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Since TC is asking about the hold in relay, I would like to ask this question: Why? Why is there a "hold in" relay that keeps the compressor engaged even if you turn off the A/C? That's how it works doesn't it? Does anyone know what the engineering reason was for operation like this? Why isn't it just a normal relay like one would think it would be? A relay activated by a switch in the control head that turns on the compressor when the control head is in the A/C positions and then turns off the compressor when it is not in the A/C position.
Sorry if this has been answered before, I couldn't find it.
Thanks Matt.
What solution have people come up with in the past? Wire in a series switch? Replace the hold in relay with a normal relay? Something else?
Take this opinion with a grain of salt because I am not a "sparky" (slang for electrical engineer) No idea why this design, but from what I've gathered this is the fix. The relay has 3 wires. The green one supplies power to the compressor and the other two receive power from the control switch on the dash. You can apparently swap these and it makes no difference to the relay function. If you place a toggle switch into one of these wires, it will open the power circuit to the relay and thereby kill the power to the compressor allowing it to disengage.
My car has this toggle switch added and it works well. I haven't looked close to see what the heck is really going on however. Just wait for someone who really understands the circuit to confirm or deny my theory before you hack into your wiring.
tc
Last edited by 4+4+2=10; July 13th, 2023 at 06:50 PM.
Basically, the way the hold in relay is wired, the relay contacts, once energized keep current flowing through the relay coil after the control head switch opens. It's because the relay contacts and the switch in the control head are in parallel. Once energized, the relay contacts continue to supply current to the relay coil. Only way to stop it is to turn the car off.
By the way, I love that old term "sparky". I just happen to be a EE (BSEE UCF '95) and I remember reading old electronics magazines growing up using that term.
Back in the 80s I installed a toggle switch in the ground in order to disconnect the compressor on my 70 Supreme. I read online somewhere since then the factory relay wiring could be modified to break the self energizing circuit, which would be a lot easier / simpler than what I did.
I love me some sparkies.. I worked closely with them in my 40+ years in aircraft. One of them in particular was quite animated and like to remind me that the airframe was just the bag to carry all the important stuff the sparkies designed.
Back in the 80s I installed a toggle switch in the ground in order to disconnect the compressor on my 70 Supreme. I read online somewhere since then the factory relay wiring could be modified to break the self energizing circuit, which would be a lot easier / simpler than what I did.
Below is the link to how to mod the relay. I have done it several times.
Ok, I just opened mine up and SURPRISE! Someone had already cut out the white wire. Now all I have to do is acquire a vacuum operated electrical switch and do the "Brian mod" for defog functionality. And it will look all stock!
Thanks all!
July 13th, 2023, 05:32 PM
