Hey everyone.

I've spent the last few days researching on this forum trying to figure out what in the world is wrong with the AC on my 2002 Ranger 3.0 liter 2WD. The main culprit is the AC compressor is not receiving power from the plug. Engine running, plug disconnected, I'm reading 0 volts.

I charged the system correctly (I believe) and while it was a little low, it wasn't exceptionally low. It's currently at about 40-45 psi on the low pressure side. I used an analog gauge connected to the hoses with the fitting. Autozone special with the blue cables but new.

• Fuse 25 checked out with a multimeter along with a handful of other PCM related ones (just in case).
• Relay / 47A and 56A I played musical relays with and swapped around the existing ones in the block with known good ones (wipers, headlights). No change. 56A there's only 3 total of that style.
• The high and low pressure switches I believe checked out. The low pressure switch is new. If I bypass the switches by jumping a wire between the two contacts in each plug (individually or both at the same time), no change.
• Compressor clutch spins freely.
• If I jump relay 47A, pins 3 and 5, the clutch activates.
• Memory is fuzzy on which terminals of the AC relay socket I checked, but two had 12V the rest nothing.
• Resistance across the two terminals of the compressor read about 3.5 ohms, which would be the resistance of the coil if I'm not mistaken.

It's not the TPS either. Reading tells me sometimes that can send a WOT signal to the computer which prevents the compressor from getting power, so that was the most recent thing I attempted to replace but no change. I feel like I'm running out of things to check. 2002 does not have a WOT relay to my knowledge so the TPS would be the only part in the system that could misinform the computer of this scenario. What am I missing? I'd like to think it's something simple and obvious but I'm not too sure at this point.

Thanks in advance.

 

Learn where the 12 volts begins. You need a schematic . Sometimes it’s easier to work backwards.
From the Ac clutch, whats next, backing up ?
Whats after that ?

12 volts leaving the AC dial ? If so, whats next in line ?

 

Welcome to the forum

In 2002 the PCM(engine computer) controls the AC, I will post the wiring but it looks way more complicated than it is

The dash AC mode switch sends a 12volt "signal" to the PCM on all settings except OFF and VENT, "AC Demand signal" violet wire, Fuse 10 in cab fuse box

The PCM has an internal relay that closes when it gets that "demand signal", which connects PCM pins 69 and 86 together

The pressure switches connect to the PCM on pin 86, they are the GROUND for the AC Clutch relay but thru the PCM, lower left in diagram, test switches for GROUND not 12v

The AC fan clutch relay's coil gets 12volt with key on, but has no ground so stays OPEN
PCM internal relay connects the pressure switch's ground to pin 69 which grounds the AC Clutch relay's coil and it closes which activates the Clutch to engage AC Compressor

 

Thanks for the lengthy reply.
The next step then would be to check power from the switch in the cab. I'll check for 12v at fuse 2 on AC and AC Max and report back.

Pressure switches make more sense when looking at the diagram. Jumping them should remove that variable.

The internal relay in the PCM is non serviceable, correct? But also the unlikely culprit?

 

Update

I'm seeing 14v at the power (right) side of fuse 2 with the AC switch on ALL settings including OFF and VENT. I'm looking at RonD's diagram and I'm not sure how that could be happening short of a broken wire being energized.

Removed the climate control switch and tested continuity on the pins for the plug.
Contacts 1 and 2 had continuity on all positions except off, air to feet, and air to face.
Contacts 3 and 4 had continuity on all positions except off.
No other contacts had continuity with each other.

The red plug behind the dash did not have any power on any of the 4 contacts when in any keyed position or running.

 

Fuse 2(should be fuse 10) has power with key on, period, only control is "key in Run", Same for Fuse 10, which is the one for AC Demand signal, I mis-spoke about fuse 2
That 12v/14v travels to the Mode selector and then OUT on the violet wire to the PCM pin 41 if Mode switch is in AC or any position besides OFF or VENT
You would test at Pin 41 on PCM

"Contacts 1 and 2 had continuity on all positions except off" this is correct for the AC demand signal, Fuse 10 12v in, then 12v OUT to PCM pin 41

Pressure switches need a Ground wire to work, test if they are grounded, ohm meter or test light on battery positive to pressure switch wires one should show a good ground

 

Pin 41 was getting power. Power going in, nothing out. Pretty much guarantees the PCM is bad I believe. I ordered a new one but I'm unfamiliar with the PATS E system this is likely to have. PCM should've been programmed to based off the VIN but it will not start with the original keys. I believe this is expected? Would I need 2 new transponder keys cut to the ignition to them sync to the new PCM for it to work? Or do I have to take it somewhere to have the the PATS "reset" to work at all?

 

A mobile locksmith can hook up a laptop to the vehicle and add your two current keys to the new PCM, need 2 keys as minimum, no Clones

Or you can get Forscan software for your laptop and a USB to OBD2 cable to do it yourself
Forscan is like a Deluxe OBD2 reader programmer

Forscan has a forum, with step by step instructions for Ford PATS


Too late now but I would have just added an external relay activated by pin 41's 12v, and it then connected the pressure switches ground, pin 86 to clutch relays ground, pin 69
Take the PCM out of the loop, all you lose is no WOT(wide open throttle) AC off control

 

I'll give that a look. Thanks!

One other concern I ave before calling someone out is the replacement computer has a different number than my original.

Original
2L5A-12A650-BD / BRN3
2 = 2002 year
L5 = Ford Ranger or Mazda B-series
A = electronic department

Replacement
XF3F-12A650-CD / YPT3

Internet tells me that's a Lincoln computer. Does that matter...? Just got off the phone with Flagship 1 (place I got it from) and it looks like they sent the wrong computer... MLC-431 vs MP2-111...

Looks like MLC-421 is...
2000 Ford/Lincoln Continental 4.6L. That's quite different than a 2002 Ranger 3.0L.

 

Yes, that IS the wrong computer

Go here: https://www.car-part.com/

2002
Ford Ranger
Computer box engine
USA
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Now you can see the computers part numbers used in 2002 Rangers
The 12A650 in the middle is not included since ALL Ford computers had that number, from 1985 thru 2023, lol
Yours is listed, 2L5A-BD
Electronic Control Module, (RH rear firewall), 6-183 (3.0L), w/o flex fuel vehicle; 4x2, ID 2L5A-BD

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Looks to be a fairly common model

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