Fixing AC in 1976 C20 3+3

[RADustin]

Gents,

General question I know, but I want to try and fix the AC in my '76 as I think it will help the sale. Don't worry, I am replacing the truck with another square.

Anyways- The heater works, and for the MOST part the blower motor does want I want as far as changing speeds. But it doesn't all the time. Sometimes it doesn't come on at all. Do the dash switches go out? or are they serviceable? I heard of a relay that goes out often, any ideas where this is and what it controls?

I don't think it's really switching modes either, from heater to AC. From what I recall, the heater works when it catches but it doesn't switch over every time. The blower motor is kind of intermittent as well.

Does anyone have an electrical diagram of the system so that I can first check that the controls are working? Also, maybe an overal functional diagram of the system so I know what is electrical and what is mechanical. I'm imagining the heater valve is mechanical. From there I will check refrigerant charge and pressures. I think I can get a handle on everything once I am sure the dash controls are correct.

There has to be some sort of electrical gremlin because while adjusting the modes the [factory] tach will jump. So I'm wondering if there is some sort of ground loop or similar causings issues. Anyone else ever seen this?

I appreciate any help. TIA!

 

[chengny]

Do the dash switches go out? or are they serviceable? I heard of a relay that goes out often, any ideas where this is and what it controls?

I don't think it's really switching modes either, from heater to AC. From what I recall, the heater works when it catches but it doesn't switch over every time. The blower motor is kind of intermittent as well.

Does anyone have an electrical diagram of the system so that I can first check that the controls are working? Also, maybe an overal functional diagram of the system so I know what is electrical and what is mechanical. I'm imagining the heater valve is mechanical.

There has to be some sort of electrical gremlin because while adjusting the modes the [factory] tach will jump. So I'm wondering if there is some sort of ground loop or similar causings issues. Anyone else ever seen this?

1. Do the dash switches go out? or are they serviceable?

The dash switches can get old and fail - but not often. Eliminate everything else before suspecting the switches. Personally, I wouldn't try to service the blower speed switch or the mode select switch.

2. I heard of a relay that goes out often, any ideas where this is and what it controls?

You probably mean the blower relay. It is mounted on the outboard side of the evaporator housing and - as controlled by the blower speed switch - is used (in conjunction with the blower resistor) to vary the amount of voltage applied to the blower motor.

3. I don't think it's really switching modes either, from heater to AC. From what I recall, the heater works when it catches but it doesn't switch over every time. The blower motor is kind of intermittent as well.

I'm unclear on what the above means.

4. Does anyone have an electrical diagram of the system so that I can first check that the controls are working? Also, maybe an overal functional diagram of the system so I know what is electrical and what is mechanical.

Two views of the electrical system:

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Vacuum system with operation chart:

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Air handling system flow diagram:

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Component location/layout in engine compartment:

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5. I'm imagining the heater valve is mechanical.

The temperature of incoming air is controlled by the position of the "temp valve" (AKA a "blend door"). This damper moves left/right to vary the percentage of air allowed to flow from the evaporator core (cold) and the heater core (hot) and into the cab.

6. There has to be some sort of electrical gremlin because while adjusting the modes the [factory] tach will jump. So I'm wondering if there is some sort of ground loop or similar causings issues. Anyone else ever seen this?

When the mode switch is moved to MAX AC two things happen:

1. The blower speed is automatically shifted to high

2. The compressor clutch is engaged.

The carburetors on A/C equipped trucks are fitted with an idle-up solenoid that is designed to maintain idle speed when the compressor is engaged. It is possible that if your solenoid is not functioning and MAX AC mode is selected, that - along with the shift to high blower speed - could cause a dip in engine RPM. Conversely, if the mode switch is moved away from MAX AC, the engine RPM might jump enough to register on the tachometer.


The thing is, you haven't really described what the problem with your system is. Is it a problem with the blower speed, temperature control, mode operation, etc? Please provide more details.

 

[RADustin]

I know I haven't described the problem...it's because I don't know- yet. It just doesn't work.

I'm going to start with the blower motor and work my way from there. I think that is a good place to enter as everything is based on the blower motor working.

I appreciate all the diagrams- they are exactly what I was looking for.

 

[RADustin]

got it to work! your diagrams are great and were a big help.

the 30amp glass fuse in the barrel fuse holder took a dump. the holder was bad, not the fuse. so I shook and tapped it and it all started working. didn't even need freon, pressures looked great. Tomorrow I'll get a new fuse/fuse holder and swap it all out.

thanks!

 

[75gmck25]

I have the same setup on my '75 GMC and the 30 amp fuse got so hot that it melted the holder. I replaced the entire unit with a new blade fuse holder and 30 amp fuse and its never failed again. It must have been high resistance in the fuse holder that was generating the heat when the blower was on high.

Bruce

 

[RADustin]

I have the same setup on my '75 GMC and the 30 amp fuse got so hot that it melted the holder. I replaced the entire unit with a new blade fuse holder and 30 amp fuse and its never failed again. It must have been high resistance in the fuse holder that was generating the heat when the blower was on high.

Bruce

I could certainly see that happening. Especially my a/c doesn't blow the hardest, so I have been running it on high most of the time.

I checked the blower fan and such, everything seems good. Maybe leaky ducting??

 

[75gmck25]

Another good maintenance item is to clean out the plenum under the dash, since a lot of junk can fall off the dash and into the ducts.

I'm writing this from memory, but I think I have all the steps.
- First remove the glove box door, which is held on by 5 or 6 screws that have a 1/4" head on them.
- Behind the door area is the plenum, and in the middle there is a vertical duct piece. It is held on by two screws in the bottom and one at the top. Remove this piece.
- Once you remove the vertical piece, you should be able to see into the plenum and look at the blend door. Clean out any junk you find in the plenum (pencils, match books, etc.) and make sure the blend door goes full travel in both directions when you slide the dash lever from hot to cold. Also look at where the blend cable connects to the top of the door and lube that area to make sure it moves freely. Also take a look inside the bottom of the plenum and check for evidence of any heater core leaks.
- Look up high behind the glove box and you will see a black wire-reinforced tube running across to connect the plenum to the right dash vent. Make sure the hose is securely on both ends and does not have any holes.

The lower speeds on the blower get power through the blower resistor pack mounted in the plenum under the hood. If any of them don't work, at least one resistor has failed.

There are several models of fans and squirrel cages for these trucks, and someone may have installed the wrong one. When I replaced mine it took two trips to the parts store before I got one that actually matched the original.

Bruce