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[JamesH]

I had rough running which then got worse and worse until undriveable. Eventually the check engine light came on but only after it was clearly a major problem. I had it scanned at a local Jeep specialist but only with a normal garage scanner and not a Jeep specific setup. They put on a MAP sensor and it was fixed.

Prior to that I had changed TPS, all the ignition parts, checked for air leaks and exhaust leaks, changed fuel pump and still not quite fixed the problem. All of these things can cause running problems.

[LairdScooby]

Hmm, best i look at the MAP sensor then. I've got a few non-Jeep MAP sensors floating around, if it comes to it i'll adapt one of those but i'll see if i can find a Jeep one first.

[bill99]

So I got a MAP sensor and fitted that but no real improvement so I draw a blank there.

Anyway I got into the cable loom with a meter and checked all the continuity from the TPS to the PCM unit and checked out the internal sensors earth link in the PCM as well, Pin-4.

It was while doing all this I realised the TPS pin (23) was sitting at 5-volts when the 3 pin connecter was off the TPS, in theory it should be low.

So check this out and sure enough it is sitting at 5-volts any time the power is on but as the TPS is normally connected the 5-volts is pulled down to 1.3-volts by the sensor ground.

 So when you check the sensor output it appears to work but it’s really just the grounding effect rising as the throttle opens.

So to cut a long story short I need to replace the PCM unit as it seems to have an internal 5-volt short circuit on to pin 23 where the TPS signal should go in.

So any help on fitting a second hand PCM unit from a scrappy would be appreciated, I hear it is programmed to the sentinel system so it wont just plug and play. Can anyone give me an outline of what is required assuming I can get one.

Also it follows if anyone wants to sell one for 1999 XJ 4.0 L petrol send me a message.

[LairdScooby]

Hmm, i had a look at the diagram i have for the 93 XJ 4.0 which also applies to my 95 XJ 4.0 and the TPS signal pin is Pin 22, not Pin 23.

I can't imagine changing the loom, the plug and the PCM just for the sake of shifting one pine along so are you sure you mean Pin 23 or is that maybe a typo, misread the diagram or something?

On a more technical note, there's a good chance the PCM has an internal pull-up resistor for biasing the signal processing, whether it's via a DAC/ADC, an op-amp or whatever. This 5.0V at the signal pin when the TPS is effectively open circuit is not necessarily a fault in itself, it could even be used byu the PCM to indicate a fault condition.
Next technical question, did you reset the PCM by disconnecting the battery for a few minutes after fitting the replacement MAP sensor? If not then it may still register the fault (even though it's not showing) and still be in limp mode. If you haven't reset itthen it would be worth doing before jumping the gun and condemning the PCM out of hand.

[bill99]

Hmm, i had a look at the diagram i have for the 93 XJ 4.0 which also applies to my 95 XJ 4.0 and the TPS signal pin is Pin 22, not Pin 23.

I can't imagine changing the loom, the plug and the PCM just for the sake of shifting one pine along so are you sure you mean Pin 23 or is that maybe a typo, misread the diagram or something?

On a more technical note, there's a good chance the PCM has an internal pull-up resistor for biasing the signal processing, whether it's via a DAC/ADC, an op-amp or whatever. This 5.0V at the signal pin when the TPS is effectively open circuit is not necessarily a fault in itself, it could even be used byu the PCM to indicate a fault condition.
Next technical question, did you reset the PCM by disconnecting the battery for a few minutes after fitting the replacement MAP sensor? If not then it may still register the fault (even though it's not showing) and still be in limp mode. If you haven't reset itthen it would be worth doing before jumping the gun and condemning the PCM out of hand.

Hi there yes definitely pin 23 for my TPS signal; (pin 22) on a 1998 onward seems to be called  (fused B+) in the diagram. I also did the battery and cleared the codes with a reader.

On my 1999 the TPS is going into the PCM connecter block nearest the headlight on the pins as below.

5-volt supply  = pin 17 (Orange wire)
Signal output  = pin 23 (orange with blue stripe)
Ground            = pin 4  (brown with yellow stripe)

Nothing would surprise me about Chrysler electrics after the battle I had to get to the transmission module under the dash, there was a possibility of that causing the 5-volts to appear on the TPS signal pin because that is shared with the Transmission control module.

You could be right about the 5-volts on the pin being a sense for errors or something, I have been looking for an internal circuit diagram of the PCM unit and a good operating description of it but no luck so far.

My PCM part number is P56041 494AD if I could pick one up and figure out how to frig the immobiliser and maybe other stuff I would give it a go just to see.

Cheers

[LairdScooby]

Trust Chrysler to fanny about with the pin numbers just for the sake of making the PCMs non-interchangeable! If you could PM me your diagram i'm fairly sure i could find the bypass points for the immobiliser (will need not just the PCM diagram but the complete car diagram too) so you could frig it - not happy about posting that on the open forum as it puts mine, yours and all the other members cars at increased risk.

Still not convinced the PCM or the TPS is at fault though but without looking deeper can't think what else. Mine behaved perfectly on a 200-ish mile round trip today until it got home and was idling on pterol after restarting and it cut out on the ASD. Restarted fine first time and then i blipped the throttle to get it back on gas - no troubles at all on that!

The pins on my PCM for the TPS (as far as i know, i'm going on the 93 diagram which i've been assured is the same as my 95 XJ) are :

4 Sensor earth - Black/L.Blue
6 Sensor 5V - Violent/White
22 Sensor signal - Orange/D.Blue

So.................. not only did they change the pinouts on the PCM but they changed the colour codes as well! Gits!

