Automotive Air Conditioning Information Forum

I am having a problem with the AC on my 2003 Ford Crown Victoria and I hope someone on this forum will be able to help with this specific vehcle.
1. R-134 pressures are good and within range
2. Compressor clutch will engage and AC will operate if clutch contacts are bridged with jumper wire and ac blows cool but not cold.
3. Low pressure sensor new and voltage is 12 volts
4. High pressure sensor is new and tests OK
5. Evaporator temperature sensor (ETS) checks out OK as resistance varies with temperature change.
6. grey/ red wire to Evaporator Temperature Sensor (ETS) is 0.1volts when engine running and AC on. This car has the EATC system control
My question is this; Where does the voltage on the grey/red wire to the ETS originate from?
Is the voltage of 0.1 volt correct? I have read that the correct input voltage should be 5 volts.
The black/white wire on the ETS is the input wire for the Electronic Control Module connector 38 but I find no reference for the grey/red wire.
any help with diagnosing this problem would be greatly appreciated.

OK, for starters
"1. R-134 pressures are good and within range
2. Compressor clutch will engage and AC will operate if clutch contacts are bridged with jumper wire and ac blows cool but not cold."

1. This has little meaning without actual pressures.
Pressures (hi and lo, thus using a professional manifold gauge set) should be taken with RPM at 1500 , max fan max cool, recirc, doors open (a true load test), and ambient stated, and center vent temp reported.
2. Having said that, you are not able to get the comp to run without jumpering.
Jumpering is NOT recommended, since with these fancy control systems you can easily damage an electronic component and be worse off than you were.

I would have the system evacuated and recharged to spec before going any further.
If that is a no-go, there is another consultant who will be along in a day or two (JohnHere) who can advise further

Thank you for the advice and yes I plan on vacuuming system and recharging it but not before I know why the clutch does not engage without jumping the relay which by the way is not defective.. I was under the impression that the compressor should operate if the relay receives ground from the ECM if the two pressure switches were closed and the evaporator temp sensor was operating. Both switches are closed when the relay is bridged. I don't leave it bridged for more than the time to check the switches as I don't want to cause damage. My understanding is the voltage to the low pressure switch is supplied by the EATC so that seems to be operating. This is a complex system with the ECM involved so I would like to know if it may be defective. My concern is what voltage should be at the ETS and where it originates. Should it not recieve voltage even if engine is not operating but ignition is on and ac dash switch is engaged with staic pressure only? I have the Ford wiring manual and the service manual but there is no mention of the ETS in either of those two hence my questions. I appreciate your prompt reply.

You have either a ground, power, or control module problem. Be sure you have actual grounding when that is supposed to happen. Check fuses, if control module is supposed to send power somewhere, check the voltage output. Ford systems are notorious for problems. Could be the ECM, but my consultant suspects the control module
We agree this question needs to be answered:
"Where does the voltage on the grey/red wire to the ETS originate from?"
There are many variables. You could try the service manual from a Lincoln Town car or a Mercury Grand Marquis...might have more detail. If using Haynes manual, they tend to not be very detailed, may need FSM. Could try a FSM from a year before yours or a year after, if car was not produced mid-year.
You can also purchase a one-month temp subscription from AllData.

Thank you once again for your assistance. My understanding is that the ECM supplies ground to the clutch relay. By control module do you mean the in dash control panel? There is a way to have the module do a self scan for fault codes and that shows no issues but it may not be complete. No faults show up on OBD2 scan either but my code readers are not professional quality either.
I have both the Ford service manual and wiring manual and cannot find source for power to the evaporator temperature sensor. I can find that a power wire from the control module goes to the low pressure sensor and that is 12 volts. The black/white wire on the ETS goes to the ECM as an input source. I cannot see a source in either the ECM or EATC module that leads to the ETS via the grey/red wire. Bridging the 2 wires to that component does not engage the clutch so the problem likely lies elsewhere. I can swap out the ECM and the EATC module with spares from another '03 CV if need be but they have not been tested. Manuals reference an addition manual which I do not have or can find online so alldata maybe necessary to locate that one missing item. I have read that the '03-04 Crown Vics are the same but the ETS was eleminated in the '05's. All relevant fuses seem to be OK and since they control other components I have confidence that they are working since the only issue is the AC clutch engagement.
I really appreciate your knowledgeable advice.

" I can swap out the ECM and the EATC module with spares from another '03 CV if need be but they have not been tested. "

This would be a worthwhile endeavor

You really need to figure out where the power is coming from, no doubt that is where you will find your trouble

Yes I will swap out the ECM and module. Black flies are out and biting now but I can lather on some fly dope and get them out of the spare parts car. Probably worth a shot. The FoRd Wiring Manual shows the ETS harness goes into the firewall and not back to the ECM so likely it leads to the EATC module. Thanks again! Much appreciated.

Please keep us posted

ECM from parts car removed but not installed. The EATC module in the parts car is junk. I tested all thre sensors again just for fun
1.Low pressure has 12 Volts (from EATC)
2. High Pressure has 5 volts from ECM
3.Evaporator Temperature shows 0.8 volts from ?

The ECM I took from the other Crown Victoria appears to be from a P71 (police version) and does not fit the wiring for my civilian P74 version so before I try to source another ECM I hope that someone on the forum can confirm that the power signal and at what voltage is sent from the ECM or somewhere else such as the EATC module. From what I have read on another site but not confirmed is the voltage should be 5 volts. Still looking for an answer from a knowledgeable Ford person. Thanaks for all the help so far.
Also how can I make a monetary donation to this helpful website. Once I sort out the present problem I am sure I will be back with more questions and guidance.