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Old 12-13-2015, 03:24 PM
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98 tj 4.0 pcm question

So after the long tracking issues of my tj losing fuel and spark and running many different tests and testing components, Finally pin pointed the issue being a fried pcm. Had a buddy come down and swapped his pcm out of his 98 xj in and it fired right up. Question being will an xj pcm from a 98 with the 4.0 work in a 98 tj or being that the trans is different and other odds and ends will it not work long term? Thanks Ethan

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Old 12-14-2015, 07:23 AM   #2
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If you want plug and play with no codes depends

If you have a manual tranny
Same year, same engine, also manual and same emissions California with purge pump or federal without peeve pump
With manual TJ or XJ ok


If you have auto then XJ auto PCM won't work right for converter lock up and will throw a code
So only TJ same year, same engine same tranny and same emissions

If you just want one to make it run and don't care about codes
If your TJ is manual tranny any 97-99 4.0 TJ or XJ any tranny
If you have auto and you want torque converter lock to work normally back to only TJ auto

Finally any 99 or newer PCM may have SKIM activated and if it does it won't work without SKIM module it is married to and it's already learned chipped key(s)

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Old 12-14-2015, 07:25 AM   #3
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However if only issue with your PCM is no longer produces 5 volt output for sensors an external 5 volt power source with your PCM is the cheapest easiest option
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Old 12-14-2015, 09:53 PM
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Yeah mine has no spark or fuel. I've tested every part and the distribution box and everything seemed fine. So I swapped a computer from an xj same year same motor and it ran so pcm it is. I have the 32RH so 97-99 with an auto will work I'm assuming then?
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Old 12-14-2015, 10:27 PM   #5
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Well if no 5 volt power from PCM to crank sensor you will get no spark or injector pulse

XJ uses different auto and a separate external transmission control module

Need TJ same engine plus auto plus same emissions plus if get 99 watch out for sentry key

I would check your PCM for 5 volt output first as that is probably most common
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Old 12-14-2015, 10:47 PM
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Yeah the xjs are the AW4. Yeah the whole chipped keys and security systems. That's why this whole thing is a pain in the ass. Excuse my lack of diagnostic knowledge but how exactly would you test to see if your pcm is putting out the 5v?
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Old 12-14-2015, 11:08 PM   #7
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PCM white plug pin 31 wire color VT/OR
And
PCM black plug pin 17 wire color OR

Both should have 5 volts with respect to ground when key is on

On these two wires all PCM does is supply 5 volts to power various sensors
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Old 12-15-2015, 09:12 PM
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How would you wire an external power source to pcm if those aren't putting out enough juice?
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Old 12-15-2015, 09:27 PM   #9
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That part is simple
You get a Walmart USB hub or phone charger that has decent amperage output and is designed to run off a cig lighter plug

Then power it from a 12 volt source on whenever the system is in start or run mode

An easy to access power source is the power wire connected to the coil

The coil receives a constant plus 12 when in start or run mode and a pulsed ground from the PCM to generate the spark

So power your 5 volt supply from the coil hot wire and a body ground




On a separate note
To check for current 5 volt from PCM can easily tap into 5 volt feed wire at the sensors with a meter and see if the five volts is miss g
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Old 12-16-2015, 08:53 AM   #10
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Never opened up a PCM myself but this might interest you

https://m.youtube.com/watch?v=uY62r-rtvls
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Old 12-20-2015, 12:28 PM
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So if I wire an external 5v to run juice to various sensors to give me power for fuel and spark will it make anything go haywire like in the guages or anything else?
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Old 12-20-2015, 01:43 PM   #12
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Not as long as on for crank and start and a decently regulated 5 volts with enough amps for the sensors which is not very much

If you want to get real fancy tie the 5 volt DC ground output to the sensor ground wire

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