I swapped out the old AC system in my 92 4D Sport with the 2001 version. I replaced the heater core inside, which was a pretty straight forward mod, and everything under the hood. But now that the system is charged up, it cycles every 15 seconds.

When the AC is switched on it runs for 15 seconds then off for 15 seconds, very consistently.

My first stop on the trouble shooting train were the pressure switches ( I had to add the low side, 92 only had a high side) but they remain constant +12v at the ECM. Bypassing them does nothing.

Next stop was over over to the relay at the Fuse box. Sure enough it and the aux fan relay pop on and off at the same time in 15 second cycles. ( it is worth noting the aux fan and mech fan were deleted and three E-fans cool the huge new champion 4 core radiator)

When I bypass the relay with power from the pressure switches it stays on Longer than I wanted to spend watching and blows ice cold.

What could the problem be?
Bad ecm/pcm?

what else can I trouble shoot?

 

Do you have ac gauges hooked up when its doing that, what are the pressures doing?

 

The tech who filled it had gauges hooked up, I don’t have any. But his determination was that the pressure was as good as he could get it with it kicking in and off.

Bypassing the switches changed nothing. While monitoring the relay the pressure switches remain constant 12+. It’s the signal on the negative side of the relay coil coming from the ECM that is switching... not sure why it’s doing that though if the pressure is good.

 

Pick up a set of gauges from harbor freight, they're like 50 bucks. I know on my other pickup it would shortcycle like that when it was slightly low on freon. Usually the shop manual will have a chart that will show what the pressures on the low / high side should be when the system is running, based on outside ambient temperature. When I added just a little bit of freon, it cycled about every 30 seconds (if I remember correctly).

 

The pressure is not the problem, the switches are shorted and it cycles... pressure is not the issue.

What else could it be?