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Discussion Starter #1
Time to sell my 86 Mustang which I have owned for over 30 years. Just refreshed the engine with new rings, bearings , valves, etc. Equipped with Kenne Bell blower , and Megasqirt ecu. Runs fine. Have put about 500 miles on it and in getting it ready to sell I just re-charged the a/c. Here's the mystery - after changing the orifice tube, filter drier, flush, etc. added 3 cans r-12 and it is blowing nice cold air. As soon as I start driving though the air starts getting hot...WTF? Pull back in the shop, open hood , start car, turn on a/c, ice cold air..WTF? Hook up gauges again, run hoses thru the cowl hood, and tape to the windshield . Don't even make it out of the shop when I discover what is happening - as soon as I put the trans (aod) in reverse, the a/c kicks off. put it in neutral, back on, put it in drive - back off.
Now I am totally baffled. I have no idea how there can be any connection between the trans /shifter position and the a/c harness. Nothing has been changed to the wiring, and it worked fine just like it is in the past. I don't even know where to start although I suppose a wire diagram would be a good place.
any body got any ideas?
 

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Take a jumper cable and hook to the neg post on the battery and the other end to the engine.
Set the parking brake and do not use the brake pedal, while shifting gears.
Just a couple of things to try.
 

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Discussion Starter #3
Take a jumper cable and hook to the neg post on the battery and the other end to the engine.
Set the parking brake and do not use the brake pedal, while shifting gears.
Just a couple of things to try.
????? you lost me on that one man ......... to summarize, the problem is that the a/c clutch is kicking off anytime the auto trans is placed in any position other than park or neutral. Obviously somehow related to the neutral safety switch. I have looked at a wiring diagram and to my surprise the neutral safety switch does go thru the ecu but I am having no luck at trying to figure out what to start testing first.
 

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There’s a wide open throttle A/C cut out switch, maybe something is making the computer think you’re at WOT when you shift in to gear?


Sent from my iPhone using Tapatalk
 

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Discussion Starter #5
The WOT cutout is controlled by the ecu - at least it is on the stock ecu, but now I'm running the Megasquirt. It has never been an issue before, but to be honest I have never owned a computer, which includes several desk tops all the way to my present smart phone, which at some point worked fine and then out of the blue just starts behaving differently for no apparent reason. I have always suspected they put voodoo in those pieces of shite. What a pain in the ass.
 

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I am pretty sure your a/c clutch is energized by one hot wire on a relay. The engine, compressor and mounts provide the neg for the clutch to work. Maybe you can see where I was going. Sometimes, with a ground loss, other circuits can provide a back feed, which would let the clutch energize, until the back fed circuit was utilized for its purpose.

Clear as mud, huh.
What type of TPS are you using? Does your throttle have a mechanical solenoid to adj for A/C rpm correction?

Just grabbing at stuff that might be involved.
 

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Discussion Starter #7
cut and pasted from another site : The WOT relay is controlled by the stock engine control computer. It is pin 54, pink/yellow wire.
The WOT relay shuts off the A/C when the engine goes to WOT. The computer provides a ground to make the relay provide power to the A/C clutch. You can ground the Pink/Yellow wire to force the relay to close the contacts and provide power to the A/C clutch.

The thing is , I am losing the 12v at the compressor when the neutral safety sw is activated. I am about 98 % sure now that the problem is in that damn Megasquirt. There was some kind of issue a few years back (don't remember what now) where I had to update the firmware and change a couple of small jumpers inside the unit. That has to be what is causing the issue now.
 

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The wiring diagram will be needed if it doesn't turn out to be simple. If it were me I'd start at the compressor clutch. With a meter to check the voltage to the clutch. You have to find out whether you're losing the 12v pos or losing the ground. If you can recreate the fault, 1st confirm the loss of voltage at the clutch, then test between clutch positive to neg battery post. Then clutch neg to pos battery post. Find out which one is missing. I'll bet it's a lost ground to the clutch. Then the fun starts. Isolating where the wire fault is.

Good luck
Gerry
 

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Discussion Starter #9
very easy to recreate the fault - key on, a/c switched on, move shift selector out of park. definitely losing the 12v pos at that point, already checked - haven't checked to see what is happening with the ground yet. will do that next
 

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FIXED! Matt Cramer, the head dude at Megasquirt support bailed me out. Told me to clip the wire from the ecu to the WOT cutout relay and that did the trick. It is still a mystery as to why it worked previously but I can live with it....:grin2:
 

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FIXED! Matt Cramer, the head dude at Megasquirt support bailed me out. Told me to clip the wire from the ecu to the WOT cutout relay and that did the trick. It is still a mystery as to why it worked previously but I can live with it....:grin2:
Well isn't that one for the books. A cut out of the compressor during WOT for a little more power? hmmm.
 

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Discussion Starter #12
actually, it just to save the compressor from over-speeding - I think it is pretty standard on all modern cars - but a little extra power never hurts...>:)
 
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