Fast & Ac

[ChrisCairns]

Perhaps someone can shed some light on this....

I "finally" got around to wanting to get my air running with the FAST system. So I dug around the wires in the connector until I found A2 and soldered on a wire I was going to use as a ground, since it was my understanding AC wouldn't work with the FAST unless you grounded A2.

Lo and behold, all it took to get the AC working was to turn on the dash switch....lol. I never did have to ground A2.

So it merrily works now just as with the stock ECM.

Can anyone explain this? Thanks.

 

[norbs]

Chris,
Maybe someone, made the change in the harness already. The stock ecm has an a/c input to bump the rpm and richen the mixture .4 secs before the compressor engages, also at wide open the compressor will disengage. With the fast system there is no A/C input to the ecm, and it will not cut off at wide open.

 

[ChrisCairns]

Originally posted by norbs
Chris,
Maybe someone, made the change in the harness already. The stock ecm has an a/c input to bump the rpm and richen the mixture .4 secs before the compressor engages, also at wide open the compressor will disengage. With the fast system there is no A/C input to the ecm, and it will not cut off at wide open.

Norbs....I appreciate the reply,

I'm the second owner of this T-Type. The first was an old one legged man. (True.) I don't think he made any changes to the wiring....lol.

You're totally correct when you say this FAST ECM won't increase rpm upon AC startup....or cut it off at WOT. Mine does that (or doesn't do that.)

The question, out of curiosity, I'd like to see answered is....

Where did the instruction that we need to ground A2 originate?

 

[TurboJim]

Chris:

POwer to the coil on the AC cutout relay comes from the brake switch. ECM pin A2 completes the ground path energizing the relay coil and closes the relay. Closing the relay, completes the circuit path from the dash switch, thru the low pressure switch, thru the AC cutout relay, to the compressor coil. Groound path is thru the high pressure switch in the back of the compressor to gnd.

I am gonna assume, either A2 is already grounded in the fast ECU, or the harness you're using may have that pin connected to ground? Maybe thats a feature built into say Harry's harnesses?

What you can do, if for some reason it stops working, jumper together the light blue and dark green wires at the cutout relay, which will take the relay out of the picture.

Funny, I fired up my car a last w/e when I threw up some flourescent lights in my garage, and to my surprise, the AC engaged in my car as well.

 

[ChrisCairns]

Thanks Jim.

Congrats on getting your car going....but don't worry, if you're at all like me, you'll be back to swearing at it in no time.

Very nice clear explanation of the A/C circuitry.

The harness that I got with it from Jack was simply the O2 sensor wires....6 if I remember correcty and a hot and a ground. Not too elaborate.

One thought I had was that perhaps the cutout relay is bad. Closed all the time. Using your explanation then that would simply keep the circuit hot but not completed until the dash switch was on.

 

[Craig Smith]

Pin A2 is an unused pin. Without looking at a schematic I can't say off the top of my head if it will provide a ground path or not, but it sounds like it is. Any ground would suffice in this instance.