Troubleshooting HVAC 1977 Cadillac

I tried a Google with not much luck, jut looking to see if anyone has gone through this before. The HVAC system in my 1977 MM LIfelienr is dead. I checked the blower: works, the fuse: good, pneumatic vent controls work and switch he air flow correctly yet no power to the fan motor fromt he stock brown Cadliiac 12 g wire.

The MM Switch Rear AC/HEAT in the over head panel has power, but I don’t think that should affect the factory HVAC function. (I will tackle the rear stuff next)

I do have the 1977 Cadillac manual, but not with me at the time of this post. My next step is going to be pulling the dash and checking the connections behind the control module.

Just though I would put it out there in case someone has found an easy fix for this symptom….
 
Jump the fan wire with a jumper from battery to fan, and see if it works. There is a resistor in the heater duct that regulates the speeds, and it is known to burn out, causing a dead fan. There is also a fan relay in the circuit, but I don't remember exactly where it is located. My fist thought is it is a plug in on the fuse panel, but even then I am not certain. If you have a FSM (factory service manual) it will give you the location. Last thing to check is if the noise suppressor on the fan motor has burned out, causing a incomplete circuit to the fan motor.
If the compressor clutch doesn't engage, there is a low freon pressure switch located on the back of the compressor that is internal to the compressor. You can jumper power the compressor clutch to verify this issue. If the clutch engages when you jumper it, then the switch is either defective or the system is low on freon. The body builders also put a jumper on the rear air conditioning unit, so when you turned that on, it would send power to the compressor, even if the front air conditioning wasn't set to "on".
 
Great !, Thanks Paul...thats just the info I was looking for. I did jump the blower motor and it worked great! The factory Brown Cadillac wire, before the noise filter is dead, no power there ...I am guessing its the fan relay...I will track it down with the manual.
 
From the FSM.......
AIR CONDITIONING 1-21
BLOWER OPERATION AT LO (Fig. 1-24)
In "Low", battery voltage flows thru the fusible link, ignition switch and fuse block to the ignition pad on the control head which is connected to the compressor feed pad by the wiper contacts. This provides energizing of the compressor clutch if the evaporator pressure switch is closed.
Blower operation is again delayed until the relay coil circuit is completed by the metal temp. or in-car switches. Once completed, a fixed low blower speed is provided as blower current travels through the single fixed lo blower resistor before reaching the blower motor. Multiple blower speeds are not provided since the programmer B+ feed circuit is not completed at the control head.

All this translates to you have a big headache trying to find what is wrong in the electrical system. I think that the first thing to do is to make sure that the system has freon and is fully charged before delving into the electrical system. Many times, a proper charge will rectify the electrical problem. All this circuitry is designed to keep the compressor from self destructing if it looses the charge.
 
It is most likely the fan relay located in the fuse box. They have been known
to melt and start burning in these cadillacs. I have a 1978 Fleetwood and have replaced it 2 times already. There is high resistance in the wiring for some reason.
 
Has anyone out there tried an alternate to R-12 like Freez12 and if so, any do it yourselfers done a freon recharge yourself with a kit ?
 
If you use "Freeze 12", that will mean that you can never go back to R12, without first decontaminating the entire system first. I have not heard of any good substitute for the original charge of R12 that has been successful in every application that has used R12 as the original charge. I just use what is original to the car, and leave all the substitute products to others. It is like using aftermarket parts, you never know for sure if it is going to work well, and the cost savings are not worth the cost of doing it over again.
 
freeze 12 is a older refregerent that was used before they came out with r12. it is a mix of methane butane and propane. you might as will say it's natural gas. they came out with the r12 because they wanted something to use in buildings that did not burn. guess there was a big school fire with the other refrigerant. the interesting thing is that now they have determined that the r12 is a heavy gas there for it can't contribute to the green house gas. but the albatross still hangs around it's neck. I have some r12 here I can sell you for 30 bucks a can. you get someone to put it in and have them add a can of sealer to your system. if you plug off the rear you can get buy for about 3 or 4 cans. the problem with the freeze 12 is if you need help on the road who you going to get to fix it. once you convert to r34 your stuck with it for life and it will not stay in the older system. that one I know I did it and now I'm regreating that I did. we just got done putting 4 cans in the 72 seville and it works good.
 
