69 a/c diagnosis

[Edward Lauman]

Just purchased a '69 350 coupe and am in the process of replacing the underdash ducts and controls that the previous owner decided were not needed. Wondering if anyone knows of a good diagnosis book or information regarding the operation of all dampers, ie., what position they are supposed to be in for bi-level, heat, vent, etc. Also the underhood system seems complete but compressor does not activate.

Thanks in advance.

Ed

[Jack H.]

Re: 69 a/c diagnosis

Chevy Chassis Service Manual gives a nice tutorial on A/C system. You should find a theory of operation section that describes the general air flow based on full sized Chevy passenger car. This is very similar in that it's also a 're-heat' system (air flows first through AC cooling then passes in fractional flow through/around the heater core).

AC compressor should have separate fuse (in-line) on large diameter orange wire under hood attaching to the bus bar (always hot) side of your horn relay. Check to see this is intact first.

Then, if fuse is not blown open, use the wiring diagram in either your AIM or the CSM (specific to Corvette) to start tracking who's not doing what. Note, AC compressors were 'open loop' until '72. But this I mean there was no safety thermal overload protection. If system was low on refrigerant/oil, it'd literally self destruct (get darn near red hot)....

Could be compressor and/or clutch are 'toast'....

[Paul E. Young]

Re: 69 a/c diagnosis

Jack does the compressor pull the motor down noticably when turned on or is my compressor showing age and/or worn bearings?

[Jack H.]

Re: 69 a/c diagnosis

Degree of engine RPM drop will depend on several factors. What judges look for is published in the Operations/Performance Verification Manual (a great compliment to your Judging Guide). Here's the skinny.

Corvette used a smaller compressor than conventional Chevy Passenger cars. I believe the displacement capacity is 10.9 cu in vs. 12.6 cu in for the standard Passenger car A-6 compressor. You'll also see the R-12 capacity of the Corvette system is slightly less than what's rated for the balance of the Chevy car line.

Most people think the A-6 is a 'Corvette Only' part. Not true! Zora didn't toss tooling $ out the window when there was another way to skin the cat! You'll find the smaller Corvette A-6 compressor was the heart of the "Chevy A/C System" (different from Four Seasons).

What the heck is this you ask? Thought all Chevy A/C was Four Seasons.... Not true, another revelation of the Chevy Chassis Service Manual. 'Chevy A/C' was the system used in Chevy/GMC trucks as well as dealer installed A/C. That's why Corvette A-6 compressor has BOTH R-12 service line taps at the compressor head manifold (access points for expansion valve were buried inside the underdash 'box' on dealer add-on units).

Now, to the best of my knowledge, nobody other than AC-Delco understands the displacement capacity difference between Corvette and Pass car A-6 compressors and all get rebuilt to the larger/more common passenger car specs. Increasing compressor capacity will provide a modestly higher engine drag silhouette, but cause no system harm....

Next, setting curb idle RPM on your car varies with the phase of the moon. Some Shark cars have underhood stickers that disagree with CSM specs and methods. What's 'right' can be a judge's nightmare of who do you believe and what spec superceeded what....

With the wrong curb idle setting (can be compounded by additional compressor capacity and system fill), the transient of compressor turn on and/or engine cruise/idle change can cause engine stall. This is enhanced by the increasing complexity of Shark cars with their emissions systems (TCS/CEC), the vacuum related loads of aging headlight/wiper door parallel systems and aging distributor vacuum advance.

Whew! Typically, a normal Corvette A-6 compressor will introduce a 100-150 RPM engine drag (STEADY STATE). All bets are off on aged/worn engine/vacuum sub-systems, compressors re-built to passenger car specs, R-12/oil fill, and the transient specifics of system turn-on and/or cruise to curb idle. This last factor can make the compressor drag 'appear' to be in the 500-700 RPM range, but it's a set-up, maintenance short lived issue and, at times, GM addressed it with a separate curb idle solenoid that was controlled by AC active/inactive status.