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  1. #1
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    Unlocking the extra features in the JTEC PCM (high idle, fan control, etc.)

    Me: 1
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    After a bit too much time probing around my PCM harness, the wiring diagrams in the ZJ and XJ FSMs and the SCT software, I now have a working XJ-style high idle switch. Idles up to 1000 rpm when the switch is on in P or N, idles normally the rest of the time.

    The process for adding it isn't nearly as simple as an XJ, however. An XJ (4.0 auto only) just needs C1, Pin 12 in the PCM harness grounded and it'll idle up. OBDI ZJs can't do it, AFAIK. OBDII ZJs can if they're SCT tuned. There's a setting in the system switches portion of the SCT software to enable "extended idle mode". Turn this on. Then do into the K1 diagnostic settings and turn off the EEPROM check (you'll get a P0601 if you don't do this).

    Now for the hardware side of the install: disconnect the black C1 connector from the PCM and open it up. Add a pin to position 12 (there isn't one from the factory) and wire it through a switch to an ignition switched 12v source. Have it set up so the switch is normally on, feeding 12v to that pin. This will cause it to idle normally. When you flip the switch and remove the 12v from C1, pin 12, the engine will idle up to 1000 rpm if the trans is in P or N.

    As an important note, make sure you do the hardware bits when you do the software bits (or before). If you set up the software tweaks without feeding 12v to that pin on the PCM, it'll be stuck idling at 1000 rpm in park and neutral (high idle will be always on).

    Next project will be attempting to get the JTEC fan control working. I know one of the fan relay outputs should be on C3, pin 2 (XJs use this). The SCT software shows settings for both low and high speed fan operation, so I'm thinking there's a second output somewhere. I have no idea where, however. If anyone has an FSM from a JTEC based vehicle that's not a Jeep, it would be helpful if I could get a copy of the PCM connector pinouts from it. If I can get this figured out and working well (probably involves adding more things to the PCM harness and adding pins to spots that don't have them in the connectors), it should mean I can trigger the e-fan relays based on the PCM outputs and the A/C pressure switch and then get rid of the t-fittings in the rad hoses and clean up some wiring in the engine bay.

    - - - Updated - - -

    I just found a pinout for a JTEC in a Viper, looks like C3, pin 2 is low speed fan, C3, pin 21 is high speed fan. At some point, I'll crack the harness / connectors open again and add pins / wiring to those 2 and see if I can actually control the fan with them.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  2. #2
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    So it turns out that I still get a P0601, even with the EPROM check disabled (although it doesn't show up until the 3rd key-on after flashing, rather than immediately when the check was enabled). There must be something else causing it to freak out over the extended idle feature being enabled. I'll poke at it more tonight and see what I can find.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  3. #3
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    Fixed it! There's an additional check to disable. It's under one of the trans diagnostics pages, called "ROM Check". Once that and the EPROM check are disabled, it doesn't seem to throw the CEL anymore.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  4. #4
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    You had my moral support throughout your PCM frustrations. Glad you got it licked.
    "magic suspension made from unicorn tears"- can you get those on ebay?

  5. #5
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    Quote Originally Posted by carlmon View Post
    You had my moral support throughout your PCM frustrations. Glad you got it licked.
    "magic suspension made from unicorn tears"- can you get those on ebay?
    I think so. The suspension is actually still a work in progress in terms of control arms and such. But so far, the rest of it's an Addco front bar, stock rear bar, cut down skyjacker lift springs up front, cut down stock front springs in back, Bilstein 7100 shocks (valved 400/100 front, 360/80 rear, going to change to 360/70 rear soon). Sits about 1/3" above stock in front, 1/2" up in the rear (still below upcountry height). Rides great, drives great, handling is very neutral.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  6. #6
    Forum CONDUCTOR Man Z88Z's Avatar
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    Do you know if it's grounds that control the Efan outputs? I assume it would be.

    You'd have to rewire the high speed slightly to have it work off a ground, but that's NBD if you're going to that much effort. (assuming you wanted to bypass or eliminate the ignition relay)

    hmmm... You could leave the Ignition Relay in place and just run the PCM outputs to their respective coolant temp sensor connectors, or you could go in right through the diagnostic connector and eliminate or tuck the wires to the sensors.

  7. #7
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    I haven't figured out just how I'm going to I'm going to wire it up just yet. I need to keep at least part of the stock e-fan harness for the A/C pressure switch (don't think the necessary bits exist to trigger the fan based on A/C pressure demand through the PCM).

