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  #1  
Old 01-10-2010, 09:45 PM
Josh Horton Josh Horton is offline
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Default no heat

My heat has not work right in my 73 Cadillac hearse for a while. Now when I put the selector in "vent", it sounds like a flap is moving under the dash on the passenger side and the blower motor comes on. When I put the selector in "low" or "high" the fan motor turns off. When I put the selector in "def" the blower motor will turn on. The a/c compressor is locked up, so there is no belt on it. I bypassed it, so I would have power steering. Any suggestions?
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Old 01-10-2010, 11:54 PM
Paul Steinberg Paul Steinberg is offline
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The blower motor relay may be defective, or the fan resistor may have burned out. You can check this unit be removing the two screws that hold it to the blower box, and visually check it for defects.
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Old 01-11-2010, 12:06 AM
Tom Hoczyk Tom Hoczyk is offline
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Default Attempt to help

If you don't have a Cadillac shop manual for your '73, that's the best place to start. A FACTORY manual, not Chilton's or something. I believe the factory manual has a step by step diagnostic procedure too. I believe it was Robinair that made a diagnostic tester for the programmer and vacuum system. I have seen them on ebay, so you may consider one of these if the source of the problem is not obvious.

First make sure that your engine thermostat is not stuck open, or missing completely. It will take forever to throw any heat if the radiator is doing max cooling. Is the fan clutch locked on? Does the engine roar when you drive it, from the loud locked up fan clutch? This would super-cool the radiator, reducing heat.

Are the water lines to the heater core BOTH hot to the touch, when the engine is warmed up? There is a water valve in the system, connected to a hose near the firewall. Vacuum closes the valve, so if no vacuum, water should flow through it to the heater core. Of course you have to have the control set to heat, and the knob turned into the red area.

I believe the default setting for the vacuum is to defrost, and you should hear the flap open and close while moving the lever between heat and defrost. Also, in defrost, the default is for the blower to come on at full speed. If you never get a full speed blower, no matter what, the blower relay may be bad or have a burned electrical wire, as it draws high amperage. Look for scorched insulation on a thick wire, probably red in color. Don't worry about the a/c compressor at this time. It's not contributing to your problem, although technically it does come on during the defrost mode, so be sure to pull the electrical plug off the compressor clutch.

These are all off the top of my head. Hope something helps. Tom
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Old 01-11-2010, 10:56 AM
Richard Vyse Richard Vyse is offline
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Quote:
Originally Posted by Tom Hoczyk View Post
If you don't have a Cadillac shop manual for your '73, that's the best place to start. A FACTORY manual, not Chilton's or something. I believe the factory manual has a step by step diagnostic procedure too. I believe it was Robinair that made a diagnostic tester for the programmer and vacuum system. I have seen them on ebay, so you may consider one of these if the source of the problem is not obvious.

First make sure that your engine thermostat is not stuck open, or missing completely. It will take forever to throw any heat if the radiator is doing max cooling. Is the fan clutch locked on? Does the engine roar when you drive it, from the loud locked up fan clutch? This would super-cool the radiator, reducing heat.

Are the water lines to the heater core BOTH hot to the touch, when the engine is warmed up? There is a water valve in the system, connected to a hose near the firewall. Vacuum closes the valve, so if no vacuum, water should flow through it to the heater core. Of course you have to have the control set to heat, and the knob turned into the red area.

I believe the default setting for the vacuum is to defrost, and you should hear the flap open and close while moving the lever between heat and defrost. Also, in defrost, the default is for the blower to come on at full speed. If you never get a full speed blower, no matter what, the blower relay may be bad or have a burned electrical wire, as it draws high amperage. Look for scorched insulation on a thick wire, probably red in color. Don't worry about the a/c compressor at this time. It's not contributing to your problem, although technically it does come on during the defrost mode, so be sure to pull the electrical plug off the compressor clutch.

These are all off the top of my head. Hope something helps. Tom
Great help for me as well Tom. And by the way, I enjoyed reading your recently published article.
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Old 01-11-2010, 10:42 PM
Danny Ryder Danny Ryder is offline
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The 69 caddy superior high top I recently got has a fan issue someone might help me with. On the fog and ice setting it works as should but on low, auto, and high it runs on low for all these settings? Anyone know why? Relay?
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Old 01-11-2010, 10:56 PM
Paul Steinberg Paul Steinberg is offline
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On the top of the heater box there is a resistor that is attached by two small hex screws. If you unscrew this unit, and look at it carefully, you might find that there is a break in the resistor wire. If this is the problem, you will need to find a replacement from a like kind of vehicle. They are mounted in the heater blower box area so the air blowing across them will cool them. After years of use, they will burn out one or both of the resistors. You might also find that the wire connections at the top have either melted or have corroded causing high resistance in the circuit. I believe that this is what is also the problem that Josh is having. It is quite common on older vehicles..
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Old 01-11-2010, 11:17 PM
Josh Horton Josh Horton is offline
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Thanks for the advise so far.
I forgot about this. Sometimes after parking and restarting, the heat will blow warm air.
I have replaced the water modulator valve and thermostat. Both water lines are hot to the touch. Could I check the vacuum to the modulator valve, by pulling the vacuum hose off the modulator and holding my finger over the hose?
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Old 01-11-2010, 11:26 PM
Paul Steinberg Paul Steinberg is offline
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Quote:
Originally Posted by Josh Horton View Post
Thanks for the advise so far.
I forgot about this. Sometimes after parking and restarting, the heat will blow warm air.
I have replaced the water modulator valve and thermostat. Both water lines are hot to the touch. Could I check the vacuum to the modulator valve, by pulling the vacuum hose off the modulator and holding my finger over the hose?
The modulator valve is located on the transmission, and has nothing to do with this problem. Are you confusing the water control valve with the modulator valve. Yes, you can remove the rubber tube to check for vacuum, however, it gets its vacuum source through the heater programmer module.
If you are getting some heat, then I would check to make sure that the engine block if full of coolant. If the coolant level is normal on the coolant recovery tank, then the next thing to try is to remove the two hoses, and reverse them If the heater core is partially blocked, this will reverse the flow through the core, and will usually remove the blockage. If you find that this resolves the problem, then you need to clean the engine block and the heater core. I explained this process in another thread a few months ago.
Your original post referred to a blower motor problem, and that is what I originally addressed. If you have a heater core problem also, then I suggest that you attack one problem at a time until the entire system is working correctly..
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Old 01-12-2010, 03:21 AM
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John ED Renstrom John ED Renstrom is offline
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then caddy is a vacuum close in the water valve. it should only close when it's on max ac. the rest of the time has hot water flowing there it. they control the temp with the out side air and the mix the heat and ac to get the temp. as I understand the system. main cause of failure is loss of vacuum from the engine to the control. the fan speed is selected auto from the control. from signals sent to the control from the dash temp sensor. but to proper diagnose the problem you need the book. as it will tell you what should be doing this when it's set here and what to check if it does not. you can't shot gun the system as everything cause something else to do some thing different. but the book has a cascading schematic the walks you threw it all.
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Old 01-12-2010, 07:44 PM
Josh Horton Josh Horton is offline
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Quote:
Originally Posted by Paul Steinberg View Post
The modulator valve is located on the transmission, and has nothing to do with this problem. Are you confusing the water control valve with the modulator valve.
Yes, water control valve. I have replaced the modulator valve also, but not for the heat problem.
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