A/C stopped working - Help?

[PerryB]

Hello - was disappointed to turn on the A/C a couple days ago and get no reaction at all from my '68 Eldorado.   Worked great the last time I tried, just a couple weeks ago.   Also get no heat, and I don't even think the vent position is working.  I suppose it may have to be cold inside the car for the heat to work, but the temperature inside the car was definitely less than the highest setting on the dial of 85.  Had been driving in town for 20+ minutes so pretty sure coolant was up to temp also. 

I'm in Wisconsin so I'm mostly concerned of course with the air conditioning as the car will never be out of the garage here during cold weather months.  Any ideas on what would cause the whole system to stop working all together?

Thanks so much for any troubleshooting advice....I just bought this car recently (my first older Cadillac) and have plenty to learn.

Perry Bush

[Fred Zwicker #23106]

My AC cut out returning form the Cadillac Legacy Show a few weeks ago.  Running nicely then nothing. (90 degrees outside so a long trip home).  It turned out that it had a blown fuse, due to corrosion on the fuse block clips.  I removed the fuse, cleaned up the clips with some emery cloth and it now works like new.  If you are getting nothing, I would check the fuse first.

Fred

[PerryB]

Thanks, Fred.  My fuse and contacts are like new.   

I happen to have a spare A/C duct sensor, part number on the black plastic housing is 7289736.  Has a sensing wire going across the diameter of a tube about 3-4" long, and that is wired across a resistor where the electrical leads are.  Honestly, I'm not sure what this even does, bit wondering if I could possibly ever get that lucky and have this be the problem....where will I find it on the car?   

Interestingly, the box the part came in has a part number on it of 7289845.  Is there a reason for this, or did someone mismatch the box and the part?

Other ideas?

[TJ Hopland]

Does the fan run?  Does  it change speeds?  If you move the lever to defrost does it blow from the defrost vents?  If you are on econ and put the dial to 85 does it blow from the floor.  On 65 it should blow out the dash.   When its in AC mode is the compressor clutch engaged on the engine?  The belt always turns, when the clutch is engaged the hub at the front also turns.  The ac compressor is the long belt driven thing pretty much on the top center of the engine.

[PerryB]

Thanks TJ - good questions (that I should have answered before throwing out a line!).  The A/C compressor is running in all settings other than "Off" and "Vent".  In the Low, Auto, Hi, Fog, and Ice settings, the clutch is engaged.   Is that normal?  I would have thought the compressor would disengage in either the Fog or Ice settings, when heat is requested?  Maybe that depends on ambient conditions in the car actually..     This is the case no matter what temperature I set the dial to.  So in Auto, even with temp set at 85, the A/C clutch is engaged.

Blower never blows regardless of all else.  According to the owners manual, the blower relay uses the same fuse (#3) as the Air conditioning amlplifier, which I 've tested as good (zero ohms).   Are there other overcurrent devices in the circuit beyond the fuse, like a breaker or fusible lnk?  Where would they be? 

What's the A/C amplifier?  Can I test this?

Is it ok to jumper a hot wire from the battery directly to the blower motor just to make sure that's not the problem?

Any other data I should provide?

Thanks again!

["Cadillac Kid" Greg Surfas 15364]

Perry,
If the compressor runs in all positions but off and vent then you at least have a primary circuit working.  Does the fan motor come on in the defrost position? If it does not, you probably have a bad connection somewhere in the blower circuit or even, could this happen? a bad fan motor.  Check the fan motor first by jumping the positive lead of the motor to a 12 volt source.  If the motor runs, just work backwards from the motor with a volt meter(with the AC on and the key on run)  until you find the point of disconnect.  Check all the connections and the wires, and I hope you have a shop manual with a wiring diagram.
Let us know. Happy hunting.
Greg

[TJ Hopland]

I think the blower motor should run on low anytime the key is on, I know for sure it did starting in the 70's.   You can use a jumper to test the motor, use a decent size wire and be ready for a big spark, full speed these draw quite a bit.   I dont have a 68 or a 68 manual but on my 78 the relay (and socket) melted.  I told a friend and he said the same thing happened to him on his 68 so its worth a look.

[Mike Josephic CLC #3877]

Have you checked the "thermal fuse"?  That's located on the plastic duct
on the firewall in the engine bay.  That's designed to "blow" if you're low on
refrigerant and once that happens, nothing in the AC system will work.

Refer to your shop manual for more details.  There is a way to "hot wire"
the system to restart it and recharge the system -- but you willl have to replace
the fuse for continued operation -- not an easy part to find.

