Need 30A aux in Engine compartment


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Jul 26, 2010
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Morris County, NJ
My Car
1971 Conv; 1 of 17; 351C, 2V, Auto
My AC compressor doesn't seem to be getting enough current to trip the clutch.

I want to run a relay from the compressor wire and then run 12v from the relay back to the compressor.

Where should I get the 12v from? Directly from the battery or is there a better source? I am also putting in an inline 30A fuse.




"My World is Fire and Blood"
Jul 4, 2010
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New York
My Car
72 Mach 1.
if you do this you will lose the freeze sensor for the a/c system the monitors the evaporator core when it freezes up.

if the core freezes and the compressor continues to run it will lock up the compressor damaging it or causing more damage from high then normal a/c line pressure.

more then that you can actually snap a Crankshaft if the compressor locks up with the engine coming off idle. i have stalled motors at 30mph because a a/c compressor locked up and actually thought i destroyed the block from it.

that said you would tap off the battery and have a extra switch inside the car to manually turn the compressor on/off. 30 amps would make me nervous i would start maybe at 15 amps.

based on the other post i would still suspect an issue with the magnetic clutch itself, you might want to also have a spare clutch to try out or a replacement compressor.



Long time vintage mustang hobbyist
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Apr 19, 2011
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Buffalo, NY
My Car
1969 Convertible Restomod enhanced.
former owner (25 yrs) of 1973 Convertible, 351c, A/C, RA
I had a buddy who was driving his 73 stang on the thruway when the compressor locked up. Blew the clutch off the unit and through the radiator. Very messy and pretty expensive to repair. You would think the belt would snap before anything else, but apparently not in his case.

It doesn't give you a solution, but the topic reminded me of the incident...



Well-known member
Jul 6, 2014
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My Car
1973 Q-Code Mach 1. Full restoration in progress.
You should trace the clutch wire back to its' source of voltage before bypassing it.. The compressor should not be running all the time and if you install a manual switch in the circuit, you may forget to turn it off some day and cause damage. The system is designed to cycle on and off as pressures change.. Search for the wiring diagram on this site and take your time with the job.


Pastel Blue

When I got my current 419cj vert restoration project last year, I found that a PO had ran a separate power line c/w inline fuse to the compressor. It was disconnected when I got the car and when I plugged the line in, the compressor engaged and there was some freon flowing as witnessed through the sight glass. I shut it down at that point in time as I knew this separate line meant there was other concerns with the A/C system. I can't recall at the moment where he had the line connected to for power, needless to say it won't be going back in. If you need more current to engage the compressor clutch, you have other issues that need to be addressed. I would not take the chance of running a separate power line in this situation, as others have noted, bigger problems could/will arise.