# A/C Pressure Switch -- Ford Fusion

#### jpanhalt

Joined Jan 18, 2008
8,710
A/C went out on my 2012 Fusion last year, 600 miles over warranty and got no help from Ford. This year, I decided to fix it. A lot of Internet experts say it is probably the temperature sensor on a receiver under the dash. I am 95% sure that is not it. My scanner shows an error for the high-pressure switch. Pressure reads ad "499.7" PSIG from the scanner, which I believe corresponds to open circuit. Refrigerant pressure is 90 PSIG static by actual measurement.

Instructions I found for testing it in circuit on the Internet are baloney. The gauge is a 3-wire gauge and is used on a lot of cars, including GM. I pulled one off an old Malibu yesterday for testing. Here are the Ford schematic for my make and model and the inside view of the one I excised from a Malibu:

The chip is labeled MLX12103EB (Melexis?) and is apparently a real number, but I cannot find a real link to its datasheet or pinout. The white wire was added to allow testing to chassis ground -- there are no direct connections to that ground; although, there are some capacitors that connect to it (one is obvious in the picture).

Questions:

2) I have identified the three leads on the Malibu switch as RED/BLK = Signal, GRAY or GREEN (can't tell) as Reference voltage (5V), and BLK as the Return Common. It does not appear to be chassis ground. Are those assignments correct?

3) The signal seems to be a DC voltage from 5V (no pressure applied) to something less than 5V depending on how much I press on the ceramic disk (white thing below the PCB). Is that correct?

4) Some cars have a Shrader valve on the port to which the gauge attaches. The Malibu did. Does anyone know where Ford does that? Haynes' directions say the system must be emptied before changing the switch. The Ford factory switch does have a protrusion to open a Shrader valve, if one is present.

Regards,

John

#### tcmtech

Joined Nov 4, 2013
2,868
Have you tried putting a 1K pot in its place to fool the computer?

Option two is to do as I did with my first car I ever had when the AC went out due to a bad electrical connection someplace in the clutch coil assy. I just ran a pair of long sheet metal screws in between the compressor clutch plates at 45-degree angles 180 degrees off from each other.

Drove it that way for three years and near 50K miles (Put the screws in for summer take them out for winter) before I came across another compressor unit that had a good clutch assy I could swap mine with.

#### jpanhalt

Joined Jan 18, 2008
8,710
Have you tried putting a 1K pot in its place to fool the computer?
That is exactly what I am considering. I just want some confidence in my identifications.

I did testing the harness as recommended by a reputable (?) automotive "diagnostics expert" on YouTube and came up negative -- meaning all of the voltages that were supposed to be on the 3-pin plug read zero. Plausible causes:
1) Maybe my connections to the plug were faulty. Got the used sensor just for its male end. Cutting that off and attaching wires to it is one of my next planned steps.
2) A d*** chipmunk chewed the harness wires -- only the 5V reference needs to be broken -- and the link is broken. That is a more difficult trouble shoot as the PCM to which the wires go is buried under a bunch of stuff. Ford had the foresight to bury the switch at the rear of the engine compartment just at the transmission tunnel. It can be reached from the top, but most everything is done by feel. On the other hand, the Malibu was right up in front at the radiator and easy to access.
3) Testing procedure was BS.

The other treasure from the junk yard was the pigtail. That will let me test the switch to see if it truly is open, once I am certain of of the lead identifications.

Good news from the Internet: One of the places that claim to have the chip identify it as a 5-channel gauge driver with serial link. A presumably related chip, the MLX12101E provides sensor protection from reverse polarity and voltages >6.8V. That makes me a little less worried about testing the sender, which I plan to do sooner. If the sender is open, then I will put the voltage divider across the leads.