    Once I get a chance to add the pins to the PCM harness, I'll check whether the PCM switches +12V or ground for the fan outputs. Knowing Chrysler, it's most likely ground switched. I need to probe at the e-fan harness to figure out if it feeds ground or +12V through the stock switches. If the current setup doesn't match the PCM's outputs, I'll just add a pair of relays that jumper the relevant 2 wires in the fan harness together (like the stock thermoswitch closing) when the PCM activates those relays.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  8. #8
    Forum CONDUCTOR Man Z88Z's Avatar
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    Quote Originally Posted by comptiger5000 View Post
    I haven't figured out just how I'm going to I'm going to wire it up just yet. I need to keep at least part of the stock e-fan harness for the A/C pressure switch (don't think the necessary bits exist to trigger the fan based on A/C pressure demand through the PCM). ....

    I was wondering about just that.

    You don't think it's in the PCMs architecture to do that? Can you get ahold of the wiring diagrams for something that uses it and see of they run something external to the PCM for that? (I think you said Cherokee and Viper?)

    Have you seen any indication that the PCM has internal protection that functions like the Ignition Relay, so only one speed can be powered at a time? I would ASSume it had that, lol - but it could still use an Ignition Relay or something external.



    Were you particularly looking to reduce the amount of Efan control wiring and/or not use the Ignition Relay?

    Easiest way - meaning least amount of work and wiring modifcations, and that's assuming it operates off switched grounds - would be to run the outputs from the PCM right to the TAN/BLK and TAN wires at the temp sensor connectors. That would allow everything to function as normal. The A/C pressure switch would still do it's thing and the Ignition Relay would still be in there for protection (if it needs it)

    Most stripped down way would be to run the outputs right to the high and low relays. You'd just have to make the high speed relay permanently powered instead of grounded like it is now, but that's easy enough - just tap into the power wire for the low speed relay next to it.
    Of course that depends on whether it has the internal protection and whether it has an internal capability to operate the Efan off the A/C demand.


    FYI I noticed the Efile version of the FSM has two Efan diagrams, one of which shows the A/C pressure switch on the correct low side. Somehow the paper FSM got the "wrong" one that shows it operating the high speed. Per the Efile FSM that diagram is actually for the Euro CRD ZJs, or maybe the 6 cyls or whatever.

  9. #9
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    I was planning to keep the stock relay setup, I just want to get rid of the t-fittings in the rad hoses. I think the Vipers (which did 2 speed fan control from the JTEC) did trigger the fan through the PCM for A/C demand. I'll check into whether that pin is used for anything on a ZJ or not and if I can determine whether the logic exists in the software. Then again, the way I'm planning to wire everything, it's about the same amount of wiring to control that through the PCM or leave it as-is.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  10. #10
    Member SilverXJ's Avatar
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    The XJ PCM triggered the fan for AC demand as well.

  11. #11
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    Hmm... Looks like 2000 - 2001 XJs have a pin for "radiator fan request". ZJs use that pin (C13) for the OD off switch. Not sure what the rad fan request is. However, pin C22 is an A/C pressure switch sense on both XJs and ZJs. So it's quite possible that it'll trigger the fan on A/C demand without changing anything (this would allow the extra sensor in the 5.9 A/C line to be disconnected, and for the cheaper 5.2 line to be used as a replacement if one is needed).

    I'll definitely try that out when I get a chance to play with this. If it works, then I'll be able to shorten up the stock fan harness a lot, so it will only have to run between the fan relays and the PCM, rather than running all the way across the engine bay.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  12. #12
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    So I'm getting back to work on this. I dug into the XJ wiring diagrams and determined that the PCM switches ground to trigger the radiator fan relays. I need to confirm whether the stock 5.9 fan setup is switching the ground line or the +12v line to the relays, get some connector pins to add to the PCM harness and wire it up and see what happens.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  13. #13
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    Found a diagram for the stock 5.9 fan setup. The stock switches provide ground to the relays to trigger them, so this should be really easy to wire up once I get the correct pins added to the PCM harness.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  14. #14
    Forum CONDUCTOR Man Z88Z's Avatar
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    So the PCM demands the fan speed based on what it reads from the water temp sensor I guess? And maybe the AC request?

    Did you figure out if it actually cuts out the ground for the low speed when it requests the high speed? Or does it still rely on an Ignition Relay?


    That would make the wiring ultra simple, just grounds from those two pins right to the relays, and adding ignition sourced power to the High Speed relay. Well, adding ignition sourced voltage to both relays actually since you'd theoretically be bypassing the Ignition Relay.
    Just divert and extend the orange wires from pins 1 & 2 of the Ignition Relay to power the High/Low relays.

    That's if the PCM does it's own cutting out of the Low Speed request ground when it request high - otherwise you'd just go Low Speed ground to the Low Relays and High Speed ground to the Ignition Relay, more or less like factory.