Latest update on the HVAC…. I have decided to take the advice by using R-12 to recharge my system. Before I get to that point, I found the blower relay…appears the factory one had melted out of the relay cluster and had been replaced with a 30 amp Bosch style relay. They seemed to have used some type of black goop that has corroded. I by passed the relay and the blower, blew…. Now it appears the B- from the control unit is not making it to the relay to activate the relay. I am in the process of trouble shooting that problem now thinking is may be the dash control unit may be defective…

I am planning on recharging the entire system front and rear. Oer my Cadillac Manual the main AC requires 3 3/4 pounds of R-12 to fully recharge and AC Compressor Oil 10.5 fl Oz. Can anyone guess how much additional R-12 for the rear? Also what kind of sealer should be used to flush the system and possibly deal with any leaking?
 
If you are going to recharge the system, you need to remove the compressor and drain the oil into a measuring container. Depending on how much drains out, depends on how much you add. The FSM should have a chart for this. There is no way to really know how much the total system will take for a full charge, until you start filling it. If you want to leak test the system, you should first draw the system down with a vacuum pump for 24 hours, and then leave the pump hooked up, and close the valve on the pump. See if it maintains the vacuum level. If it doesn't, then there is a leak somewhere in the system. If you want to charge the system with an inert gas first to check for leaks, then you would use nitrogen. All this new high tech stuff is things that I have learned, but have never done. I used to do air conditioning service by the seat of your pants method. Pull a vacuum for 24 hours, and then fill with freon. Leak test, and hope for the best. If it leaked out in less than 6 months, search for the leak. If it didn't, then add freon and top off the system the following year. Back then, freon was cheap, and there were no certification requirements. I also had a liquid that I would pour into the lines to flush them, but I don't remember what it was. If I dig out some of my old FSM's, it is listed in there. Back in the stone age, we would use a propane tank and burner with a rubber hose pick up for leak testing. If the rubber hose picked up a leak, it would change the color of the flame. Today, I use a electronic beeper type tester. Freon is heavier than air, so you check the connections at the bottom of the fitting first.
 
there is a lot of stuff to that automatic temp control unit. I taking it that you have a book. the service manual has a cascading list in it that if this does this then go here kind of thing. vary handy to have. with out it I would have been lost redoing the Seville. I would think you will need to add at least 2 lb for the back unit. but most people now add until you get to a temp coming out the vent. the man that recharged the Seville for me put it at 42 deg. they have a system flush and a system sealer they are different things. if the system has not been taken apart then the flush is not necessary. the sealer will pick up any small leaks you have or develop in the none moving parts. most ofthe people recharging things today use it. they say it will stop the r34 from walking out the back of the compressor on the ambulance. we will see. by the time I get to flint it had better still be working. but on yours the thremal cut off switch must be burnt out at least.
 
OK... I replaced the relay with a larger 60 amp and soldered the connections.... I found the problem to be the resistior module under the hood was the issue...blows & works fine now and ducting/pneumatics seem to work good too with different speeds and duct direction....Now....No cooling (I expected that) but or heating also........ Thoughts ????
 
Cooling.......low or no freon... Heating, should be OK, unless the mixing doors are not working. Hard to get heat when it is 90 degrees outside. Try the defroster mode and see if there is heat. If you have heat, then you know that the water valve is working.
 
the vacuum valve may be not working in the heat system. most of the caddy's were closed with vacuum. when you wanted max AC it would close off the heater water. yours may be stuck close.
 
OK, need to check the vacum line to the heater door and/or water valve...Also I have a rear ac/heater dpdt switch in the front overhead console. Switch has power and runs to the rear blower rotary speed switch. Each wire has a large diode that prevents power from being back fed to either the a/c or heater circuit...In order to get the rear blower working, I diconnected the heater circuit...(cant imagine I would ever need it) and bypassed the a/c diode and conencted it. So whent the front Overhead A/c swicth is on, it provides power to the rear blower switch....

I do have the 1977 Cadlillac manual but not easy reading... Hopefully I can get the system to the point where I can take it to an AC guy and get it reapired for a decedent price... both front and rear.

As awalys, thanks for the ongoing help....
 
The switch in the console up front is a bypass switch that over rides the front air conditioning controls. It is designed to allow the rear air conditioning to be operational without the front compartment air conditioning to be working. I have no idea of exactly how the system was electrically engineered, however, I believe that by disconnecting the switch, you might also be interrupting the circuits in the front heater / air conditioning control panel. I would reconnect everything the way that it came from the factory, and see if that might give you the desired results. You darn near have to be an electrical engineer to understand how they wired the interior dome lights, so I am certain that the air conditioning circuits are much more convoluted. They must have done these wiring changes in consort with the factory wiring engineers based on what changes I have observed in the wiring in my 1978 Superior Cadillac Ambulance.
 
Thanks Paul...thats the stuff that is beyond me...I will find an ac guy ...My mechanic has a buddy thats going to check the rest of the system out...when im ready and I have fixed most of the bonehead stuff
 
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