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engine dies when heat/ac is turned on?

outracing said:
Okay everyone. So here is an update on what I have done. I got the thing to start, the problem is still there. And dare I say, getting worse. Now, I can only start it once every few hours.

So I have tried pulling the blower relay. As soon as I pull it, the Jeep dies. Same goes for the blower fuse under the hood. (YES, I PULLED THE RIGHT FUSE.) It, as is should, dies also when I pull the "Start Run" fuse under the hood. I then pulled out the JB. Completely pulled it apart, layer by layer, I was surprised at how clean it was. There is nothing visually wrong that the naked eye could see.

I unplugged the blower itself, as soon as I did this, the Jeep died. I took apart the power distribution center under the hood. I was extremely careful. Every wire and every fuse is in good shape. No water nothing.

I even pulled apart all the knobs and inspected the circuit board behind them.

I am at wits end with this thing. Please, anyone, HELP!! :banghead:

Photobucket.com seams to be down for the time being. I'll post pictures as soon as I can.
Click to expand...

How aggressive were you in hosing down your engine bay? Did you get water on your PCM?
 
ehall, i have gone through every wire i could find. both inside and outside i even removed and inspected that fuse box which is in great shape.

lry, could you please define cps?

sidewinder, it was just a rinse job. i definitely got water on everything, but not anymore then if I was to change the oil in the rain. is the PCM the thing located on the fender by the air box? if so, would one from a 99 fit? I could swap it in and just see what happens.
 
outracing said:
ehall, i have gone through every wire i could find. both inside and outside i even removed and inspected that fuse box which is in great shape.

lry, could you please define cps?

sidewinder, it was just a rinse job. i definitely got water on everything, but not anymore then if I was to change the oil in the rain. is the PCM the thing located on the fender by the air box?

Yes.

if so, would one from a 99 fit? I could swap it in and just see what happens.
Click to expand...

A '99 PCM won't work. You need a '97 PCM. Look for a decal on the side of the PCM. It will have the Chrysler part number on it. That's the part number PCM you will need.
 
IMG_20150314_190441_723_zps7yk6h8vb.jpg


is any one of these the part number?
 
So, are these reprogrammable? I checked a lot of the junkyards. Apparently these are hard to find for the xj's with the ax-15 which is what mine is. I got one from a 97 automatic. The last five charecters are different in the part number. Now the odd thing with the blower is gone, but the no start is still present. If I went to a stealership, could they flash it or something?
 
Nearly every diagnostic procedure in the diagnostics manual has you wiggling the wiring harnesses as a preliminary step to solving the problem. You should try this.

Wiggle ALL wiring harnesses and see if the engine starts. If it does, wiggle them again to see if the engine quits.

One thing you should do is remove the three connectors from the PCM and inspect the pins and pin cavities in the PCM and the connectors for water intrusion and/or corrosion.

Try eBay for your PCM. Call the dealer and ask them if they can 'flash' a Auto trans PCM to manual shift.
 
I tried wiggling all the harnesses, I have pulled every plug under the hood, including the computer. the local stealerships don't want to flash a used or aftermarket computer and they tell them that they are vin number specific and crysler has discontinued them. I'll cross my fingers that I can find one on ebay.
 
outracing said:
So, are these reprogrammable? I checked a lot of the junkyards. Apparently these are hard to find for the xj's with the ax-15 which is what mine is. I got one from a 97 automatic. The last five charecters are different in the part number. Now the odd thing with the blower is gone, but the no start is still present. If I went to a stealership, could they flash it or something?
Click to expand...
Yes they can be flashed/reprogrammed, but you don't have a software issue its hardware.

The PCM isn't a Windows Computer that gets so bloated and bogged down and stuck with registry errors, etc, that backing it up, wiping the hard drive and installing the software fresh results in fixing all the problems and speeding it up again. The PCM is running basic software, very clean and tight, they don't get viruses, they just don't change their programming by themselves.

I would suspect you've got a shorted crossed wires under the dash, that the switches in the environmental control is shorting the ignition switch. Perhaps its under the hood, but you've checked there and NOT under the dash.
 
Before you get too far, I would check the alternator output. I had a no start issue with a 96 that I couldn't figure out last year. She would crank and crank but no fire. Finally I floored it and got it too run around 2500 rpm and noticed that the voltage on the dash was just below 14v. So after fighting the CPS change and spending money that I didnt need to, it was the alternator after all.

I have had 2 other separate occasions that have killed an alternator, both were water related.
 
