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Juan Crespo
Juan Crespo, Tech Trainer
Category: Mercury
Satisfied Customers: 1526
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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I have a 2005 Mercury Mariner 3.0 V6. On a trip a few weeks

Customer Question

I have a 2005 Mercury Mariner 3.0 V6. On a trip a few weeks ago the AC started having issues. It would work then it wouldn't. I've done some troubleshooting and basically what I've found is there is no ground going to the clutch relay from the PCM. What tells the PCM to ground the relay and kick in the AC clutch?
Submitted: 1 year ago.
Category: Mercury
Customer: replied 1 year ago.
Right now the AC clutch will kick in sometimes and sometimes it won't. There seems to be no pattern to it. I can unplug the PCM connector and ground the wire going to the relay and the clutch kicks in.
Expert:  Juan Crespo replied 1 year ago.

Hi there.

Upon receiving an A/C ON request from the control panel, the PCM looks at engine temperature (ECT), and throttle position (TPS). If those are within parameters, it then looks at system pressures (dual pressure switch) and evaporator temperature (cycling switch). If those are also OK, then it grounds the compressor clutch coil relay.

On an intermittent operation such as you describe, I would look at the dual pressure switch or the cycling switch to see if there is a faulty connection (you can test by wiggling the wires/connectors. See attached graphics for location))

Easiest way to diagnose this system would be with the help of a capable scan tool that could show the signals (PID) from the different components. Not having access to one of those would reduce our diagnostic capabilities to just jumpering the cycling switch to see if that makes the compressor run continuously.

Customer: replied 1 year ago.
Thank you for the quick response.First I would like to add some information I should've added in the original post. When going down the road it's easy to tell the clutch has disengaged because it starts blowing hot. When and if it re-engages you can feel it in the motor. It will jerk a little for a few seconds like it's running out of fuel then evens out and blows cold.I went to do some more testing with the information you provided, but the AC was in a working mood. I drove around a while and it never cut off. When I got back home I tried wiggling wires at both switches to get the clutch to disengage, but nothing happened. You think I should go ahead and replace both switches? They are both inexpensive parts.
Expert:  Juan Crespo replied 1 year ago.

Oh, man... you asking an old time mechanic whether you should replace something that ain't broke? ;)

Seriously now; replacing the switches won't fix it if it's a wiring/connection issue. I would wait until it acted up, then I would test it and see what's happening before replacing anything.

Best Regards.

Expert:  Juan Crespo replied 1 year ago.

I have to log out for the day. I’ll be back online ready to continue our conversation tomorrow from 2 to 8 PM EST. Please contact customer service at http://ww2.justanswer.com/help if you need immediate assistance.

Customer: replied 1 year ago.
The AC hasn't worked at all today. I've wiggled wires and unplugged and replugged in connectors making sure they are seated good. I also made a jumper wire and jumped the connectors to the switches and still got nothing to happen. Normally I can narrow things down pretty easy but this has got me confused.
Expert:  Juan Crespo replied 1 year ago.

Can you disconnect and by-pass/jumper the cycling switch and see if that makes the compressor work? If it does, then we'll know the system is low on refrigerant. However, if it doesn't, then we need to do some more testing. Can you get access to a scan tool capable of reading A/C data? Also, we will need a set of gauges to read both Hi and Lo pressures and a voltmeter to check out voltage at different points in the circuit.

Customer: replied 1 year ago.
Multimeter and gauges I have, I do not own/have access to a scan tool that can read AC data. Jumping the cycling switch did not engage the AC clutch.
Expert:  Juan Crespo replied 1 year ago.

Ford designed this system to be diagnosed with a scan tool, so it's going to be tough to do so without it...

With the engine running at idle and the A/C on, check that fuse 26 (5 Amps) in the smart junction box (below center of dash) has 12 volts going through it, then check that the circuit from that fuse to the control head (Black/Light Blue wire) also has 12 volts. Then check the REQUEST circuit from the control head (Dark Green/Orange wire) to the pressure switches and on to the PCM to see if there are 12 volts there as well. If not, replace the control head.

Expert:  Juan Crespo replied 1 year ago.

I have to log out for the day. I’ll be back online ready to continue our conversation tomorrow from 2 to 8 PM EST. Please contact customer service at http://ww2.justanswer.com/help if you need immediate assistance.