Good morning everyone!

I’ve been searching the last couple nights with no luck to figuring out the AC problem on my wife’s truck. Thank you in advance for helping me troubleshoot this to get it figured out and fixed.

I just recently got married and noticed my wife’s truck didn’t have a working AC system. So I decided to fix that (maybe that is where I first went wrong…haha).

I noticed when you started the truck with it on max ac the clutch on the compressor would activate for two seconds or so, and then deactivate and not engage unless the vehicle was shut out and restarted the same process would happen again. So I made my first mistake by assuming it was a bad compressor without troubleshooting. I noticed the pressure on the low side was high and went to get it evacuated. Changed out the ac compressor and accumulator. Put on a vacuum and got negative 25* for 30 mins and waited for 20 mins and the pressure showed no leaks.

I started charging the system (with the truck running and max ac on and the dial turned to cool.) the ac compressor never kicked on as I watch the dial slowly go past 30 psi, and then 50 psi even up to around 80 psi. I checked the relay for the compressor. When she turns the switch to max ac I can feel it click. I disconnected the electrical connector on the side of the ac compressor. Put the meter to voltage. The only time I get voltage to the compressor when right after she started the truck with max ac on it would briefly hit around 8.6 volts and drop back to nothing. Checked the low pressure with with ohms and it showed it to be closed. Same with the high pressure switch. Checked the other two fuses and they are good.

Im thinking about testing the switches inside the truck as I am still trying to figure this out even though I’m started to feel defeated. Thanks for your time and hopefully we can figure this out!

 

Welcome to the forum

First there is no difference electrically between AC and Max AC, just FYI, and that's universal not a Ford thing, AC is either ON or OFF it has no high or low
Max AC closes outside air vent and pulls air from inside the cabin to re-cool it, so its cooling already cooled air for "max" cooling, and that's the only difference
The outside air vent is controlled by a vacuum valve, as are all vents in the cab, i.e. defrost, panel, floor

The AC electrics is pretty straight forward but was made slightly more complicated in 1998 and up, as Ford add the PCM(engine computer) as the controller
The switch in the dash will send 12volts to the PCM with AC(either) is selected, this is called the AC demand signal, from FUSE 27 in CAB fuse box(check it)
PCM will then Ground the AC Clutch Relay, which will send 12volts to the AC Clutch and compressor should start

The "Ground" the PCM uses to activate the clutch relay is from the TWO Pressure switches
Ground to body--------------high pressure-----------------cycling pressure---------------------------PCM
So when AC is activated either pressure switch can deactivate compressor as pressure rises or falls, this is the cycling which is normal on any AC system

I would test Grounds first, make sure each pressure switch shows 0 ohms to ground(battery negative)
Makes sure one of the compressor's 2 wires is also a good ground

Fuse 25(10amp) in ENGINE fuse box powers the AC compressor relay, so compressor when its closed
With volt meter check that fuse 25 has 12v, its not a key on fuse, has 12v all the time, and test fuse with ohm meter to make sure is good, "looking at a fuse" doesn't mean its good

Pull out the AC Clutch relay, you can get 2004 owners manual here: https://www.ranger-forums.com/genera...1-models-3747/
It has the location of all the fuses and relays diagrams

With key off test each slot in relays base(plug in) for 12volts, only 1 should have 12v, from fuse 25
With key ON test again and there should now be another slot with 12v, from another relay
Remember those 2 slots
There should be 3 slots left

Now you are going to test if PCM is grounding this relay when AC is selected
Unplug the AC Compressors connector as that would read as a false ground
Start engine and select AC on dash switch
Test the 3 remaining slots with ohm meter, one should be 0 ohm to Ground(battery negative)
If so then turn dash selector switch to OFF, must be OFF
Retest same slot, should NOT be a ground now
If this all happens then PCM is working as far as AC is concerned

If no slots are testing as a ground then shut engine off and retest the pressure switch's ground, if they are OK then problem is most likely in the PCM


This is the wiring diagram for 2005 but same as 2004, but not 2003
2.3l engine has electric cooling fan but your 4.0l won't have that relay or wiring but the rest is the same

There isn't alot to it, it looks way more complicated than it is
When you select AC on the dash switch, a relay closes and AC Compressor clutch is activated by 12volts, and that's all there is to AC electrics on any vehicle
There is no high/low just on/off
AC compressor is ON in any setting on the dash switch EXCEPT Off and Vent, that's to dry the air in the cab
Temp control is done by an electric door, the Blend Door, in the cab, it sends air from the fan thru the heater core(HOT) or around the heater core(COLD), or anywhere in between

 

Ron,

I followed your steps. I made it to where I unplugged the power connector to the ac compressor. Checked and found power three ways (no key, key in ignition, and truck running with ac on.). I had a total of 5 ports for the relay (only four with active pins), and couldn’t find a ground within where the relay plugs in with ac no matter what. I checked the low and high pressure switch with ohms as well at got .2 so not the best ground but the circuit is closed.

