2007 Mercury Milan - V-Ref Problems FIXED!

[protraxrptr17]

Car came in with the complaint of no A/C.

I made a quick check of the data and found the evaporator temp was reading -40. I thought, "this is gonna be easy, unplugged or bad sensor." The sensor is deep under the dash, but is pretty easy to change. It was pretty crusty so I replaced it. Put everything together, still no A/C but the sensor was now reading correctly.

After the car ran a few minutes the A/C kicked on. Went for a test drive, sluggish right off a stop. Got the throttle system wrench once. A/C now works sometimes. Baro is like 18. ACP is about 62 when it's off. Found reference voltage about 3.62V. I have unplugged the pcm and checked resistance to ground on all v-ref circuits and found no continuity. I cleared the throttle code and forgot to write it down. Never came back. The v-ref runs all the sensors that have skewed readings. What should I look for? Gotta be a bad PCM right? I'm thinkin the A/c works sometimes because its right at the threshold and sometimes it sees just enough pressure to kick on.

Thanks for any help. I'm losing money fast on this job. The customer swears the engine ran perfectly and never had a CEL before I worked on it. You know how that goes.

[steven kiser]

there could have been an underlying issue here. it wouldn't surprise me if repairing the a/c just intensified the no power issue. the compressors will add a good amount of drag on the engine. un plug the compressor and i'll bet that the power drain will go away and as far as the customer is concerned the power is back. unfortunately for him or you it's not. it was probably such a slow loss it wasn't noticed. you never know the water is too high in the tub until it hits the floor.

[ricmorin]

The reference voltage could be low from one of the sensors pulling it down. You can cut the wire at the pcm and see if it's still bad on the pcm side.....then it's a probable pcm. If it's good, then you need to go lookin.....

I know cutting wires sounds drastic, but they're easily soldered and can save tons of time. I just got burned on a short in a Nissan because of poor diagrams. I called the ECU bad when it wasn't, so I started cutting wires.....BINGO. Found the circuit and fixed the car. Should've done that the first time, but I didn't and now paying the price.

[protraxrptr17]

Thanks for giving me the courage to cut wires. I wanted to yesterday but backed out. Turns out the fuel tank pressure sensor was shorting. Funny thing was that it wasnt showing a problem by testing continuity to ground without power going to it. Luckily I cut it first. As soon as I cut the wire, voltage jumped to 5v. I hadn't unplugged it earlier because shopkey said that it was on top of the fuel tank. It was pretty easy to get at. Why isn't there a low V-ref code?

[ricmorin]

Good job. Snippers are now our best friend.

[liljoe]

Cutting is OK, but in most cases you can remove a terminal from a connector to achieve the same result and not have to fix any wires. Don't take this as me saying to not cut, just think first and see if you can do the same thing with out cutting anything. Again, I am NOT saying cutting is bad, so don't think I am,

[ricmorin]

Absolutely! Whenever it's easy to pull a terminal I'll do it. Some are tough though, especially at the PCM or other module.

[protraxrptr17]

I dont like pulling terminals. They are too fragile and too hard to get if you mess one up. And real hard to find if it causes one of those dreaded intermittent drop outs

[steven kiser]

cutting wires is fine as long as you're not a wire nut and tape kinda guy. i've seen more issues where sensor wires are held together with scotch locks or just twisted and bare. i opened a lower knee pad on a nissan and saw more splice connectors i 10 feet of harness than i have in my winzer drawer. the same wire was butt spliced numerous times. i think i was told that b@#t b*y put a remote starter alarm combination in it along with a stereo with an amp and boom box speaker. closed it up and sent it on it's way......

[protraxrptr17]

Yeah, I solder everything. I dont even use thefactory crimp and heat connectors. Nothing frustates me more than to work on something that has those dang scotch lock things. First ting I do is cut all that junk out and start over.

[steven kiser]

almost forgot to thank you for posting the fix. happens so rarely it got me by surprise. btw i haven't gotten my problem child back in so i'll post the fix when done.......