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Ask A. Penland Your Own Question
A. Penland
A. Penland, Ford Senior Master Technician, ASE
Category: Ford
Satisfied Customers: 6212
Experience:  ASE Master Certified, ASE Advanced Gasoline Engine Performance
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Trying to determine if my 99 Ranger 4.0 is having PCM issues

Customer Question

Trying to determine if my 99 Ranger 4.0 is having PCM issues related to the AC clutch.
Submitted: 1 year ago.
Category: Ford
Customer: replied 1 year ago.
Checked and metered all connections. Everything checks out. AC works until engine gets to operating temp, then PCM will shut it off.What other sensors does the PCM look to in order to determine if the AC should be engaged or not? (other than ac switch, cycle switch and pressure switch?) No CEL, no drivability issues, no other symptoms. Could my PCM be bad?
Expert:  A. Penland replied 1 year ago.

Are you loosing power to the AC compressor clutch on the GRAY/WHITE striped wire?

Customer: replied 1 year ago.
the relay will not engage, opening that circuit. However if I use a jumper at relay the gry/wht wire gets power, or if I ground the PCM 69 PNK/YEL, the gry/wht gets power
Expert:  A. Penland replied 1 year ago.
if you unplug both pressure switches and jump the wires in the connectors, does the AC compressor still turn off after it warms up?
Customer: replied 1 year ago.
still turns off. Also if I ground the pressure switches directly at the PCM, still turns off. This eliminates wiring issues between pressure switches and PCM as far as i know, and eliminates freon charge issues.
Expert:  A. Penland replied 1 year ago.

That eliminates everything but the PCM. You verified it is not the switches, sensors, wiring or AC clutch. The PCM internally after it warms up is cutting ground for the AC clutch WOT relay.

Customer: replied 1 year ago.
That's what i'm thinking, unless there is any other tests I can do. What other sensors would the PCM rely on to turn on/off clutch? Could there be another sensor unrelated to the ac but the PCM looks to it to see if the AC should be turned on/off, such as another engine sensor or rpm sensor?
Expert:  A. Penland replied 1 year ago.

If it sees the engine overheating or a severe miss fire, it will shut the AC off. Check the ECT (Engine Coolant Temperature) sensor. If it shows the engine is overheating, it will disable the AC to cool the engine down by taking the heat away from the radiator from the condenser.

Customer: replied 1 year ago.
Is the WOT ac relay the same as the ac clutch relay? How does the PCM know when to shut off AC when i accelerate quickly?Also, where is the ECT? Can it be easily checked with a DMM? I doubt it is a misfire issues because it runs pretty smoothly and has no CEL
Expert:  A. Penland replied 1 year ago.
They are the same relay. The PCM will turn off the AC when it sees Wide Open Throttle or your foot pushing the gas pedal to the floor by reading the TP (Throttle Position) sensor.
The ECT is located on the front of the engine just behind the thermostat on the intake manifold.
Customer: replied 1 year ago.
Okay, i doubt it has anything to do with TP because it probably would run horrible if the TP has issues.I see the ECT, but looks like theres 2. My temp gauge always starts low and ramps up to about halfway once thew truck is warm. Should I still consider swapping out the ECT? I get no overheating or check engine lights. Is there a way to test the ECT?
Expert:  A. Penland replied 1 year ago.

I would try to find a scan tool that can monitor it when the compressor turns off just to make sure. I would hate for you to replace it and still have the same problem.

Customer: replied 1 year ago.
What would I look for, like if the ECT is reporting overheat but the engine is actually at operating temp? Wouldn't that trigger a CEL?
Expert:  A. Penland replied 1 year ago.

You can have a hot running engine or an ECT showing hot, and no codes will be set.

The scan tool will actually show the temperature the ECT is reading.

Customer: replied 1 year ago.
Okay, so if the ECT is reading hot, the PCM may shut off the ac? What kinda temps should I be expecting out of the ECT?
Expert:  A. Penland replied 1 year ago.

Should be around 180 to 190 degrees. If it gets to 250, it is too hot.

Customer: replied 1 year ago.
Okay, so my todo list is scan the ECT, check for high temps. If it is high, replace ECT and/or thermostat, then see if that cures the problem. Can you think of any other sensors that may affect the AC clutch circuit?
Expert:  A. Penland replied 1 year ago.

Nope, that is all.

Customer: replied 1 year ago.
I see two sensors that look like an ECT. One is 1wire, one is 2wire. Do you know which I should focus on?
Expert:  A. Penland replied 1 year ago.

It should have a light green/red striped wire and a gray/red striped wire. The other one is for the instrument cluster.

Customer: replied 1 year ago.
I see it. So it would only take me maybe 2 hours and 30$ to replace the thermostat and ECT. Do you think I should just replace them and see what happens? Finding/buying a scan tool and trying to find where the ECT readings are would probably take me more than 2hrs or cost me several hundred.
Expert:  A. Penland replied 1 year ago.

Try the sensor first. If the thermostat was bad, the gauge would show it. Just make sure they give you the correct sensor. There are tabs on the sensor that only align with the connector for it. You may want to take it is and have them match it up.

Customer: replied 1 year ago.
Great, thanks. WIll do. So, I've never used this service before. Can I put this topic on "hold" until later tonight or over the next few days when I try the ECT and possibly thermostat/scan tool? When will I be billed?
Expert:  A. Penland replied 1 year ago.

Sure can. Just reply back when your ready and I'll get an email when you do. I do work at a dealership during the day as a service technician. So I will not be able to get back to you till the evening when I get home from work.

Customer: replied 1 year ago.
10-4 thanks. I hope this is it, as I've been stressing out about the possibility of a bad PCM for the past few days
Expert:  A. Penland replied 1 year ago.

Ok.

Customer: replied 1 year ago.
the ECT was switched out with a new one, no luck. The AC's clutch still disengages as soon as the truck reaches standard operating temp.
Expert:  A. Penland replied 1 year ago.

Where is the temperature gauge reading when it turns off?

Customer: replied 1 year ago.
Almost exactly half way.
Expert:  A. Penland replied 1 year ago.

That is normal, but I wonder if it not reading properly.

Customer: replied 1 year ago.
Sure seems like it is, starts at bottom, creeps up as engine warms up, after about 5min driving with AC on it hits halfway then ac quits. I could try replacing the 1wire sender too. I've never noticed it ever acting strange in the past.
Expert:  A. Penland replied 1 year ago.
If they are not that expensive. But I'll run this past a few of my friends at the dealership tomorrow.
Customer: replied 1 year ago.
Ok, Thanks. I wonder if the thermostat is stuck closed which triggers the ECT. Let me know if you come up with anything,
Expert:  A. Penland replied 1 year ago.

Ok.

Customer: replied 1 year ago.
Problem is fixed. Last night I installed a new thermostat, radiator cap, and also the gauge ECT, and today I drove it for atleast an hour on the way to work and it worked flawless the whole time. The thermostat I pulled out was real nasty and crunchy, it was really hard to open with my finger. I still don't understand how my PCM thought it was overheating and my gauge or CEL didn't reflect that, but regardless, it's running cooler, smoother, and the AC works fantastic. Thanks for the help, and pointing me in the right direction with it being a cooling issue.
Expert:  A. Penland replied 1 year ago.

Your very welcome and thank you for the update.