91to99, Thanks for the advise. I took it down to my local AutoZone, they tested my battery and alternator. the alternator, they told me, is functioning like it should. the battery, a Napa Legand, was starting to go, but just barely. It was at 93%. I explained to the AutoZone worker in great detail, what was going on. He seamed to believe that it may be computer related as well... Bottom line being, it starts fine when cold, (computer has different instructions when cold) and runs fine there after. Switching the HVAC on causes the computer to "reboot" there by killing the engine. When the wrong computer is installed, it starts every time, and the heat and ac work on all settings, it just runs really bad until about 50 or 55 miles per hour. He guessed, and I might be inclined to believe this to be correct, that at about 50-55 the engine is warm and the computer believes that the tranny is in overdrive and kind of relaxes on trying to shift gears. I think I'm going to keep calling some junk yards and see what I can find as far as computers go. The stealership said that they were supprised the Jeep even starts with the wrong computer as they are VIN specific. I have a local junk yard searching for me, they said if they can find one they can get it via the parts exchange program and even have it "flashed" and warrenty it for 30 days if it does not just "plug-n-play. I told them to look, at this point, it is about the only thing that I have not tested. 50 bucks and at least I will know for sure.
 
outracing said:
...Bottom line being, it starts fine when cold, (computer has different instructions when cold) and runs fine there after.....
Click to expand...
Ummm, sorta true, when the motor is cold the computer operates in open loop, all it does is ignore the O2 sensor feedback. It will also set a much richer A/F ratio, which is needed for a cold motor until it warms up. The O2 sensor is ignored for two reasons, 1. its NOT accurate at all until it gets warm, 2. its only useful for indicating stoichiometric A/F ratio, which you need richer than stoichiometric during warm up.

outracing said:
...Switching the HVAC on causes the computer to "reboot" there by killing the engine. When the wrong computer is installed, it starts every time, and the heat and ac work on all settings, it just runs really bad until about 50 or 55 miles per hour.
Click to expand...
From your video, it appears the whole cars reboots, be careful NOT to confuse cause and effect. Shorts in the wire harness that connects to the PCM often blows the fuse (in most Chrylser's the PCM and Fuel Pump share a fuse and its labeled Fuel Pump), but a short can kill the PCM and make it reboot. If the electric system suffers some huge disruption it can kill everything in the car, including the PCM and cause the PCM to reboot.

That doesn't mean a PCM is rebooting all on its own, there is no easter egg programmed into PCM's that when you turn a knob it makes it reboot itself, there has to be some problem with the vehicle that causes the PCM to reboot.

outracing said:
...He guessed, and I might be inclined to believe this to be correct, that at about 50-55 the engine is warm and the computer believes that the tranny is in overdrive and kind of relaxes on trying to shift gears. I think I'm going to keep calling some junk yards and see what I can find as far as computers go. The stealership said that they were supprised the Jeep even starts with the wrong computer as they are VIN specific. I have a local junk yard searching for me, they said if they can find one they can get it via the parts exchange program and even have it "flashed" and warrenty it for 30 days if it does not just "plug-n-play. I told them to look, at this point, it is about the only thing that I have not tested. 50 bucks and at least I will know for sure.
Click to expand...
This would NOT be the first dealership that has no idea what they're talking about, but they may be correct.

What is this "wrong computer", as in what vehicle is intended for?

PCM's being VIN specific can mean two things.

All PCM's are Part Number specific, manufacturers use the same "hardware" PCM in dozen of different vehicles, but the same PCM has different programming specific NOT only to the vehicle, but the options for that vehicle, and it changed from year to year as well. So, a part number is assigned and affixed with a sticker for each PCM according to the software loaded in it. So you can have two identical PCM's but they will have different part numbers because they have different software loaded into them, and that assures you get the right PCM with the Right Software for your vehicle. (Yes, the flashing goes off part number, the tools that do the flashing have that built in, so even the dealer can't flash the PCM with software for a different vehicle or different options, either intentionally or by mistake). All the Manufacturers/Dealerships have gone to VIN tracking of their vehicles, if you want to order a part you give them the VIN and they check a database to tell you the exact replacement part number you need (the database has all the info from the factory when your vehicle was put together). If this is what the dealer was thinking when he said VIN specific, he was correct, although he could have explained it better and that you could find what you're looking for by part number.

PCM's have the VIN programmed into them, but depending on the vehicle and its options, that may or may NOT make any difference at all. Most XJ's, if NOT all, if you have the wrong or NO VIN number programmed into it, it won't effect anything at all.