Mike

[Glen]

I don’t believe the ’68 A/c system used that thermal fuse.  It’s not in the shop manual.  I think that came sometime in the ‘70s.

I sent Perry a schematic of the A/C system from the shop manual. 

Perry, The system is designed to run the compressor except at max heat.  The idea is to dehydrate the air. The temperature is controlled by the temperature door.  All the air goes through the A/C evaporator to be cooled to the max.  Then the air is divided by the temperature door so a portion goes through the heater core to be reheated, it is then recombined with the air that bypassed the heater core.  The ratio of reheated air and cooled air sets the temperature of the air coming out of the vents.  By simply moving the door a little the system can go from cooling to heating.  When it goes into heat mode the mode door redirects the air to the floor vice the dash outlets. 

Glen

[Guidematic]

 No, there is no thermal fuse on the '68 system. That was not used until The VIR system came out in '73 or '74 and lasted through '76.

I would first check to see if the blower motor is receiving power. It may be something as simple as that. If it is, tap the motor lightly and see if it starts.

Mike

[Gene Moscicki]

Perry; the a/c  & heating system are operated on vacuum. You should have a switch- looks like a shoe covered in rubber that turns on the fan.located on the firewall engine compartment, after you position the in dash to the desired operation, not sure if car needs to be running, anyhow you can operate the firewall switch by pushing on the rubber bellows. IF this gives the desired results a thorough check for a vacuum leak could also be the problem, either way good luck.

[Rusty Shepherd CLC 6397]

Thanks TJ - good questions (that I should have answered before throwing out a line!).  The A/C compressor is running in all settings other than "Off" and "Vent".  In the Low, Auto, Hi, Fog, and Ice settings, the clutch is engaged.   Is that normal?  I would have thought the compressor would disengage in either the Fog or Ice settings, when heat is requested?  Maybe that depends on ambient conditions in the car actually..     This is the case no matter what temperature I set the dial to.  So in Auto, even with temp set at 85, the A/C clutch is engaged.

The compressor does run in all positions except "Off" and "Vent" at temperatures above 40 degrees F.

[TJ Hopland]

Perry; the a/c  & heating system are operated on vacuum. You should have a switch- looks like a shoe covered in rubber that turns on the fan.located on the firewall engine compartment, after you position the in dash to the desired operation, not sure if car needs to be running, anyhow you can operate the firewall switch by pushing on the rubber bellows. IF this gives the desired results a thorough check for a vacuum leak could also be the problem, either way good luck.

I forgot about that!   Maybe someone can tell you the location of that vacuum line in the engine compartment.  On a 78 its the same one that feeds the parking brake.   Is it the same feed that runs the air doors and such?

Was changing the 'vent' to 'econ' a 70's fuel saving thing?  I assume the functions were the same.

["Cadillac Kid" Greg Surfas 15364]

Perry,
I hope you are not getting too crazy with all our input.  Although the vacuum circuits , and all the other devices play an important part in the proper temperature control, IF your compressor is running and the fan is not, and UNLESS someone has rewired the system from original, I will repeat my suggestion that you check out the electrical circuit to the fan motor.  Check the motor with a 12 volt lead to make sure it is not bad, and then work backwards with a volt meter until you find the fault.  It will probably be in the last place you look.
Patience is truly a virtue on these systems.
Greg

[Rusty Shepherd CLC 6397]

Was changing the 'vent' to 'econ' a 70's fuel saving thing?  I assume the functions were the same.

Yes, sir, "Econ" was a '70's and 80's fuel saving notation.  Some of the manual systems of that era had "Econ" in a bracket over both "Vent" and "Heat" to indicate that the compressor didn't run in either position.

[PerryB]

Thanks very much to you all!  My master vacuum switch is indeed the culprit and I have a replacement on the way.  With the blower motor jumpered to 12V,  I get cold or hot air blowing depending on where I dial in the temperature. 

Another member forwarded a link directly to me for CadillacTim.com, and I was so impressed with the tech help section of his website, I had to share with you all here.  Tim has done a truly wonderful job of not only explaining all the detilaed workings of how the HVAC systems work, component by component, but also includeds a troubleshooting guide with most common root causes, in some cases broken out by model year.   He also sells new and reconditioned AC (and other) parts at what I've found to be very reasonable prices.  He also has an equally impressive write-up on troubleshooting the 6-way power seat circuits.   Check this out - if you appreciate straight forward and comprehensive language to guide you through a problem, you will be impressed also.

Thanks once again - I really appreciate the help!