Still working on the a/c on my 71. Its in the shop at the best auto electric place around and still they can't fix it. The guy tells me there is a blocking relay and an anti-diesel relay/switch that he can't find without taking a bunch of stuff apart and charging me labor...and he says one or both of these are bad and not sending juice to the compressor. Anybody have any ideas on this, where they are. I'm told most schematics and manuals are wrong. Wrong diagrams, wrong color-coded schematics. Trying to get the a/c to work is getting ridiculous, any help greatly appreciated.
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Re: C3 air conditioning
Yes, it's child's play to find them! Look straight up on the passenger side, below the lower dash pad adjacent to the interior light. Both the anti-diesel and blocking relays are there, screwed to the RH side of the interior Harrison air box plenum.
The anti-diesel relay triggers the AC compressor clutch to engage for a fixed time period on engine shut down. It's job is to add drag to the motor to thwart run-on. Even though the shop manual and emissions sticker on the car make no distinction between A/C and non-A/C equipped cars in terms of idle RPM, the 'feature' was added to allow A/C equipped cars to have a slightly higher idle RPM than non-A/C equipped cars (see theory of operation discussion in 1971 Chassis Service Manual).
When you switch off the engine, the anti-diesel relay over-rides the settings on the heater/AC control panel to FORCE the compressor to engage. Unfortunately, the side-effect of simply turning the A/C on would be the inadvertent start-up of the heater blower motor. Apparently, designers found this to be a surprising/annoying trait, so the blocking relay kicks in at the same time to kill the blower motor.
The electrical information that's published in the 1971 Chassis Service Manual is CORRECT for the system. The only thing in print that's INCORRECT is the engine wiring harness for ALL 1971 Chevrolet cars. It lacks the proper definition of the CEC (Combined Emission Control) system that apparently went to production AFTER the rest of the documentation was already released.
Plus, Catch-22, the wiring harness that runs to the two special A/C relays has a single, ring loop terminal, ground wire on it. To make the system work as it was designed to, that ground wire HAS to be connected to the mounting screw holding the anti-diesel relay (the relay's case is grounded and, in Corvette, the relay mounts to the fiberglass air plenum). This is clearly shown in the AIM.
To test his hypothesis of a faulty relay (anti-diesel or blocking), simply remove the RH lower dash pad and disconnect the wiring harness connectors at the two relays. The A/C system should run just fine as controlled by the center console switches and thumbwheels WITHOUT the added benefit of the compressor being triggered on engine shut-down.
Sounds to me like your guys knows a bit about what he's doing, but isn't intimately familiar with the system.... So, get the book (1971 Chevrolet Chassis Service Manual), read it, and both of you should be good to go. It's all documented! -
Re: C3 air conditioning
Thank you Jack!! Going to the book now!Comment
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