a/c relay

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Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=127397
Printed Date: May 04, 2025 at 2:03 PM

Topic: a/c relay

Posted By: agent13
Subject: a/c relay
Date Posted: May 19, 2011 at 5:01 PM

Ok guys this is a little difficult to explain...

I have a 1990 Corvette that originally had a 350 TPI engine in it. I swapped that engine out for a LS3 from a 2010 Corvette...

My only problem is that I can't get the A/C compressor clutch to switch on.

Originally the A/C clutch was activated by the original engine's ECM which has been removed from the car.

What I have is the "A/C Request" wire (Circled in dark red in the diagram) that supplies 4.950 volts and 0.001 amps and I was wondering if there was a relay out there that would be triggered by that? I need to use that relay to ground the wire (Circled in bright green) that triggers the A/C clutch relay when grounded. The wire circled in green is 12 volts and 0.250 amps.

I tried using a 5VDC relay but the A/C request wire won't trigger it.

Does anyone have any suggestions? Thank you!!!!

Replies:

Posted By: KPierson
Date Posted: May 19, 2011 at 10:04 PM

You most likely won't find a electromechanical or even reed relay that will trigger with only 1mA of current. Your best bet here is a transistor.

Get a general purpose NPN transistor (like a 2n3904). Wire the 5vdc (when on) output to the base of the transistor. Wire the emitter to ground. The collector is now an "open when off, ground when on" output. You can most likely tie the collector directly to the where you would connect the relays (-) output.

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Kevin Pierson

Posted By: howie ll
Date Posted: May 21, 2011 at 12:07 AM

Why do I think this is something to do with engine management and the AC is incorporated into that rather than good old old school off the heater blower and trinary switch?
Methinks lots of problems here.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Posted By: howie ll
Date Posted: May 21, 2011 at 12:15 AM

I've just re-read the post and I think you are going to need an new EM ECU
Have you changed the management ECU?
Does the new engine start and run?
Ref my previous post, you might have to go back to old school AC.
Someone tried to involve me recently on a 2002 SLR Benz with a new 2011 front end*. I ran away but I've been called back about 5 times to sort out the installer's work.
* Two front end looms, 2 x new HID headlights, 4 x new h/light levelling motors, 2 x new SAM ECU's would have been cheaper to swap in the old car for new.
Plus trying to make sense of 40 pages of factory wiring diagrams

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.