Rotory heat dial
Had that problem with my caddy. The dial on the heater had issues and the vacum would not seal off enough to hold the mode doors over when you wanted cooler heat. Due to the way the heater lines are hooked up there always flow through the heater core and how they control temp is the air doors.The only time the vacum pod is activated with vacum is when a/c is called for. This then shuts off the water flow to heater core and the evap core then cools everything down as the mode door swings over to let air now be pulled through evap core also. Here in Washington you need heat in the am and some days you wanted a/c to keep the leather cool. I would check the heater controls for proper function and then go to the temp sensor in the heater air box. It just a potentiometer if my mind recollects correctly.Hope this helps.

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