Electrical Ac Issue Help Needed

ggradtech · Jun 17, 2016

My 92 Mustang has recently been converted to R134. When that was done, car was using a Megasquirt standalone ECM. I decided to go back to a stock ECM....long story. When the car uses the "stock" ECM, my compressor will kick on only for a second or 2. It will run normally with the Megasquirt computer and not turn off though. Could it be that the Megasquirt somehow bypasses a relay like the WOT clutch one that the stock ECM needs? My low pressure relay was changed when they did the R134 conversion and it's not low on refrigerant. Any ideas? Thanks!!

Davedacarpainter · Jun 17, 2016

ggradtech said:
My 92 Mustang has recently been converted to R134. When that was done, car was using a Megasquirt standalone ECM. I decided to go back to a stock ECM....long story. When the car uses the "stock" ECM, my compressor will kick on only for a second or 2. It will run normally with the Megasquirt computer and not turn off though. Could it be that the Megasquirt somehow bypasses a relay like the WOT clutch one that the stock ECM needs? My low pressure relay was changed when they did the R134 conversion and it's not low on refrigerant. Any ideas? Thanks!!

I know nothing about the MS system, but @a91what will know, he's our guru on the MS stuff.

By the way @a91what , @madmike1157 will be wanting to talk to you.

a91what · Jun 17, 2016

Davedacarpainter said:
I know nothing about the MS system, but @a91what will know, he's our guru on the MS stuff.

By the way @a91what , @madmike1157 will be wanting to talk to you.

I'll get with mike.

The MS is a fantastic system if you know what your doing. Check the high pressure switch could be kicking the system off. Or you could try the system a few times and then pull the codes off the stock ecm. This will help @jrichker figure out what you may need to do.

As far as the MS and AC is concerned it's up to the installer to figure out what the system needs when wiring the unit. Check out some wiring diagrams and see what switches go back to the ecm. Jrichker can help with that as well.

a91what · Jun 17, 2016

Oh and if your having issues with the MS I can help, we have a MS help thread here.

jrichker · Jun 17, 2016

86-93 Mustangs only have a WOT relay and a low pressure cutoff switch in addition to the A/C control panel on the console above the ratio.

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.
This is for test purposes only!!! Do not go to WOT with the pink/yellow wire jumpered to ground, it may damage the A/C compressor.

Diagrams courtesy of Tmoss & Stang&2birds

Fuel, alternator, A/C and ignition wiring

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs

Electrical Ac Issue Help Needed

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Electrical Ac Issue Help Needed

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