Air Conditioning PCM Control

[WhiteFalcon]

My ac stopped working and I traced it back to the pcm failing to issue a ground command to the clutch relay. Is this a common issue and do I need to replace the pcm? I don’t see any issues with the wiring from the relay back to the pcm.

 

[Hamfisted]

What are your freon charge pressures ?

 

[WhiteFalcon]

The pressures are good and I can get it to run by grounding the 30 prong on the ac clutch relay. So I am suspecting that the pcm is not commanding a ground for the relay as it should. Coincidentally (or not) I and now getting a p3051 code and it is running like crap. This in combination with the out of nowhere ac issue is starting to make me think PCM. I’ve never had one go out on any car. Do they generally fail all at once or do they loose functions gradually over time?

 

[stamp11127]

If you ground pin 30 you create a direct connection(short) to ground. What are the high and low pressures with the system running after 10-15 minutes once you jump the relay?

 

[WhiteFalcon]

The high and low side pressures are a little under 100 lbs static. I have not running pressures since the pcm won't let the compressor turn on.

Right after the ac quit, I starting getting p00351 code and misfires on cylinders one and two. Truck basically goes into a limp mode if you try to drive it. I have replaced all coil packs and plugs when I did the timing chain, tensioners and phasers back in April of this year. The truck ran great until just last Thursday when the AC failed it developed the misfires. This is leading me to believe that the pcm is starting to fail. It seems too coincidental that all these functions would fail at once unless there was a common source for the issues. Could be wiring too I guess.

 

[stamp11127]

What was the ambient temp when you read the static? Was the engine run and we are dealing with a hot ac system? Expy parked out in the hot sun or shade?
Earlier you stated that when you jumped the relay it worked, need to do the same in-order to get the operational pressures.

From repairpal.com:

P0351 Symptoms

• Check Engine Light will illuminate
  
• Engine idles rough
• Engine misfires on acceleration
• In rare cases, the engine may not exhibit noticeable symptoms

Common Problems That Trigger the P0351 Code

• Defective Ignition Coil(s)
• Defective Spark Plug(s)
• Intake Manifold Vacuum leaks
  
• Carbon buildup in the Throttle Body air passages
• Defective Idle Air Control Valve or Electronic Body

No pcm problems listed. Don't assume it is the pcm. One way to do a simple check of the pcm is to connect a scanner and command functions. PCM's that have issues may not connect to the scanner or fail to perform and tests.

 

[WhiteFalcon]

I found the information below and this is exactly what is happening except mine in the #1 and 2 cylinders misfiring. When I get home I’ll check the wiring. In the meantime, does this sound like a plausible explanation?
https://www.ford-trucks.com/forums/962359-ac-and-p0354-ignition-coil-d-primary-circuit-fault-2.html

 

[stamp11127]

Yes

 

[WhiteFalcon]

Ok. Finally solved it. After checking all the wiring, it turns out to be a faulty connector plug for cylinder one. Changed it with a new one ($29.00 for Motorcraft plug - Ouch) and the misfired is gone and the ac works. Thank you to everyone who helped out. I appreciate all your advice.