Electrical Stalling, revving, and theft light issues on 1998 GT. Stumped.

Discussion in 'SN95 4.6L Mustang Tech' started by ANaughtyTugboat, Mar 19, 2013.

  1. Hey guys, I've been posting this on multiple forums seeing if anyone could point me in a direction for the strange problem my car is having. It seems the shop I'm at is even having trouble.

    Basically what's going on with my car is it starts up fine, drives around fine until the engine starts warming up. Once the engine warms up, the car basically tells me to enjoy being stranded somewhere. If the car is in neutral(It's a 5 speed) and i'm just sitting somewhere idling, the car will at random start revving itself like someone is playing with the throttle or as if someone was trying to record an exhaust video to show off. I took a video of it. My legs are nowhere near the pedal, this is just what is does.

    It will also do this in gear sometimes, which can be kind of scary. Normally when it revs like that, it only does it for a short while, and it stalls itself like someone turned off the key. Also it never revs any higher than it does in that video. I've never seen it go to 2500 rpms by itself. You know when it's going to stall because the check engine light starts to flash, and then it just dies. It didn't die in that video, i ended up turning it off because it was late at night and didn't feel like getting noise complaints from my neighbors.

    Once it stalls the theft light comes on and flashes repeatedly. The car cranks but wont start. If you scan it, it comes up with codes U1147, B1681, and P1260 which all have to do with the PATS system. There is no aftermarket alarm on this car. The code the theft light flahes out is 1-6 "Faulty link between PATS and EECV. I tried it with both keys and the same thing happens. If I let it sit for a little while the theft system will turn off and let me restart the car. This video is of the car after it stalls. There's a clicking sound coming from the CCRM. The fan also turns itself on after it stalls.

    Brought it to a shop, and they had an electrical guy come in and check the wiring. They couldn't find a problem there, but reinforced the grounds anyway. They also replaced the crank sensor, but that didn't fix the problem. They also had that guy from Auto Tech On Wheels come check it out today, but just like with the electrical guy, the car would not act up when they were there. It's a totally random thing. He seems to think it's a PCM issue. What do you guys think? I'm at a total loss at the moment.

    Also as extra information, I've cleaned the MAF and the IAC, replaced the plugs in July, and put all new ford racing wires on about 2 months ago. Car ran great up until last month when all of this randomly happened. I tried testing the MAF and IAC by unplugging one at a time while the car is running, and the car runs worse(as i think it should) with the MAF unplugged and stalls within a few seconds with the IAC unplugged.

    Car warms up and revs itself, stalls, theft light flashes, cranks and doesn't start, eventually starts back up, and everything happens again.
  2. You might expose the PCM by pulling the passenger footwell side trim and, with the engine running, use your fist to firmly rap the PCM a few times to see if there is an onset of symptoms at all. This might point to, say, a bad solder joint in the PCM that gets worse as the PCM warms and things expand a bit.

    You might also try manipulating the wire harnesses leading to the PCM -- wiggle the harness and wires a bit -- to check for bad crimps or broken/fatigued wiring.
  3. So basically you're saying start the car and let it warm up and move the pcm around or grab it tightly along with the wires to see if anything happens? The PCM is that big gray box correct? I'll probably run up to the mechanic and try that now since I have nothing better to do and the car is sitting outside anyway haha.
  4. So I went to the mechanic today, found out there was a chip in the computer. Apparently they ran the ran before pulling out the PCM and when they started touching it the car started acting up. Guess that should say the PCM is the issue, correct?
  5. When you say there "was a chip in the computer" do you mean an actual chip inserted into the board at the J3 service port? That is, is there a device hanging out the side of the PCM?

    If so, it's possible that the connection point is simply dirty. Disconnect the battery and remove the PCM. Open the PCM and remove the chip and look at the connections. The PCM comes from the factory with a clear conformal coat on the board including over the contacts. It's important to clean all traces of this stuff from the contacts or it can result in intermittent connections, the sort of thing that will cause the engine to run like crap or stall completely.

    Got any pics of the setup?
  6. I would assume that's what they meant. There are other locations for chips to go that have to do with the PCM? I'm not too sure on all of that. The chip has been in there since long before I bought the car, so unless it got corroded somehow recently it must have had a fine connection. I'll make sure to post pics of the setup as soon as I get a chance.
  7. Just an update. I got the car back today. They told me the chip in there was shorting out the computer and **** up the board. They replaced and updated the PCM with the newest info. Car's been running for about 2 hours today and no problem yet. Hopefully this is all over with.