engine dies when heat/ac is turned on?

[Rick Anderson]

Yea, all the speculation about the PCM thinking its in overdrive, etc.... means nothing.

Overwhelmingly likely, if he just troubleshooting by dropping in a similar PCM that is NOT for his vehicle/engine, its barely adapting and working in some conditions and the highway speed is outside the PCM's ability to adapt, so its runs like crap, probably the wrong spark advance and/or fuel injector pulse width for that condition.

 

[Sidewinder CC]

Rick,

For future reference, below is a pin out of the AW4 Transmission Control Module for '97+ TCM's. You can see the various inputs to the TCM. The TCM communicates with the PCM through the CCD Bus.

The TCM has a self diagnostic program and when a malfunction in the transmission control system is noted, the TCM will store the DTC. The PCM will 'see' the DTC through the CCD Bus and set a P0700 DTC, indicating there has been a DTC set in the TCM. Most of the time the actual TCM DTC will be displayed as well as the P0700 code.

The PCM does have inputs that govern the operation of the engine that are related (somewhat) to the TCM. Note the inputs to the TCM from the Transmission Range Sensor (called NSS hereabouts). If the TRS fails in the Park or Neutral positions the PCM will not allow the engine to be cranked.

If the Vehicle Speed Sensor input to the PCM (only) fails, the TCM will not be able to compare vehicle speed to transmission speed and shift points will be lost.

TCM pin outs:

Relationship of the TCM/PCM on the CCD Bus:

 

[outracing]

Okay, so I have replaced the computer with a reman unit with the proper software installed. It solved the reboot/shut down when the HVAC blower is moved out of the off position. However, now, after driving it for about 30 minutes, it will not restart? Again, I have no power to the coil. Is it possible that my Crank Position Sensor has also gone bad? It starts just fine when it is completely cold. Also, after it dies I still have power on two posts of the ASD Relay. Against the wall here. What do I do?

 

[outracing]

Okay. So there are two fuses under the hood that are marked for ASD. I do not get power to either one of them. Before some one asks, I did swap in a known good relay into the ASD slot.

 

[mustang_gt_350]

Have you checked the grounds on the rear of the engine block on the passenger side. They ground right where the dipstick mounts.

 

[Sidewinder CC]

Have you checked the grounds on the rear of the engine block on the passenger side. They ground right where the dipstick mounts.

That ground point is for the Data Link Connector.

 

[mustang_gt_350]

That ground point is for the Data Link Connector.

On my 96 that i had problems with That is where Pins A31, A32 ground. Just throwing out something for him to check since it looks like most everything else has been looked at.

 

[outracing]

Hey Mustang, thanks. I have been through every wire and connection, for right now everything is running like it should. I'm gonna avoid thehighway for a bit, as long as I get a couple tanks of fuel through it i will resume normal driving with this thing. if problems crop back up, i know where i'll start looking. Keep your finders crossed for me that I don't have to update this thread. Thanks everyone for your help.

 

[Automechnic077]

I know this is an old thread but prior to me posting a new one and asking all the same questions has your issue been corrected? I have a 99 sport same issue. Heat, cold, 1,2,3,4 no matter the setting as soon as I turn on the switch the Jeep dies. Dash needles fall, everything freezes. Cycling the key on off while the blower is still running won't let the fuel pump kick on. Turn off the blower, cycle key it fires right up.HOWEVER if I apply the brake everything is kosher. Heat AC compressor works blows cold. But as soon as I depress the brake. Bam dead... I feel like I've been through everything but a new computer..

 

[outracing]

So apparently quite a few people are having the same issue as I was so long ago. It is been awhile since I have logged onto this forum and apologize for not posting what was my final fix.

The PCM in my beloved XJ head finally bit the Dust. After countless hours of research and talking to anybody that would listen I finally decided it was time to replace the PCM the problem with that was it is very hard to find an exact match. A mechanic that I talk to at one of the junk yards compared it to donating a kidney. Sure, they all do the same thing and will operate the same way but your XJ will have to accept the donation. The only way to make sure that it will not reject the new PCM is to get one from the same here with the same engine and transmission and even the interior package and amount of miles will matter.

There are tons of serial numbers model numbers and barcodes printed right on the PCM. All of these numbers must matches well. Once you have the proper PCM it is a simple plug-and-play operation.

It was such a good feeling to be able to turn on the blower for heat or AC and have the Jeep stay running.

 

[Automechnic077]

Welcome back! Thanks for the reply, I replaced my PCM as well after a year of fiddling with everything. My XJ has never ran so well. I wish I took the $300 gamble a year ago!