Tell me everything about computers

I have a stock 99Sport/AT/NP231.

Looking at the 2000FSM and the only computer reference I found was in the emissions section where PCM was spelled out, but ECM was not...Search for either on this website, returns zero results.

The question is:

How many computers my XJ has, whats their function and where are they physically located? Pictures are more than welcome.

I was told by my junkyard gury, that if I want an "engine computer" I need to get the numbers off mine, otherwise he wont be able to sell me the right one.

Also, I read in the Haynes manual that before changing any of the sensors, I should be calling the repair facility, because due to Federal Emmission Standards, they would be replaced for free. Anyone had sensors replaced for free? Does anyone have a list of the sensors that are being replaced for free?

Thanks in advance

Generally, the emissions system components are good for 8 year/100,000 warranty.

You have an Engine Control Module, mounted on the inner drivers side fender next to the air box. You also have a Transmission Control Module, mounted inside the cab under the insrument panel (on a 96 anyway). A PCM or powertrain control module is usually used when the same module is used to control the engine and transmission. You may also have a GEM or general elecronics module that controls interior lighting, buzzer, and other random things. I'm not even sure if you have it or not, someone else may chime in. There may be a few more that are in a 99 that arent in my 96 so I wouldnt know for sure.

I think on 99's the 'computer' is the little corroded aluminum thing between the airbox and the edge of the engine compartment, mounted to the fender with the giant wiring harness going to it.

What happened to yours?

TacticalFats said:

I think on 99's the 'computer' is the little corroded aluminum thing between the airbox and the edge of the engine compartment, mounted to the fender with the giant wiring harness going to it.

What happened to yours?

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I am down to a controller module, I think. I have a fuel delivery issue. It bucks from time to time. I have changed TB and all sensors around it ($30 for the whole package, that was a sweet deal..., and the two oxygen sensors (brand new). Each time the behavior is different, for example: after I changed the O2 sensors, it started to die right before it reached its normal temperature, but it went away after a week and it bucks way less. So it makes me think that it is the control module. Before it all started, some idiot had it "reflashed at the dealership"...coz he couldnt find that the real problem was severed fuel pump harness...

P.S. I also changed the fuel pump at 140K as part of preventative maitenance

Engine Control Module (ECM) is a generic term for the Engine Computer.

The Chrysler Specific term for their Engine Computers after 1994 or so, became Power Control Module (PCM).

The software for a PCM is loaded like firmware, it is tailored for specific equipment and options of each vehicle and by year/make/model. At least for Chrysler, a specific Part Number is assigned a PCM according to what software is loaded and the flash tools that load the software will NOT load software for other part numbers. For example, if you put a PCM for a manual equipped vehicle into your automatic equipped vehicle, likely the transmission will burn up, because the PCM will NOT lock the torque converter since the software was NOT designed for that. So you have to have the right part number for the PCM, the wrong PCM might very well plug-in and even work, but its likely it won't work right and may even do damage to the vehicle. Some part number PCM will cross over the years and different configurations of option/equipment, rarely for models.

A major supplier of rebuilt Engine Computers stated in an industry magazine once, that 95% of the cores they receive for Engine Computers have absolutely nothing wrong with them. Most people in the industry will agree that probably the number one misdiagnosis is replacing the Engine Computer, 95% of the time the ECU is just fine and the mechanic/owner simple was stumped and just jumped to conclusion that the engine computer must have gone bad.

Rick Anderson said:

Most people in the industry will agree that probably the number one misdiagnosis is replacing the Engine Computer, 95% of the time the ECU is just fine and the mechanic/owner simple was stumped and just jumped to conclusion that the engine computer must have gone bad.

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Usually, if the problem is intermittent its not the computer. Usually an engine computer failure is obvious, like no spark or an injector not firing, etc. The tranny computer is pretty bullet proof too, although I have seen a few reports here of confirmed failures. Knowing that 95% of the time the computer is fine, I feel slightly guilty pulling computers at the junkyard and reselling them on eBay for a profit.

Anyway, if you have another computer handy its a nice way to verify. I'm willing to bet your problem is still a bad sensor or a wiring issue.

You're past the emissions warranties, so I would actually avoid the dealer (they're likely to tell you its a bad computer and want to charge $720 for a new one).