No A/C.....ECU is not amused....

[Joe Schmoe]

Car has roughly 60K miles. Using a low-level shop computer I can see that the dash controls signal is reaching the ECU (A/C request signal), the A/C pressure sensor is reporting sufficient pressure, but some other criteria isn't being met and the ECU is not commanding the A/C clutch to engage (A/C compressor commanded state). The A/C fuse is OK, and I swapped the A/C clutch relay for one of the other similar relays and got no change. Anyone know what other criteria the ECU is looking to to be satisfied? I've found stuff about an ambient air temp sensor and a possible cylinder head temp sensor that need to be within spec, but the only thing I see odd in the shop computer is a IAT sensor 2 value that looks wrong. Don't we have just one IAT? Could that be the post-intercooler temp sensor?

 

[Joe Schmoe]

Oops, I think I got it figured. The 2nd IAT value is the temp in celsius, derived from the first value, the voltage. The IAT temp reading is -40 celsius here in 70-degree California. That would be probably too cold for the ECU to command the A/C, and the likely cause of the rich running I've noticed lately. New MAF it is. I'll update the post if this is successful.

EDIT: nope, the shop computer says that value is the post-intercooler temp sensor. I am running an aftermarket intercooler. The investigation continues.

 

[Joe Schmoe]

So...anyone know if the charge air temperature is from the sensor in the cold pipe, or from the sensor in the intake manifold? Also noticed that 128kpa equates to only 18.5 PSI freon charge on the low side. Conflicting indicators of trouble, but I think that may be the issue.

 

[CarGuy]

I like your diagnosis path.

Sent from my SM-N960U using Tapatalk

 

[TyphoonFiST]

How much is Freon in the A/C system? If it is not the correct amount the system will not function it will need to be vacuumed need then refilled and injected with a Dye. If you don't have access to Freon I'd suggest bringing the FiST to garage of your choice. The freon in a can isn't worth a shit as most of it escapes into the atmosphere. A machine is the best way to diagnose and fill a vehicle. Good luck!

Sent from my SM-N975U1 using Tapatalk

 

[Intuit]

He says the pressure sensor is reporting sufficient. (there should be a high and low side sensor BTW)

If you want to eliminate the modded parts as being a factor, easiest way is to reinstall the OEM.

 

[Joe Schmoe]

There's a possibility there are two things wrong. First, I might be incorrect about sufficient pressure - I realized that after I converted the KPA reading in the shop computer to PSI. Assuming it's correct to do the conversion, low side pressure is only 18.5 PSI. I didn't see a PID for high side pressure. Second, the post-intercooler charge air temp sensor is reading as if it's faulty. It's reading -40C...which is what I see from the IAT in the MAF when I unplug it. Thing is, all of this is assuming the shop computer is displaying KPA, not PSI and that the 2nd IAT value is true, not just a function of how the 2nd IAT works. Lastly, I was able to confirm that the A/C clutch does work when manually commanded by the shop computer (engine off only).

EDIT: can someone who has Torque please let me know what values they see for the post-intercooler IAT with the engine running?

 

[TyphoonFiST]

There's a possibility there are two things wrong. First, I might be incorrect about sufficient pressure - I realized that after I converted the KPA reading in the shop computer to PSI. Assuming it's correct to do the conversion, low side pressure is only 18.5 PSI. I didn't see a PID for high side pressure. Second, the post-intercooler charge air temp sensor is reading as if it's faulty. It's reading -40C...which is what I see from the IAT in the MAF when I unplug it. Thing is, all of this is assuming the shop computer is displaying KPA, not PSI and that the 2nd IAT value is true, not just a function of how the 2nd IAT works. Lastly, I was able to confirm that the A/C clutch does work when manually commanded by the shop computer (engine off only).

EDIT: can someone who has Torque please let me know what values they see for the post-intercooler IAT with the engine running?

Do you have a set of A/C compression measuring gauges? That hook up to both high and low side?

Sent from my SM-N975U1 using Tapatalk

 

[Joe Schmoe]

No, I don't. At the rate my wife uses the A/C (her ST), I'm prob better off just using that money to have a shop do a recharge by default. After that I can use the shop computer to compare to the PSI they report to me they charged it with to see if it's a problem with the sensor, then monitor for leaks. That said I'm open to recommendations for a quality set of gauges for later.

Still curious about the 2nd temp sensor tho.

 

[scooter123]

On a stock Fiesta the Intake Air Temp Sensor is mounted to the lid of the airbox. Discovered that one day when I swapped the air filter prior to getting an oil change at the local Ford dealer. Got the warning light because I didn't get the plug fully seated and Fixd reported a failure of the Intake Air Temp Sensor. The fix was real simple, I just had to push the connector until it locked in.

 

[CarGuy]

No, I don't. At the rate my wife uses the A/C (her ST), I'm prob better off just using that money to have a shop do a recharge by default. After that I can use the shop computer to compare to the PSI they report to me they charged it with to see if it's a problem with the sensor, then monitor for leaks. That said I'm open to recommendations for a quality set of gauges for later.

Still curious about the 2nd temp sensor tho.

You can rent A/C gauges from AutoZone or of course buy a set of A/C gauges from them.

Sent from my SM-N960U using Tapatalk

 

[Joe Schmoe]

On a stock Fiesta the Intake Air Temp Sensor is mounted to the lid of the airbox.

That's the IAT built into the MAF assembly. Does the Fiesta not have charge air temperature sensor after the intercooler to account for the density/temperature of the air after being cooled? There is a sensor on the cold pipe that looks like a thermistor, but when I look up the part number Ford labels it a MAP. There is also a sensor on the intake manifold that doens't look like a thermistor with a different part number that is also labelled as a MAP by Ford.

 

[Joe Schmoe]

Issue was diagnosed as a bad high side schroeder valve. Replaced and recharged. The shop computer shows higher refrigerant pressure levels now. I'll wait til it's cold to get another pressure reading and use that is a baseline for monitoring for any other leaks. Thanks for everyone's help.

(952 kpa = 138psi)