Team -

Need help understanding function of circuit 198 on PCM for the A/C Cycle Pressure Switch. I believe the PCM functionally grounds this signal for compressor operation.

Can anyone validate?

 

Not until you validate what truck you're talking about. But if it's a truck covered by this forum ('87-96), then NO; the PCM has nothing to do with the A/C operating.

 

1. It's a 1995, so yes covered under the forum
2. dispute your assertion "the PCM has nothing to do with the A/C operating" considering the attached diagram reflecting
   1. Pin 10 - Purple - Circuit 348 - A/C Panel Input
   2. Pin 43 - Dark Green / Orange - Circuit 198 - A/C Cycle Pressure Switch Input
      

 

The PCM absolutely controls the circuit and has for quite a while
At WOT the processor cuts compressor operation
(1995 may not have a WOT relay, If / then the processor just uses the TP sensor signal voltage)
There are other parameters that must be met on the newer ones before the processor will allow the compressor clutch to operate

 

You're absolutely wrong about his truck, and you have been for quite a while. His truck hasn't changed, regardless of how newer ones are built. And monitoring a voltage has NO EFFECT on the A/C or the compressor clutch because the clutch circuit in his truck doesn't pass through the EEC, or through anything controlled by an EEC output. You don't seem to grasp "input", either.

 

Appreciate the reply. In looking at the pressure switch and compressor clutch circuit, I believe the PCM grounds pin 10 to enable the clutch based on voltage level of pin 43. That is what I am attempting to validate. This description is in line with your comments. I’ve searched for a PCM schematic and block diagram but to no avail.

 

I know there is not one on his 95
I said they have been controlling the circuit for a long time.
The WOT relay is just one way they did it
Means you are wrong in saying the processor never had anything to do with it
You are right about his truck

 

So, regarding my original question, where does this leave me?

 

Concerning the 198 circuit?
The processor just monitors the clutch
They test us and teach us using the latest Lincolns
Your problem is exactly what?
The Cycling pressure switch has no power unless the processor okays ity last I heard
There are several system types and Yes I have a HVAC master from Ford
That plus 50 cents will buy a cup of coffee.
Nobody knows it all including the engineers
I believe Steve 83 thinks the compressor will run in limp mode
Good luck
007

 

You need a PCED to be sure of the processor pin out diagram, I might have one for a 1995. I will post a picture of the diagram if I have one
The EVTM will just show the wires to the PCM and no further data
Also the PCED might have the data you are looking for
Not a lot in there about processor strategy, they just tell us in screwell
I will look
007