Actually, this question isn't about my old truck, but about my 97 Grand Marquis. Hopefully someone can point me in the right direction anyway

Anyway, here's the story: Driving down the road the other day, my A/C suddenly stops blowing cold air. After I get it home, I notice that the compressor is not engaging. So, I ran the EATC through the self test and it returns two fault codes (30 and 125). I could not find a reference to code 125, but code 30 indicates that the inside temp sensor is shorted to ground. Aside from being dirty, the sensor looked fine...so I cleaned it off and reinstalled. Still no compressor, but the self test no longer shows a fault with the sensor.

Other things I've done:
Checked Fuses
Replaced relay
Jumpered low pressure cycling switch

Still the compressor will not engage. I get almost no voltage (like .09 volts) at the electrical connector on the compressor.....so apparently something is preventing the signal from being sent to the compressor.

Could this indeed still be the inside temp sensor causing this? I'm not sure, but my understanding is that by setting the EATC control to A/C with the temp all the way down, the A/C would run regardless of the interior temp. Is this incorrect?

Thanks in advance for any help.

 

Check for power on the purple wire at the low pressure cycling switch when the clutch is supposed to be engaged. If there's no power there, then the EATC isn't requesting clutch operation, perhaps due to an internal fault or a sensor problem.

On your system, the series circuit of the EATC, low side and hide side pressures switches directly controls the clutch unless the PCM disconnects it via the WOT relay. A bit different than most Fords' clutch control circuits of this and later vintages.

 

Thanks for the reply. I forgot to mention that I did check this as well. I get around 12.5 volts across the terminals at the low pressure switch. I don't seem to be getting much of anything at the compressor coil pigtail though.

 

Try jumpering across the Low Pressure Switch connector and see if the compressor engages. If so, you are low on refrigerant or the switch is bad.
I believe your 97 had the accumulator mounted down by the right front suspension. If so, check the bottom for corrosion/leakage.

 

That isn't supposed to happen if the readings were correctly made. There's supposed to be battery voltage at the switch, but not across it (which would indicate the switch was open).



Try jumpering the switch momentarily as suggested.

 

Thanks for the replies. Just to clarify, I have battery voltage at the connector that plugs into the cycling switch. As I mentioned in my initial post, jumpering the switch is one of the first things I tried.

I have since tested the resistance output of the interior temp sensor, as well as the voltage from the connector, and found them to be well within specs....so I believe the sensor is good.

Still, I don't seem to be getting any voltage at the connector that plugs into the compressor.

 

The problem is not the EATC or the sensor. If it was, the EATC would not have sent its request to the low pressure switch. From there, it's just a series circuit through the high pressure switch and the normally closed contacts of the WOT relay to the clutch.

 

I replaced the relay to no avail. So, outside of a short/open in the harness somewhere, what else could it possibly be? I checked the resistance between the ground wire on the compressor pigtail and the negative battery terminal; and all is good there. It's a little more difficult to check the continuity of the supply wire to the compressor though.

I assume that the only influence the PCM has here is to open the contacts on the relay during a WOT event.

 

That's correct.

With power off, you should be able to make continuity checks back through the rest of the circuit from the compressor plug's hot terminal. See post 7.

 

Hold on here guys.
Looking at the AC clutch wiring for 98, which I'm sure is the same as a 97, It's not a straight circuit from the control head through the pressure switches to the clutch.
The EATC sends voltage through the pressure switches then to the PCM. The PCM grounds the WOT relay (Normally open) to engage the clutch. 2 separate circuits.

http://i899.photobucket.com/albums/ac191/rtg143/AC2.jpg

The clutch wiring would be the same for Manual or EATC.

 

You need the 97 diagram, it's different. 98-on diagrams don't apply.

 

I found 96 and 98 and they're both the same. 97 is different?

Found a 97 diagram from Mitchell, it shows the same clutch circuit as the one I posted. Which diagram are you using?

 

Factory EVTM, Cell55-003.

EVTM shows 96 and 97 are the same, 98-99 are different as I described. However, the 98 EVTM Cell 55-003 shows the same operating note as the previous years, the 99 does not. One way or another, I'd say that the 98 EVTM has at least one error on that page, but I don't know which, the schematic or the description. The 98 diagram you posted is identical.

The 98 WSM Pinpoint tests, section L, indicates that the PCM has to energize the clutch relay for operation. The 97 WSM is the opposite, that the PCM DE-ENERGIZES the WOT relay to cut off the clutch.

 

Just to update my progress here....I find that if I supply voltage to the compressor supply wire at the relay, the compressor kicks on without a problem. As I mentioned earlier, the relay is new, and jumpering the cycling switch will not engage the compressor in itself, but there is voltage at the connector. Not sure where to go from here.

Additional Info: The supply wire running to the relay to terminal 30 (Orange w/ green stripe) is only supplying about .5 volts when the A/C should be on.

 

That's your problem. Did you ever check the high side switch or make the continuity checks earlier recommended? Pin 30 comes in from that switch.