Engine Size: 4.0 V6
Refrigerant Type: R134a
Ambient Temp: 100+
Pressure Low: 33~65
Pressure High: 250~325
Country of Origin: United States
Hello everyone. I know it has been some time, but many regulars know of the problems with this vehicle. I have some resolution on the poor cooling, and some service information for other technicians working on similar problems.
First, a thumbnail of the problem. Poor cooling, not very good on the highway, and nonexistent at idle. High evaporator pressures, and too much hysteresis in the cycling switch. The dealer pronounced the system to be "within spec", and generally blew me off.
It became obvious with testing that the compressor did NOT have enough capacity for this vehicle, especially at idle. By manually selecting a lower gear in low speed traffic, cooling improved - as long as you could keep the engine speed above 3000 rpm. Since there is no smaller drive pulley available for the compressor, and no room for a larger crankshaft pulley, altering the drive ratio was not an option. My only option was to change the ring and pinion gears (front and rear) to increase engine speed. I installed 4:56 gear sets. This did help on the freeway, and around town, rasing the 45mph engine speed to around 2600 rpm in drive, with the overdrive switched off. It still did not cool very well at idle, but it cools down now once you start moving.
The idle issue aside, it still took a long time to cool down after hot soak - it just did not make very cold air, even at 4000 engine rpm. Once it started cycling, it was "all over the place" I wasted hours on a spreadsheet, looking for a trend. I obtained a new cycling switch from the dealer, and tested it carefully on the bench. It reliably cut out at 33psi, and back in at 46 psi. Once installed, the cooldown improved somewhat. I bench tested the old switch, and found it to be defective - depending on how you rolled it around on the bench, it would cut out anyplace from 38 psi up, and back in 5 psi above where it cut out. Clearly defective, at 9,000 miles.
It still did not cool very well overall, and I decided that adding an adjustable cycling switch was the answer. With the very dry climate of Las Vegas, I know from experience that a low pressure cutoff of 21 psi is usually ok. A connector and metric switch was obtained, and installed. The switch was set to 21 psi / 38 psi on the bench before installation. This did improve cooldown after hot soak. The vent temps dropped about 10 degrees on average. The next road trip I took revealed a new trend however - decreasing cabin comfort over time. A check with a recording thermo-hygrometer showed a slow rise in humidity.
More testing. Since the switches had been bench calibrated, I had not looked at the system pressures while running - my mistake. Cutout was a rock steady 21psi. Cut in was a different story - it was all over the place.... another bad switch?? I removed the switch, and re-tested on the bench - test OK. I installed several thermocouples and started collecting temperature / pressure data ... AGAIN. After a week, i started looking for a trend, but could find none. I even locked the fan clutch up, suspecting it was not responding the same way all the time.
After studying the electrical diagrams Tim had so graciously sent to me (looking for bad connectors and diodes to check), I decided it must have to do with the PCM. After checking virtually every sensor, and badgering the dealer into running the full diagnostic on the PCM, it hit me. I pulled the connector off of the cycling switch, with the system running. The compressor stopped immediately. I re-installed it, and it took several seconds before the compressor started again. A quick look at my watch, and a few test cycles showed the answer. Some software MORON had included a restrike delay of 7 seconds. That's right, SEVEN WHOLE SECONDS after the system reaches 38 psi, it re-starts the compressor. depending on conditions the low side pressure can be anywhere from 45 to 90 psi by then. With 100F air blowing over the evaporator, it warms up quickly - quick enough to pick up the moisture in the evaporator and case, sending it into the cabin.
The best cure so far has been an additional relay, and removing the actual cycling authority from the PCM. The OEM cycling switch connector is shorted with a solid wire, and taped up securely. The clutch coil wire from the PCM is used to power the relay, and the relay is grounded through the cycling switch. A new power wire from the fuse block to the relay provides the power to the relay contacts, then to the clutch coil. Cycling is now controlled by the switch without delay. it cycles at the set pressures, regardless of ambient conditions. Now, for the first time in 12,000 miles - water actually drips from the evaporator box drain. It cools much better now, as long as you keep the engine speed high.
I sill believe that my compressor is defective, but Ford says no. it has a 30 psi "dip" once per revolution, that threatens to tear the needle off of any high side gauge connected to it. The pressure pulse is visible in the discharge line motion, especially at idle. A chalk mark and a strobe lamp confirm that the pulse occurs at exactly the same pump shaft location each time. Even with the clutch control temporarily shorted, it will not get cold enough to produce ice on the suction line at the firewall - it just does not have enough capacity.
Sorry for the book folks, but now you know all I know about this....
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.
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