Could well be the TCU is causing the 5V pull up on the signal pin of the TPS but given that exctra information, i would expect the gearbox to do something odd with the shit patterns. Does your box shift normally? If so i'd hazard a guess the TPS is ok.

[bill99]

I think they made a big upgrade around 1997 / 1998 so lots of parts would change, that also ties in with the first Grand Cherokee models.

Anyway i was looking at module locations on in the books and there is a sentinel module I was hoping to maybe just remove that to try a new PCM  (wishfull thinking maybe).

with the replacement TPS doing the exact same issue and the TPS error code still coming straight back on after being erased I would still like to pick up a PCM just to try and eliminate it.

Cheers

[LairdScooby]

I think the upgrade/update/facelift was about 96/97 and i saw a P reg G/Cherokee today while out and about. That would be 96-97 registered so ties in with the facelift/upgrade etc.

I see where you're going with the PCM idea, i hadn't realised it was still throwing the TPS fault code after a reset.

[Jonny Jeep]

In the US vehicle electronics had to be OBDII in petrol models so a PCM redesign was required.

As far as the security on a 99 is concerned, the SKIM (Sentry Key Immobiliser Module) and PCM use rolling codes and both have the VIN programmed. If they do not recognise each other the system will not allow the engine to run for more than a few seconds. A PCM from the US, where SKIS (Sentry Key Immobiliser System) was optional, that did not have SKIS enabled would work. The SKIM needs disconnecting before the non SKIS PCM is connected or SKIS will auto enable in the non-SKIS PCM.

If you wanted to try a PCM from the UK you would also need it's corresponding SKIM.

[JamesH]

 

What he said. Removing the module in the steering column won't be enough. A non SKIS PCM from the U.S. might be a good idea though, I don't think they are too pricey.

[LairdScooby]

All of that makes a lot of sense and explains the need to alter the pin-outs on the PCM - what would be the point of having the immobiliser if it could be bypassed by fitting an earlier (non-immobiliser) PCM. Annoying (for you and your problem) that it has the software immobilisation too (rolling codes etc) as that makes life harder.

[bill99]

Thanks for all that about the immobiliser situation that explains why I see some jeep PCM’s on ebay that also include the remote key fob but it looks like I would also need the SKIS module as well.

Oh well if there is a way it may be an option if I get in quick when the right jeep comes into a scrappy and I can get everything from the same car of the right year.

I did read up a bit on the sentinel system and there seems to be a chip in the actual key as well so maybe I would need keys and the main steering column ignition lock as well.

Hmmm, a bit daunting.

Thanks Guys

[LairdScooby]

Sounds like a plan to find a complete runner in a scrappies - or keep your eyes peeled on ebay for a spares/repair car.

[Jonny Jeep]

Keys. Of course, they would be needed to match the SKIM. Forgot about that. It would be relatively easy to rekey your ignition barrel for the different keys if you did end up going that route.

[bill99]

Bit of an update.

I got my XJ connected up to a professional diagnostic tool running Jeep software and sure enough it came up with the 0123 code of the TPS circuit high.

So this tool can monitor all the sensors and their voltages in real time so with the engine running we can see that the TPS signal never moves so the PCM cannot read the signal even though the multimeter shows it going up and down on the PCM connector pin when the throttle is opened.

All the other sensors read and change with engine revs, a really good tool it can measure all the supply and signal voltages to everything but it's £1500 plus on going software upgrades charges.

So that kind of does it, it seems that the PCM unit is faulty.

We are looking into a repair from a specialist to see what costs are but I have had a few warnings from people about an attempted repair making it worse.

Just wondering has anyone here ever had a repair done with good or bad results?

[Mike Pavelin]

Borrow another PCM (any '98-99 XJ or TJ one will do, plug it in, don't bother to try to start it, just do the same test again and see if it can see the TPS working. You'll have to ignore the SKIS errors it'll throw up, but it'll prove it isn't a weird wiring fault without the aggro of programming a new unit.

[bill99]

Borrow another PCM (any '98-99 XJ or TJ one will do, plug it in, don't bother to try to start it, just do the same test again and see if it can see the TPS working. You'll have to ignore the SKIS errors it'll throw up, but it'll prove it isn't a weird wiring fault without the aggro of programming a new unit.

Well that's a good one Mike.

So in theory i could buy an American one off eBay just to test, they seem to be more available there.

Cheers

[Mike Pavelin]

If you getting one from the 'states, you can get them without SKIS, as it was an option out there. It would need to be from an automatic XJ 4.0 of the same model year. It might throw up some strange codes for some odd emissions related stuff as they have different equipment for some states, but you can just ignore those. The other option is to try one for a UK spec 1996 model, which is the last of the pre facelift with a 3 plug OBD2 PCM, which didn't have SKIS or VTSS enabled so should in theory plug and play. 

[bill99]

My replacement used PCM unit has arrived from the US and it does not have the SKIS enabled so I get that I need to disconnect the cable off steering column module for the SKIS system before I plug in the PCM.

Up to that point im ok is there anything else I need to know about before I plug this PCM in and power up?

What about the VTSS that Mike mentioned, I take it that is an alarm system but is that not part of the immobiliser (SKIS) module or have I got that wrong is there another module somewhere on the car for a 99 XJ and if so will that be an issue with the US sourced PCM

Having read up on the central locking I can’t see any problems and I think that will still work, as it seems to be independent of the SKIS

Any help or tips appreciated because if possible I would like run this new PCM permanently if I can, and not just confirm or dismiss the error codes that I have with the current PCM and TPS error codes.

bill