Code 79, Pin 10 shorted to power

[Brokex]

Been lurking and learning for quite a while but first question.

I've been having an issue with my 91 Explorer (4.0) for a long time. A couple of times a week, at least I'll be driving along and the check engine light comes on, vehicle loses power and often stalls. I've learned that if I put it in nuetral and shut off the key I can usually restart it with no problems.

The only code I get is a code 79, which means AC was on during the self test or pin 10 on the PCM is shorted to power. The AC wasn't turned on so I can only conclude that pin 10 is shorted to power.

What doesn this mean?

From what I can see on the wiring diagram, pin 10 is associated with the AC compressor circuit and I noticed the AC compressor was cycling on and off. I unplugged it for now and will try it out. It's hard to imagine but could my AC being causing my truck to stall out??

 

[MrShorty]

KOEO 79 is exactly what it says it is: it merely indicates that the PCM is seeing power at pin 10, which indicates to the computer that the AC is turned on. It doesn't monitor the AC operation/efficiency, or whether the compressor is on/off/cycling. If you know the main AC switch is off, this means that power is finding it's way to the AC on/off circuit by some other means.

I don't expect that the "AC" part of the equation is the reason for the stalling. The same fault that is triggering the KOEO 79 could also be interfering with something more critical (fuel pump, main PCM power, ICM power, or whatever), and that would explain the stall.

 

[Brokex]

Thanks, I'll keep looking at it. I noticed that the PCM is supposed to cut off the AC when the throttle is at wide open and the AC WOT relay, fuel pump relay, and PCM relay are all in the same daisy chain so you're probably right.

 

[mechjames]

If you`re running the code test with the defrost setting on, there will be power to pin 10. Turn all the HVAC controls off and have the lever on Panel mode, not defrost.

It could be the AC compressor seizing up. Unplugging it might help. Time will tell if your truck starts running better.

Check engine light is coming on because the engine stalls. You know, it comes on for a few seconds when you turn the key on, that is what is happening.

 

[Brokex]

If you`re running the code test with the defrost setting on, there will be power to pin 10. Turn all the HVAC controls off and have the lever on Panel mode, not defrost.

It could be the AC compressor seizing up. Unplugging it might help. Time will tell if your truck starts running better.

Check engine light is coming on because the engine stalls. You know, it comes on for a few seconds when you turn the key on, that is what is happening.

You're right, there's power with the knob set to defrost.

It doesn't always stall, although most of the times this has happened I'm on my way to work. Check engine light comes on, with some loss of power etc. I throw it in neutral, turn the key off for a second. Restart, put it back in drive and press on, check engine light gone, running normal till next time.

Last night, the check engine light came on and I managed to drive all the way home that way. Got it in the driveway and let it idle that way for 15 minutes tugging on this, that, and the other. Idled rough and smelled terrible. Finally gave up and shut it off. Restarted AOK, no check engine light. Shut it off and thought surely I would have a code since the check engine light had been on for 30 minutes. Nothing but 11's.

This morning, I did some checks for a no code situation I found on

http://fordfuelinjection.com/index.php?p=44 Everything was good except the alternator voltage was 14.9 the first time I checked. It settled out at 14.4 when the engine warmed up.

According to the checks, it's time for a PCM but any ideas would be appreciated.

 

[mechjames]

Camshaft position sensor POSSIBLY. Depending on the year, if the camshaft or crankshaft sensor is wearing out, stuff like that could happen, but it should store a code 214.

Personally, my 94 has been showing code 214 On and off ever since I got it 3 years ago. I know the camshaft position sensor is wearing out, because I can tell the difference in power / mileage / idle quality when the light is on. MY TRUCK runs with the camshaft position sensor disconnected, I have tried it. Older explorers, possibly your computer since it gives out two digit codes, have shown some no start situations when there is absolutely no signal from the sensor.

Its in a tight spot by the firewall, and that is why I have been putting it off to replace (I did pull a bosch one from the wrecking yard from a 94, so I know it wasn't original, so its newer). Everyone seems to have a different idea on the calibration of these things during reinstall. Some say, if its not calibrated right, it won't run at all. Some people have just simply swapped them, and their truck runs fine.

I have taken a quick look at the 2 digit code list, and I don't see one for camshaft position sensor, but code 19 should be the same thing (Cylinder Identification Circuit), which you aren't getting.

I have had an engine computer fail. It was 17km up on a mountain, and I was heading down from a ski hill. The truck stalled out just before going around a corner. Imagine my surprise when I had no power steering. At the time, I did have BCAA service, but I only had 5km of free towing, then $2.50 a km after that. I tried to get it to run, I was getting intermittent Check Engine Lights during self test, and tried starting fluid / carb cleaner, no luck. I decided to coast it down really slow down the mountain road, and got towed home, otherwise it would have been an exorbatant amount to get a tow home.

My wrecker was selling engine computers for $19.95 one week, they were regular $29.95. If you're lucky, its worth a swap. Try and find one with the same date code on your old engine computer. I've had 2 late 1993's and 1 early 1994 computer in mine, and it all runs the same. My original was late 1993.

 

[Brokex]

You're right, there's power with the knob set to defrost.

It doesn't always stall, although most of the times this has happened I'm on my way to work. Check engine light comes on, with some loss of power etc. I throw it in neutral, turn the key off for a second. Restart, put it back in drive and press on, check engine light gone, running normal till next time.

Last night, the check engine light came on and I managed to drive all the way home that way. Got it in the driveway and let it idle that way for 15 minutes tugging on this, that, and the other. Idled rough and smelled terrible. Finally gave up and shut it off. Restarted AOK, no check engine light. Shut it off and thought surely I would have a code since the check engine light had been on for 30 minutes. Nothing but 11's.

This morning, I did some checks for a no code situation I found on

http://fordfuelinjection.com/index.php?p=44 Everything was good except the alternator voltage was 14.9 the first time I checked. It settled out at 14.4 when the engine warmed up.

According to the checks, it's time for a PCM but any ideas would be appreciated.

One thing I forgot to mention (because I didn't think it was important at the time) was that even though it restarted AOK, it was missing. I pulled plug wires one at a time until I didn't get a change in the way it ran. It was one of the wires that runs around the back of the motor. I repositioned it (the portion by the firewall) and the miss went away--in fact the motor surged a little and even startled me a bit. Inspected the wire and couldn't see anything wrong with---didn't see it arcing in the dark either; however, I think it had been touching the engine to firewall ground strap.

Maybe too soon to tell but I drove 80 miles today and didn't have any problems.

Is it possible that there was enough ignition "noise" feeding into the ground system to make the computer freak out and go into HLOS mode?