EFI 225 - "ac pressure sensor signal" - DTC Topic is solved

Post by **Chuck W** » 20 Jul 2019, 22:37

Doing a scan with the 850 OBDII app, this code comes up. (Haven't seen it put up a CEL, however).

Would this be the high pressure one behind the PS headlight?

Dealing with an issue that the cooling fan isn't coming on when the AC compressor is commanded, but unless the engine is at operating temp and the fan has kicked on, or the car is moving, it's causing the pressure to vent from the AC system relief.

This is on the wife's '97 GLT with a "flashed" '98 M4.4 ECU. On the stock ECU, the cooling fan always kicked on when the AC compressor was engaged, but I was told that on the '98 M4.4 stuff, the cooling fan won't kick on unless the pressure in the AC system demands it, even if the compressor is running.

Post by **Chuck W** » 21 Jul 2019, 13:06

OK, well I went back to this today and tested the operation of both the HI and LO speed fans. First I tested them by directly grounding the relay and both worked. I then used the 850 OBDII app to test them, and the ECU engaged them properly, so the wiring is fine.

I have a new AC High Pressure (linear) sensor on order. Perhaps the current is non-functional, seeing as that is what signals the ECU to turn on the LO speed fan when the AC compressor is running.

Post by **Chuck W** » 25 Jul 2019, 16:07

Following up.

Since the new Hi-pressure switch is taking the long way here, I swapped out the one from my '97 T5, which I know works. (The car is sitting for a few days while I do some exhaust work anyway).

The fan comes on/off when it should (I think). I had the blower fan on about mid and what sounded like the low-speed cooling fan came on. When I cycled the blower fan to very low, the rad fan shut off, and when I cranked it up, the rad fan came back on. The compressor even began to cycle a couple times. Sounds about right from what I've read.

I still have NO idea why the fan came one EVERY time I turned the AC on with the original ECU, but as long as it works like it should now, that's fine with me.

I'll install the new switch when it shows up and put the original back on my car.