Air con problem with A/C wide open throttle relay input

[SimonhSmith]

Hi all,

Couple of tasks this afternoon were to replace HCV (done) and investigate why A/C clutch wasn't coming on. I won't bore you with the diagnostics but will come straight to the issue.
I assume that the purpose of the A/C wide open throttle relay is to drop out the A/C (if on) when you hit WOT so you do not have the A/C sapping power. Looking at the wiring diagram (see pic) this relay is the last thing in the chain before the A/C clutch



I removed the relay and checked out the pin voltages with A/C on and blower on and this is what I found:

Pin 1 [Coil live] 14v (as expected)
Pin 2 [Coil EMCU] 8.56V
Pin 3 [live in] (other A/C parameters OK) 14V OK
Pin 5 [output to A/C clutch] 0V (obviously as relay removed)

The relay itself works fine when tested.
If I link pins 3 and 5 or manually operate the relay the clutch comes in and A/C works great.
Clearly the issue is pin 2 which gets its signal from the Engine Management Control Unit. I assume that this output should normally be grounded until you hit WOT. If the EMCU wiring is the same as for a 1.6 Zetec SE Fiesta it is pin 54.

To get the A/C working I can simply leave the relay out and link out pins 3 and 5 but I'd like to find the root cause of the issue. Presumably the ECMU gets an input to tell it you are at WOT and I guess this is where I should start.

Has anyone else had this issue and / or can give me some pointers?

Thanks

 

[Wild E. Coyote]

Have you checked the high speed fan cooling relay (it should be pin 17 on PCM)? I think the A/C won't work if the cooling fan can't reach high speed..?

And, BTW, A/C compressor cycling switch is on pin 41 in 1.7 PCM, and A/C WOT relay is on pin 69 of PCM.

Hope that helps

 

[SimonhSmith]

Thanks for the help.
The relay itself works fine (swapped with hi-beam relay to check) but the fan speed does not increase when a/c turned on.
Only had a quick look today as the HCV was leaking from both front pipes when fully up to temp - combination of excessive mould marks and indented pipes, fixed with wet-and-dry and some plumbers tape.