Im trying to figure out what the problem and fix is. On my 2000 4.0, when i turn on the air, either heat or ac, it does it on every setting. The ac compressor clicks on and the clutch turns. Then a second later it clicks off and repeats that about every ten seconds. No cold air comes out. Could it be just the compressor is shot or some wiring problems? Thanks.

 

You may want to start here:
https://www.ranger-forums.com/genera...-switch-15872/

 

AC is used to dry the air in the cab, so it should be on when Defrost is selected.

AC compressor shouldn't be on when selector is OFF, PANEL, FLOOR, or PANEL/FLOOR

It should be on when MAX A/C, A/C, FLOOR/DEFROST or DEFROST, is selected

It won't blow cold air if FLOOR/DEFROST or DEFROST is selected


The PCM(computer) is what turns on and off the compressor, or more accurately, PCM turns on or off the A/C relay in the engine fuse box, it is also called WOT relay, wide open throttle relay, because it was used in the past to cut power to compressor clutch when "throttle was wide open", and more power was need to the wheels.

When you select one of the A/C on options above the PCM gets 12volts from switch on Pin 41, it then grounds A/C relay.............if it can, there are two pressure switches that are the ground for Pin 41, if one of the switches is "open" then there is no ground so A/C relay can't close and compressor stays off.
These switches monitor pressure in the system.

Cycling on and off is normal if the air is just being dried.
If it is doing it when you want cold air then it could be a few things.

If it only does it when stopped then it could be the condenser(in front of radiator) is clogged up with debris, so it can't cool the refrigerant and pressure goes up opening a pressure switch and turning off compressor.
Condensers often have an electric fan for cooling, it could be bad so when stopped there is no air circulation and pressure goes up.

Could be low on refrigerant, pressure switch opening because pressure drops too much when compressor starts pulling from the low side.


If compressor is cycling when FLOOR is selected then it could be a switch problem.
If you have a 12v test light pull off compressor wire, the Grey/white stripe wire is the 12volt contact, hook up test light to that and to a near by Ground.
Start engine.
turn A/C on and test light should come on, switch to FLOOR and jiggle switch a bit and see if light flickers, is so you have a short inside the switch.

 

RonD, you go above and beyond with every explanation. I wonder how the love gets returned, or if that just doesn't matter.
Either way, Sir, you are awesome.

 

Thanks, got the love when I was learning, just paying it back, like paying off a STUDENT LOAN, lol.

 

Ron D. My ranger compressor does not come on when switched to ac or max ac but runs in every other switch position including the OFF position. The truck has a remanufactured PCM in it and also having the speed sensor code with flashing o/d light but all checks good. Could the PCM need flash to this vehicle to solve both problems. Thanks in advance ...

 

Speed sensor on '98 would be on the rear axle, signal from it goes to ABS(anti-lock brake) module where it is corrected for ratio and tire size and then sent to speedometer and PCM, so PCM is the end user of the signal.
Is the ABS light on when driving?
If not then speed sensor is probably OK

You can test the rear axle sensor by removing it, 1 bolt, make sure o-ring seal is still intact, look at the sensors tip for signs of rubbing, shouldn't be any rubbing.
Set OHM meter to 1,000, measure sensor's ohms, should be .8-1.4k ohms

If speedometer seems to work, the ABS module is sending out corrected signal, and assuming wire to PCM is OK then yes you could have a PCM issue.

A/C issue reads like the switch in the cab has a problem.
The switch sends the PCM 12volts when A/C is selected, so it should only get 12v when switch is at AC, AC max and Defrost, those are the only 3 positions that should send the PCM pin 41, 12volts.

But this could also be a PCM issue as you surmise, since you have 2 separate issues involving possible PCM issues, OD light flashing is directly related to speed sensor signal at PCM and AC control.
But PCM was already replaced.....................why?
Could there be an underlying problem causing PCM to fail?
Out of the hundreds of thousands of Ford PCMs sold every year, one per vehicle, very very few fail, to get 2 failures would be very very long odds indeed.

One thing to check if possible is the Grounding of the PCM wires, PCMs have several Ground wires, these are NOT shared, so one unhooked Ground wire could effect 1 or 2 PCM controlled systems but not others, there is no common ground in PCMs.
Since PCM was replaced maybe someone missed a ground connection.


As far as I know PCMs can not be tested, they can be assumed "bad" if they will not responded to OBD II readers, so you might try going to an auto parts store and have them read the codes, they do this for free, make sure to have pencil and paper to write down any codes, and don't buy any thing to fix codes if it has any, post them here first.

 

I have replace the speed sensor and the abs and the speedometer works just the p0500 code for speed sensor and intermittent shifting issues... As far as the ac switch I also swapped it with one that works and same results. Compressor run in off position and does not in ac position.... Was told that a wire for the fuel pump shorted out to the frame and fried the PCM so the previous owner put in a remanufactured PCM and the truck ran again. When you check for PCM info with the scanner the PCM info nor the Vin shows up so was thinking let ford reflash it in hopes of solving both problems. Everything else seems to work find no other dash lights or codes... Also has the five dings five times when first started but no lights and the abs andnchexk engine light bulbs do work because they illuminate at start up.....

 

I would check what a local dealer will charge for the reprogramming.
If it will be $100-$150 it might be worth a try.

In the passenger door jab or in the engine bay there may be a PCM software version sticker, it will have the original version # for the factory PCM