I am very cautious on cars, because a small error can ruin a part that can only be replaced by getting a whole unit. I learned that lesson many years ago on a German-made car ($) of which there were none in the junk yards. Thanks for the reply. John #### tcmtech Joined Nov 4, 2013 2,868 Ford electronics are pretty forgiving and pretty much everything is designed to handle both dead shorts to ground or to live 12-volt power without burning anything down so I have doubts that a 1K pot between 5 volt supply and the system common line is going to do anything. One this with Fords though is they don't use, or at least they never did before, a nice neat 0 - 5 volt signal from most of their sensors like GM and many others do but rather a variable frequency or PWM signal instead. #12 Thread Starter #### jpanhalt Joined Jan 18, 2008 8,710 I have done that. A 1K pot (5-V reference to return) had no effect with the signal line connected to the wiper. I measured between the 5-V reference line and return line and got no voltage. I built up a small 5-V battery pack, connected the negative to the return and the variable output to the signal. Still no effect. It is either a harness issue or PCM issue. Tomorrow, I will dig out the PCM and test continuities to the appropriate pins. I have not tested the sender yet but am assuming that is not the culprit for the time being. I hope it is just a harness issue. I will definitely update on the continuity tests. Hope it is not the PCM per se. John #### #12 Joined Nov 30, 2010 18,190 Try to find online forums about Ford,your car. I learned a lot more there than I could here. These people are electronics nerds. That gives them a huge advantage over your average shade tree mechanic, but people who work on Fords hang out in other places. They will know about wiring harness issues, bad sensor batches, etc. Thread Starter #### jpanhalt Joined Jan 18, 2008 8,710 Been there , done that. There is a lot of misinformation from people too eager to see themselves on YouTube. John #### #12 Joined Nov 30, 2010 18,190 Been there , done that. There is a lot of misinformation from people too eager to see themselves on YouTube. John Are you talking to me? I didn't say anything about You-tube. In 6 months of rehabilitating a Ford Explorer, I watched about 3 You-tube videos. Two were correct demonstrations about how to get at the parts behind interior panels. The third was about using a slam-hammer to pull an axle bearing, and I'm sure glad I didn't try to get by with the wrong tools. Thread Starter #### jpanhalt Joined Jan 18, 2008 8,710 @#12 Well, yes. I avoid YouTube too, but for some subjects, it is hard to avoid. So many of the forums on this subject are irrelevant too. No attention is paid to model year by anonymous, so-called experts. No insult intended. John #12 #### #12 Joined Nov 30, 2010 18,190 So many of the forums on this subject are irrelevant too. No attention is paid to model year by anonymous, so-called experts. I got lucky and found a useful and relevant forum about my bad choice of cars. I suppose the likelihood of finding a good, active forum about any particular car depends on how bad the car is. "Ford Explorer, the most popular SUV in America for 13 years." That's a lot of people with the same problems I have. But it makes a large enough population to get a good forum going. Your mileage may vary. Thread Starter #### jpanhalt Joined Jan 18, 2008 8,710 Had I known about the Fusion problems, I would not have bought it. My previous car was a 2000 Honda that served me well for over 150,000 miles. My Fusion is falling apart at less than 50,000 miles. John #### #12 Joined Nov 30, 2010 18,190 I feel the same way about the Ford Explorer. It's at 137k and it already had a new transmission, needed lots of parts in the suspension, and uncountable problems in the myriad microprocessors. Thread Starter #### jpanhalt Joined Jan 18, 2008 8,710 (Update) PROBLEM IDENTIFIED After much head scratching and chasing dead ends offered by experts on the Ford forums, I found a real (i.e., Ford factory) wiring diagram book for my 2012 Fusion -- no more guessing. Lost a week waiting for the mail, then a few more days for other projects and the heat wave to abate. Pulled the PCM body plug (C175B, which conveniently is not labeled) this morning and tested continuity between the pressure sender wires and the female plug contacts (pins 32,52, and 56). All were open circuit! So, I questioned a lot of things, re-ID'd the PCM connector by finding known pins to ground. Then