Heater AC Problems

Last Updated 09/21/06

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Troubleshooting AC and Heater Problems

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Heater and A/C problems were the FIRST things I tackled when I bought my 77 about 2 1/2 years ago.  Everything was screwed up but I got it working.

Even when the A/C is working properly as mine is, I feel it is still not great by today's standards.  
I had all the following problems at the same time to solve:

  • Vacuum leaks causing flapper valves not to operate properly
  • Bad hot water shutoff vacuum switch followed by an incorrect, unusable switch
  • Bad hot water shutoff valve
  • Bad AC Compressor
  • Bad AC Hoses
  • Bad AC Receiver/Dryer
  • Leaking heater core
  • Bad High Speed Blower Relay
  • Bad NEW heater control mechanism
  • Leaking heater box seals

So I've been there and done most of it.
Have you checked the Freon level?
Do you see any bubbles in the sight glass?  Right fender, inside, inline with the pressure hose along and just below the edge of the hood.  Clean it and take a look.  There should be no bubbles.

After all the repairs, I vacuumed and refilled with R134.
To verify that high is really high, you can connect a wire to bypass the high speed blower relay under the hood.  You can unplug the top connector on the relay and make this temporary connection.  Connect the red wire to the purple wire on the high speed blower relay.  With this temporary connection, this is as fast as things will blow.  If there's a change to a higher speed, the relay or the trigger to the relay might be bad.  
If the low and medium settings are not working, it's likely to be the switch or some connection problem to the resistors that slow the blower.  The second is probably the case.  When you slide the lever to Max A/C, there's a relay on the firewall that kicks in to bypass the resistors and make the blower run full speed.  The connector for the resistors is under the hood near the right hood spring.  They are attached to the heater box.  The 4 wires colors are Brown, Green, Lt. Blue, and Dark Blue.  The relay for high speed blower is just above them on the firewall.
The 25A Heater A/C fuse is probably good IF you have full speed on Max A/C.  That's where the power comes from to kick the relay.
The connection into the switch (yellow wire) is good if you have  any speed to the blower in any position (vent, heat, a/c).  From the switch, three wires run to the resistors under the hood.  This output goes to the relay and on to the blower.
If the wiring checks out, maybe the blower motor is weak.  I think I've seen articles on how to replace it with something stronger.
The complete electrical troubleshooting guide is on my website.  The A/C and Heater stuff is on pages 16 and 17.
Vacuum leaks and binding doors can cause flappers to not close, not close completely, or be flakey.  Air doesn't flow where it should.
There's a hot water shutoff valve to keep the water out of the core when the temperature is in the cold position.  The on/off valve is on the heater box in side the car.  The shutoff is inline on one of the heater hoses.  Vacuum closes it.  When it sticks due to age/rust, it usually sticks open.  If you have heat, you have an open valve.  With the temperature control set to full cold, you should feel vacuum if you disconnect it from the inline shutoff valve.  If not, adjust or replace the switch inside the car.  Important.  You cannot buy a replacement vacuum switch.  The one people are selling is for the wiper door on earlier chrome bumper cars.  The spring is too strong and will keep you from moving the properly adjusted cable to the cold position.  Someone posted a tip where they use the old spring on a new switch.  I decided to convert to an electrical shutoff system.  Either is fine.
The temperature control is by cable.  It might be out of adjustment.   You might be blending some hot air into the system.  If this cable is not adjusted properly, it won't activate the switch above.  There's an adjusting mechanism inline on the cable about midway down the length.  You might have to remove the right side console panel to find it.  You twist to shorten/lengthen the cable.
On Max A/C, internal air is recirculated.  On normal, external air is brought in.  This is done with a vacuum controlled valve on the right side near the passengers foot behind the kick plate.  If this isn't functioning, you might not be getting the recirculation you expect on Max.  An upstream vacuum leak can be a pain to find.  I had one of these.  Flapper valves were not functioning properly.  Turned out to be a leaky hot water shutoff switch.  I replaced the mechanical vacuum switch with a microswitch and a vacuum solenoid.  Perfect now.  I'll have to document this.

With A/C, this vent is the intake for recirculating air.  No fresh air flows through it.  All fresh air goes through the dash vents.  If it is open, you are recirculating what's inside the car.  This happens on MAX AC only.  
Any vacuum leak is deadly to the A/C on this car.  You've got to fix it or none of the flappers work.  All vacuum comes through a tiny tube.  It enters near the heater box and there's a T connection next to the hot water shutoff switch inside the car on top the heater box.  In my car, this valve was leaking slightly.  Even a tiny leak bleeds off the vacuum before it can open a valve, door, etc.
The vacuum leak might be causing the defrost door to flip in the wrong direction or the vent / floor to go the wrong way.  I can't recall which direction is the default (no vacuum).  I'll see if I can find my notes on this.  My leak was causing the door between vent and floor to operate slowly or not completely.  It was a mess.
Vacuum Diagrams from 77 Assembly Manual


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This site was last updated 08/14/06