I want to preface this with the truck only has 1 can of refrigerant since once I got the one 12oz can in and not compressor activating at all I stopped. If I am understanding you correctly there should be a problem in the PCM?

thanks for your time and help!

ryan

 

0.2 ohms to battery negative is a good ground, just depends on the scale you are using, 200 ohms, 2,000 ohms, 10,000ohms

So we will assume PCM has a good ground to use for the AC Clutch relay

Yes, only 4 slots of of the 5 would be used in this relay setup

Ford used two types of relays, micro and mini
Mini relay seen here: http://www.gtsparkplugs.com/images/m...iring-840x.jpg

Micro relay seen here: http://www.gtsparkplugs.com/images/m...lay-wiring.jpg

Electrically they are exactly the same, only difference is in the slot layouts

A relay has a coil of wire inside wrapped around a metal core, when the coil has 12v AND a ground(85 and 86) then the metal core becomes a magnet and pulls down the relay bar and connects 30 and 87
There is no polarity, as long as 85 or 86 has 12v and the other one is a ground relay will close
Most car makers use Ground control for relays, so the coil gets 12v all the time or with key on, then a switch or control device Grounds the coil to activate the relay, they do this to limit the amount of 12volt wires running around the vehicle

30 and 87 connection also doesn't matter in this setup, either can be the 12v(fuse 25) and the other connected to the AC Clutch on compressor
If relay is activated then the 12v passes thru relay to clutch

Ford also uses a 1 2 3 4 5 numbering
1 and 2 are 85 and 86
3 and 5 are 30 and 87
4 is 87A

87A is used to power two different things but only 1 at a time
Say Fog lights, you could have head light powered on thru 87A then when relay is activated head lights go off and fog lights power on thru 87
So they both can't be used at the same time
That's the 5th slot and not needed for a simple on/off setup

So you found the two 12volt slots?
So you now know which slot is 12v, 85 or 86

But no ground with AC on on 85 or 86, which ever is not the 12v

 

I disconnected the power going to the ac compressor and with the truck off, and key in the on position, and truck running with the ac there was no ground in any of the 1,2, 3, 4, or 5 spots where the relay goes.

Where do I go from here?

 

You will have to pull the PCM connector and test the AC wiring at that connector next
Have a good look inside the connector for corrosion

Unhook one battery cable before disconnecting PCM's 104 wire connector
Then hook it back up for testing

I assume fuse 27 in that cab fuse box is OK, that fuse provides the 12volts that the AC switch sends to the PCM, AC demand signal, so PCM ground the AC relay
But if your fan works then fuse 27 is OK

You will need to look at the diagram in post #2
Lower left has the PCM wiring you need to check
Pin 41, purple wire, should have 12volts with key on and AC on, that's the demand signal to PCM from dash AC switch
Pin 86, black/yellow wire, is the Ground from the two pressure switches, should be 0 ohms to battery negative, or read 12volts if you test pin 41 with pin 86
Pin 69, pink/yellow wire, should read 12volt with key on and AC relay plugged in, pass thru voltage, means AC relay coil is OK, with key on if you ground this wire the AC relay should "click" closed, just as a test of the relay

Those are the 3 wires that are used by PCM to activate AC compressor, PCM has an internal relay that connects pins 86 and 69 together when pin 41 has 12volts
You can add your own relay to by-pass the PCM's internal relay for AC control, and you use these 3 wires to do that

A member did that and it works fine, vs replacing PCM which can be a pain on 2001 and up Rangers

Pin numbering for PCM connector seen here: https://easyautodiagnostics.com/imag...99/image_1.jpg

 

41 and 69 have 12vdc

86 ohm tests at 24.9 at 200 ohms.

thanks for helping us. We just tested them all and this is what we got.

