1971 Airconditioning Idle Speed Issue.

[Rick Church]

Gents, I tried searching the archives but nothing happens when I hit the "search" button. I am straight (I think) on the 1971 CEC system but I have another question. I have a 71 small block (base engine), four speed, 270 Comp Cam, with factory air. I have the idle set at about 750 RPM. When I turn on the air conditioner while at an idle, the engine does not speed up and stalls (quits). I believe there has to be a system on the car that increases the idle speed when the A/C is turned on. Is it an additional function of the CEC (TCS) solenoid or am I missing something else somewhere? I am a little mystified on this which is probably some obvious thing I am missing. Thanks for the help!

Rick in Tennessee

[john38706]

Re: 1971 Airconditioning Idle Speed Issue.

rick, there should be a solenoid in front of the carb linkage with a green wire going to it. when you turn the ac on this sends 12v to the solenoid. do you have this part on your car?

[Dave S.]

Re: 1971 Airconditioning Idle Speed Issue.

Rick,

I don't see anything in the 68-72 operations manual about speed up upon A/C compressor engagement. It does mention A/C clutch engagement on engine shut down as a thing unique to 71 corvettes. Another 71 complication.!!!!

[Jack H.]

NO!

The '71 CEC system was apparently rushed to release to meet model-year change over and that's why (my best guess/opinion) there are lots of errors in the supporting factory documentation (AIM, various stickers and charts). ALL of the information that's published (including NCRS Performance Verification) indicates curb idle is to be set at the 750 RPM level you're trying to achieve with the A/C OFF. I can almost guarantee you the car will stall if you set it that way....

The CEC does NOTHING to boost curb idle with the A/C engaged. And, the load of an A6 compressor on the engine will vary based on system configuration (75-200 RPM drag). The original Corvette A6 compressor is DIFFERENT from a standard Chevy passenger car's A6. It was based on the 'Chevrolet AC' system vs. the Four Seasons AC system. The 'Chevrolet' system was used in some trucks (small cabin to cool like Corvette) as well as what dealers stocked for add-on A/C (remember the underdash systems?).

Instead of having a 12.6 cu-in displacement compressor (like passenger car), the Corvette A6 had a 10.8 cu-in displacement. Also, passenger car systems used a 3 lb-12 oz refigerant fill while Corvette was less--3 lb-4 oz. NONE of the mass AC compressor rebuilders make a 'Corvette' A6 compressor (same PN's in their catalogs for passenger car vs. Corvette). Hence, if you don't have the factory original A6 on your car OR you sourced your rebuilt compressor directly from AC Delco, expect it to be a larger capacity passenger car compressor with an appropriately HIGHER engine drag silhouette....

The Chasis Service Manual's theory of operation text regarding the CEC system 'hints' at a different curb idle setting for A/C equipped cars, but the tune-up sticker on the Corvette and service text does NOT document one. The CSM says:

"To prevent dieseling on air conditioned vehicles with automatic transmission, which is caused by the LARGER throttle angle required when setting idle with the air conditioning on...."

Then, they go on to describe the special relay set that engages the A6 compressor, regardless of cockpit control setting on engine shut down. This is reinforced in the 1971 CEC Dealer Training Program Booklet. There, they single out Chevelle, Nova and Corvette vehicles equipped with AC (regardless of manual vs. automatic transmission) to describe the companion smart relay set.

I 'think' you have to bump curb idle above factory indicated specs on a 1971 AC equipped Corvette to prevent stalling during idle. And, the degree you have to 'boost' curb idle is tricky. It depends on the drag coefficient of your compressor (do you have the real McCoy small displacement A6 or do you have a generic passenger car compressor installed?). Also, if you bump curb idle TOO much, you'll 'tickle' the low end of the factory stock distributor's vac advance engagement.... This can be a devil to diagnose!

When the CEC system cuts out, it vents distributor vac through the CEC solenoid. IF your engine RPM doesn't fall below the vac advance can's cut-in theshold, the car will NOT idle down! You'll observe a Jeckyl and Hyde situation where you tune and get 'er right in the garage, take 'er for a ride and she won't return to the correct/low curb idle once vac advance has been engaged. You can tell if you've got this problem pretty easily though.

When the engine's in it's 'high idle mode', put 'er in gear, slowly let out the clutch slightly to FORCE drag on the engine, then throw 'er back into neutral. If the engine now assumes the original low idle you set during tune-up, you're a bit too high and not falling enough to get below the distributor's vac advance kick-in threshold.

[Dennis C.]

Re: NO!

Jack - You have just described in 7000 words why no REAL Corvette should have AC. Thanks, Dennis

[Jack H.]

I was glad to be terse....

but, when you have the correct factory pieces put together the way designers intended it DOES WORK, Dennis! Both Chuck Berge and Terry McManmon can testify to cooling their brows in my cockpit during Founder's Ops checks. And, if memory serves, I seem to recollect passing a certain Big Brake SA car on National Road tour(s) with my top up and A/C observing certain occupants of the referenced SA car sweating in the summer sun....

[mikemccagh]

Re: I was glad to be terse....

Jack: does the abreviation SA (in your response to the dipstick) stand for smart ass? the dipstick has been so accused in the past. mike

[Dennis C.]

Sweating In The Summer Sun...

can rid your body of the dreaded A/C desease: Always Complicated. Anything in the GM Service Manual that takes up more than 500 pages IS going to cause you problems. If you want comfort, buy a used limo... or stay at home... Dennis

[Jack H.]

Think....

your 'old friend' Howard drives his A/C equipped '82 Mark of Excellence/Bowtie 'limousene' and leaves his Mark of Excellence '58 at home ruling its trailer on similar treks in the summer sun. And, Mike; SA = Straight Axle....