Author Topic: 74 Eldorado Heater Valve question  (Read 112 times)

Offline danjarrett1

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  • Name: D Jarrett
74 Eldorado Heater Valve question
« on: January 03, 2018, 08:46:06 AM »
I recently replaced the heater coolant line from the radiator and the engine block to the heater core. There is a external dash-pot looking valve that is in line with the return line to the back of the block.  There appears to be a stem in the bottom side that looks like it requires a vacuum line for some purpose. I don't see any loose vacuum lines near it. Can someone tell me if this is an overflow valve or requires a vacuum to open or close a diaphragm or otherwise? Thanks?
1974 Cadillac Eldorado convertible
2007 Cadillac Escalade
1980 Cadillac Seville “Bustleback”

Offline TMoore - NTCLC

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Re: 74 Eldorado Heater Valve question
« Reply #1 on: January 03, 2018, 09:13:38 AM »
Others who are more familiar with your model will chime in, but I can tell you that yes, there is supposed to be a vacuum line on the top of your valve.  The line will originate inside of the cabin, and come through the firewall to attach to this valve - you should see a grommet in the firewall not far from the valve that has a number of wires and the vac. line.  The amount of vacuum sent to the valve regulates the flow of coolant to the heater core based on the ambient temp of the cabin, and the setting of the climate control, in order to maintain the desired temperature.

Offline danjarrett1

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Re: 74 Eldorado Heater Valve question
« Reply #2 on: January 03, 2018, 09:54:14 AM »
Thank you for that info
1974 Cadillac Eldorado convertible
2007 Cadillac Escalade
1980 Cadillac Seville “Bustleback”

Offline danjarrett1

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Re: 74 Eldorado Heater Valve question
« Reply #3 on: January 04, 2018, 11:16:59 PM »
I appreciate the last response but every car that I have owned uses vacuum driven blend doors to regulate the cabin air temp. am I to understand that the heater/water valve attached to the heater core via heater hose, in the engine bay, near the firewall,  acts to regulate the amount of coolant that is to pass through the heater core? That would seem odd! Anyone else have an idea where the vacuum line from the heater core valve is supposed to go????
1974 Cadillac Eldorado convertible
2007 Cadillac Escalade
1980 Cadillac Seville “Bustleback”

Offline TMoore - NTCLC

  • Posts: 316
Re: 74 Eldorado Heater Valve question
« Reply #4 on: January 05, 2018, 05:26:36 PM »
Not odd at all - most GM cars with climate control used a similar system.  There are still blend doors that direct the flow of air, but in addition, there is this valve which regulates the amount of flow to the heater core.  The source vacuum should be delivered from the controller inside the cabin -

Offline bcroe

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Re: 74 Eldorado Heater Valve question
« Reply #5 on: January 05, 2018, 05:31:15 PM »
I am pretty sure, that valve is either open or closed, no
regulating the flow.  Bruce Roe

Offline danjarrett1

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Re: 74 Eldorado Heater Valve question
« Reply #6 on: January 05, 2018, 06:29:06 PM »
Thank you gentlemen, the service manual vacuum schematic does show a vacuum line from the valve returning to what I presume to be the programmer. I haven't jumped into that area just yet, trying to sort out engine bay vacuum, rebuilt carb, new fuel pump, wires, ignition components and such. My understanding is that if there is no vacuum on the valve, it defaults to the open position, it only closes with vacuum commanded by the programmer I suppose.
1974 Cadillac Eldorado convertible
2007 Cadillac Escalade
1980 Cadillac Seville “Bustleback”

Offline "Cadillac Kid" Greg Surfas 15364

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Re: 74 Eldorado Heater Valve question
« Reply #7 on: January 05, 2018, 10:25:42 PM »
That is correct. The heater valve is only closed when the system is calling for Maximum cooling.  The actual temperature control is via the temperature mix door which is controlled directly by the shaft on top of the programmer.
Greg Surfas
Cadillac Kid-Greg Surfas
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-