Later vehicles have anti-theft measures added into the electronics, things like immobilizers, etc. The electronics in the vehicle will compare VIN's programmed in the different electronic modules, including the PCM, and will NOT let the vehicle start if VIN's do NOT match (i.e. you can't get around the immobilizer to steal a car by swapping in different PCM). I really doubt any XJ's got this feature, it was only on the most expensive Chrysler vehicles when the XJ had finished it production run, but I could be wrong. Today its on even the cheapest economy cars. If this was what the dealer was talking about when he says its VIN specific, he's dead wrong and an idiot.

Any old PCM, the connectors might NOT even connect up to it. So you at least have got a similar PCM to connect the connectors. The pin-outs for the vehicle and PCM change, so you may entire circuits disconnected from the PCM. The PCM can have programming for different engines/transmissions.

So if you put in a PCM from another vehicle, that is NOT the specific part number for the PCM you're replacing, you are lucky it even ran. The problems you encounter from this new PCM are likely all rooted in the fact that NOT all the circuits are connected to the PCM and this PCM is programmed for another vehicle and engine, it is incorrectly controlling your vehicle and engine. Perhaps the circuits for the environmental control are NOT connected to this PCM, that is why it doesn't die, and the problem won't be fixed to find and fix the circuit problems in the environmental controls.
 
outracing said:
...When the wrong computer is installed, it starts every time, and the heat and ac work on all settings, it just runs really bad until about 50 or 55 miles per hour. He guessed, and I might be inclined to believe this to be correct, that at about 50-55 the engine is warm and the computer believes that the tranny is in overdrive and kind of relaxes on trying to shift gears...
Click to expand...
Someone correct me if I'm wrong, the XJ never had any electronic transmissions, they were all hydraulic. As well, an electronic transmission would have its own ECU, which the PCM would talk to, or at least be connected to. Usually the only thing the PCM would control in the trans is the torque converter lock-up. Nor is the Programmed to "think" if the trans is in overdrive, and shifting would take the trans out of overdrive. (Overdrive is just another gear, its only called overdrive because it has a ratio of less than 1:1).

If you have a PCM for a different vehicle and engine, what you're seeing is the result of trying to control a vehicle and engine like its a totally different vehicle and engine. It might be close enough in some conditions it works OK, but in other conditions it works very poorly or NOT at all. Looks like driving 50 to 55 mph is a condition where the PCM programming is way to off from what your vehicle and engine need. If the PCM takes speed into account at all, the different tires sizes and axle and trans ratios, would throw it off horribly.

As far as I am aware (meaning I might be wrong) the HVAC controls are NOT connected to the PCM in anyway, other than a circuit to tell it to turn the AC on. The AC is monitored and controlled by PCM. It monitors AC pressures and it signals to engage or disengage the AC clutch. Within the HVAC control will be a switch that turns the AC on, the Mode Knob will have contacts to close the circuit for AC. NOT only in AC modes, but for defrost modes and sometimes recirculate modes as well (regardless if you turn the AC on or not).

The 3 knobs on the HVAC control head:
-The fan switch is just connected to the fan.
-The temp switch is just connected to a door and/or vacuum switch that redirects air over the heater core and/or shuts off water flow to the heater core. Maybe later XJ's had an electric door, like most vehicles today, but the electronics for that are all in the control head with the switches, NOT the PCM.
-The AC switch (if your HVAC has it) and the mode knob have electrical contacts that turn on the AC according to what position you turn the knob and/or press the switch. That simply closes the circuit to the PCM, that the PCM starts the AC and monitors it.

So, you're problem (in my mind, i.e. my guess) has 3 possible root causes.

-Something physically failed in the PCM, burned out, so that anytime AC is commanded it kills the PCM. (no it didn't reprogram itself) (No commanding Fan speed or heat can NOT do anything directly to the PCM, they aren't even connected).

-Something shorted with the wiring to the HVAC control panel, that when you turn switches and knobs on the HVAC its shorts that critical circuits and kills the motor, with the knobs to the off position, that short is no longer there.

-Something is shorted in the HVAC system that is high power and turning on that high power system results in disrupting the electric system so badly it kills power to the PCM.

Replacing the PCM with a similar unit, might have eliminated the circuits that are shorted, that is why its NOT dying.

With the substitute PCM does the AC actually come on and blow cold? Does the electric radiator fan or auxiliary fan (if you have either) come on?

Perhaps something did fail internally in the PCM, and when you replace it with the correct PCM, everything will go back to working properly. OR when you replace it with the correct PCM all the circuits will be connected back to the PCM and problem circuit will just kill the new PCM again.
 
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