Ac Help


Dec 15, 2001
1986 Mustang 5.0 factory air I owned for 27 years.

My ac has been acting up it works on cold startup freon reads good, then when it cycles or the switch is turn off and back on the clutch doesn’t kick back in and has no volts at clutch. When working the cycle switch has 12.2v on both sides, the wire at the wot relay from cycle switch has 12.2v, the red computer power wire has 14.1v, the computer to wot trigger is what I not sure about it reads 9.6v, then clutch wire reads 12.2v. Then when not working only thing different it no volts to clutch black/yellow wire. I tried two new relays still samething. (All volts reading are from a power probe III). If I use the power probe to add 12v to clutch wire it kicks in when not working.

Anyone got any ideas?
  • Sponsors (?)

When it stops, will the clutch kick back on if you bypass the cycle switch?
If so, you may have a failing switch or something not right with the charge/internal pressures
The cycle switch has power on both side when working and not working. So the cycle switch wouldnt be the problem, but was replaced because that was my first thought.
My saleen was doing a similar thing a couple months ago. Eventually it quit working because the compressor wouldn’t kick on. The engine would surge at idle when it wouldn’t kick on. The sn95’s do that when the low pressure switch goes bad. I bought a new switch from Napa and it fixed my problem. Ac works better than ever now. The low pressure switch is one thing I did not replace when I got the AC working again.

Does your engine surge at idle when you have the AC on and the compressor does not kick on?
The dash switch would be good it feed the power to the first side lower pressure cycle switch. I have power across the pressure switch when working and not working.
Here is a little drawing, computer wire 54 wot trigger wire is my biggest question but the three wot relay control wire read the same work or not working. Only the black/yellow wire changes kick in clutch and if I add 12v from power probe it kicks in. Also the wiring in relay plug looks all good no sign of melting or hot wires.

Thanks Jerry


  • 3C8FDEB8-D32E-44B2-B19D-D6A4A46F87C8.png
    211.7 KB · Views: 53
1986 wiring diagram

The 2nd pic shows the wiring of the WOT relay. Wire 361 should have battery voltage and wire 73 (to pin 54) should carry the same voltage when the ground on the ecu is closed (Pin 54 is a trigged ground).

Last edited:
  • Like
Reactions: 1 user

I think I traced it down to a bad ecm, the pin wire 54 had 9.6-10.2v. The clutch should be hot on the relay unless the ecm gounds it out to kill power. Just swapped in the original computer that was taken out for bad capacitors, that has been fixed and pin 54 now gets 14.1v just like pin 57 and all seem good. Emc that was in the car is also showing signs that the three capacitors are leaking to And need to be replaced.

pic on