tested another circuit that was presumably working, the MAF/IAF connector, which shared a common signal return with the pressure sensor. That tested as predicted. A simple short or broken wire shoud not affect all three wires, I thought. Are there any connectors or other splices from the sensor to the PCM connector? No, not that I could find. I have a cheap, old HF video inspection camera and was able to thread the lens end down and along the sensor cable. Voila! The cable had been chewed in half. Picture from the video camera is too dark to post here. The break is buried behind the engine and down low. I was able to get a blind shot of the break with my regular camera. I think the main harness can be reached from underneath and a repair made. At this point, it would sure be nice to have a car lift. Can't crawl under like I used to do. I suspect it is the work of a chipmunk when my car was parked outside. For now, at least there is some humor in it: I will get da chipmunks! John Thread Starter #### jpanhalt Joined Jan 18, 2008 8,710 My halfhearted apology to Chipmunk. There is evidence of gnawing on some harness casings, but in this case the harness was almost a clean cut. Perhaps a bit or road debris got kicked up and slammed the small pressure harness against a steel heat shield to cut it. Fortunately, the harness only has 3 (maybe a 4 th for local ground) wires all the way to the PCM connector. I was afraid it might enter the main harness at place I couldn't reach, but once I found the PCM end, I was able bend pressure sensor harness backward and toward the connector. I can now reach it from the top for splicing. I have ordered some XLT wire (cross-linked high temp auto wire) to complete the repair. John #12 #### #12 Joined Nov 30, 2010 18,190 Auto-electric: one of the nastiest, dirty jobs nobody wants to pay for. #### BReeves Joined Nov 24, 2012 410 Do you have wild rabbits where you park. I had the speed sensor wire cut like someone cut it with a knife in a Dodge Caravan by a rabbit. Know it was a rabbit because I saw the sucker running out from underneath the van the day it wouldn't start. Never would have believed it if I hadn't witnessed it first hand. Thread Starter #### jpanhalt Joined Jan 18, 2008 8,710 Yes, wild rabbits, but they stay in the woods. It's a chipmunk. Parts have been removed, and I am waiting for the TXL (high temp) wire.* On closer examination of the sensor end, I cannot rule out chewing vs. hitting a road hazard. I have seen the little rodents in and out of the engine area when I part outdoors. It should be an easy repair from here on in. BTW, for those who haven't tried Tesa tape or Coroplast tape (both are used today for OEM harnesses), I highly recommend getting some. I will also add a little extra protection against chafing as it passes over a heat shield. John *Cross-linked LPE insulation, SAE J-1128 Last edited: Thread Starter #### jpanhalt Joined Jan 18, 2008 8,710 Well, here is the finished product: That is the harness pulled out where you can see it. I went with new TXL wire all the way back to within about 3 or 4 inches of the PCM so there is only one splice. Had to use different colors than original, but I followed a pattern and made a notation in the wiring manual (e.g., orange-violet = orange). Love that TXL insulation. I wish regular hook-up wire came with it and silver-tinned wires. It behaves with heat just like Kynar. Staggered splices using 3M waterproof butt joints (shortened a bit). Used same 1/4" HT (nylon) split loom as original and Coroplast automotive tape to cover it. Only difference between my fix and the original is the Ford tape is a little more shiny. Getting the proper crimp-on Delphi terminals for the plug was a PITA, as Ford will not tell you. A local Chevy dealer let me scrounge through his collection, and I found a perfect match. His computer showed a$1.30 per terminal, which is 10X elsewhere, but I didn't feel bad as he spent the time with me. So, the wires on the plug go all the way back to the splice to the PCM.

Most important, the A/C works like a charm, and error codes are no longer in my scan.

John

#### #12

Joined Nov 30, 2010
18,190
Elmer Fudd would be proud of you.

#### jpanhalt

Joined Jan 18, 2008
8,710
I'm honored.

Rodents seem to come in batches. Last years, lots of chipmunks. This year, its groundhogs and far fewer chipmunks. I keep my special squirrel gun next to the door (air power, cal 177). Groundhogs require something bigger.

Sure glad I did not take it to the dealer.

John

#12