 

Ron,

We are still at it on the ac. I ended up putting jumpers in both the low and high pressure switch and tested the power going into the compressor…nothing. Then all of a sudden it went to 12vdc for a couple seconds back to 0 and then once back up to 5. No rhyme or reason as to why. I then pulled the relay. 12vdc is coming from battery and ignition, ground is good. I’m not getting any power now from the ac being turned on. I ohm tested the controls for the ac and they are good. Thoughts or ideas? Blower motor works well.

 

The AC Clutch relay should have 1 slot with 12volt key OFF, from fuse 25 in engine fuse box<<<this is the 12v that should go to AC Clutch
Then a 2nd slot with 12v with key on, from PCM relay, it powers most engine parts so its OK if engine runs

The PCM controls the AC clutch relay, by grounding it, so from your description PCM is not grounding that relay
Or the wire from that relay to the AC clutch is bad
You can put your finger on the relay and have someone turn AC on and off, you should feel it "clicking" on and off, if not then problem is on the activation side
If you feel it then problem is the wire to AC clutch

 

Ron,

Thank you again so much for everything! I keep researching and feel like I’m getting closer. I know I’m learning so much and I’m grateful for that.

I can get the ac working beautifully if I bypass the relay. Here’s where I’m at:

I get 12vdc from the battery into one leg of the relay.
12vdc with key on into the opposite side of the relay. Then I’m only getting .08 vdc when I turn on the ac. That is the line coming from the pcm number 69 which I ohm tested and it shows that it’s closed so that line is good. I checked the low pressure switch wiring (since both the Lo and High pressure switched ohm test closed), and I get 12vdc coming from one leg of the wiring. I tested the high pressure switch and I’m not getting 12vdc I only get the .04 vdc. I am not sure if This is normal or not. All advice would be great as I’m wondering if I should take it to the shop.

I pulled out the pcm and everything looks good to the eye as I don’t see anything inside the pcm on the circuit board that looks burnt. Once again we are so grateful for your time and help. Thanks for teaching us so much on how this system works. Please let me know if you have ideas/suggestions?

 

Should be NO voltage at either pressure switch

Ground---------------pressure switch-----------------------pressure switch-------------------------PCM pin 86

This is the ground the PCM uses for the AC relay, when AC switch on dash is activated PCM connects pins 86 and 69 together, ground the AC relay


PCM pin 69-----------------------------------AC relay coil-----------------------------------12v key on

AC relay gets 12v the whole time key is on, whether AC switch in cab is on or off
What turns on AC relay is when PCM grounds it using "pressure switch ground"

Ground---------pressure switch-----------pressure switch---------------PCM pin 86>>>>PCM pin 69-----------------------------AC relay coil------------------------------12v key on

Yes, one slot in AC relay base should have 12volt key off or on <<<powers AC Clutch
And one slot will have 12v only when key is on<<<powers AC relay coil
One slot will ONLY show as a Ground IF PCM has connected pins 86 and 69 together(and pressure switches are good)

Ford used two different types of relay
Micro seen here: http://www.gtsparkplugs.com/images/m...lay-wiring.jpg
And mini seen here: http://www.gtsparkplugs.com/images/m...iring-840x.jpg

Both do the same job just have different slot lay outs
85 and 86 are the coil, if one is positive(12v) and the other negative(ground) then relay closes and slots 30 and 87 are connected together
Most automakers use the ground to activate relays, because its cheaper and safer, the fewer 12volt wires you have running around the better
If a ground wire "shorts to ground" the worst that happens is a relay or device might come on
If a 12volt wire shorts, its a SMOKE SHOW worst case, best case a fuse blows

 

Okay, so us having 12vdc on the black and yellow wire leading to the low pressure switch when we turn the ignition on means I need to trace the wire back and figure out how it’s getting voltage and correct that? That might be why my relay isn’t crossing over?

Any idea where that wire runs to or schematics that would help me trace it. Maybe it goes to the PCM?

 

It appears the 12vdc is coming from pin 86 in the pcm. I pulled the negative terminal and ohm tested wire 86 going into the pcm and the black-yellow into the low pressure switch and it showed closed. Tested for 12vdc without the pcm plugged in and nothing. Plugged the pcm in and I show 12vdc on that black-yellow wire. Maybe it’s constant and the pcm grounds it out? Maybe there is something wrong with the pcm? Thoughts? Suggestions?

 

Yes, you would see 12v from the PCM via the internal relays coil(VREF), test pressure switches with key OFF