    Did you ever figure out if the JTEC will actually run an RH tranny?

  15. #15
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    I don't have a way to test the RH trans stuff, unfortunately.

    As far as the fans, I haven't tested whether it cuts the low speed when high kicks on. I'm probably going to just leave the ignition relay in place so I don't have to screw with the wiring for the fan relays (laziness comes in at this point). I've got some chopped off PCM connectors coming in from ebay soon, so I'll have pins and such to use to do this at some point in the next couple of weeks.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  16. #16
    Forum CONDUCTOR Man Z88Z's Avatar
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    So is that actually how they are set up to work on the Viper, and/or the XJ? Off the PCM? (I'm really not familiar with XJs even. Didn't know they had electric fans)

    Maybe the Viper or XJ wiring diagrams would show if there's still an Ignition Relay, or something external from the PCM that serves the same function?


    On YellowJacket I basically kept the system functioning like the stock system but was pretty much able to make everything invisible by incorporating it into the PDC and main harness through the C131/C132 connectors. C144 connector no longer exists! lol!
    (*That's with the exception of the high and low relays which are now hung on the side of the PDC - those and the wiring from there to the Efan are the only remains of the Efan harness.)

    One of the stock type relays in the PDC was added into an empty slot and now handles the Ignition Relay functions.

    I had the same idea as you about finding a way around having to use the 5.9 specific AC line, although in my case it was just to clean up the engine bay a little.
    I added another of the stock type relays into the PDC. It has two permanent grounds, one for the coil and one for the contactor. The other side of the coil gets power from the output of the AC compressor clutch relay when the AC compressor clutch is powered, which closes the new relay and sends a ground to the Low Speed fan relay like the AC HP line switch would.
    The AC compressor clutch only operates when the demand conditions are all set, so the new relay is only powered when it's running - so theoretically this duplicates the function of the AC line switch. I never charged the system, therefor it hasn't been tested, lol

  17. #17
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    If I can find a Viper wiring diagram, that should answer how they did it for 2 speeds. XJs only have a single speed e-fan (as a secondary to the mech fan and for A/C demand), as their mech fans only cover about 1/2 the radiator (the rad is wider and much less tall than a ZJ).

    The A/C line switch definitely leads to the fan running less often than the compressor does. For example, doing 50 mph with the compressor running, there's enough airflow cooling the condenser that it doesn't hit the threshold for fan activation.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  18. #18
    Forum CONDUCTOR Man Z88Z's Avatar
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    Quote Originally Posted by comptiger5000 View Post
    ....The A/C line switch definitely leads to the fan running less often than the compressor does. For example, doing 50 mph with the compressor running, there's enough airflow cooling the condenser that it doesn't hit the threshold for fan activation.
    hmmm... interesting.

    I'm not all that knowledgeable on AC I guess.

    I thought it just cycled on/off in relation to the pressures. So there's times when the compressor is running that it's putting out different amounts of pressure? I guess in response to changes in pressure from the amount of cooling maybe?

    Any idea what the Efan AC switch closes at in relation to the variance in pressure? I just kinda figured it closed with any increase in pressure rather than at a well defined range or minimum. Maybe that's in the FSM description of the system - I'll have to check that out.

    I always figured the Efan mostly served to keep air moving over the condenser when the vehicle was stopped. So if that's the case mine would just run more than it needed too - could be worse.


    Also, if that's the case (...and with Efans working off the PCM) then there'd be no way to duplicate that function without still having to use that AC line switch right? The PCM only knows your request for AC/Defrost, and cycles the compressor clutch as the high/low switches are in their proper range.
    So there'd still be no way for the PCM to know when the line pressure was in the right range to close that Efan switch, right?


    The XJs and Vipers that use this probably still have an AC line switch I guess? I'll have to look around the webz and see if I can find some wiring diagrams
    Last edited by Z88Z; 03-28-2015 at 08:05 AM.

  19. #19
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    No idea what pressure threshold triggers the fan for A/C demand. The high side pressure climbs as the condenser temperature goes up (more heat in the system means more pressure), so it only runs the fan as needed (in hot weather, it'll be on pretty much any time you're under 10 - 20 mph, it'll be off when cruising with A/C on, as the condenser stays cool enough without the fan running).

    I'm not sure how the XJ and Vipers trigger the fan for A/C use, unfortunately. I can't think of an input that duplicates the extra switch's functionality. I might just leave the additional switch hooked up and not worry about it, it's still simpler (and the better control over e-fan for coolant temps as well as removing the t-fittings is most of what I care about).
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  20. #20
    Forum CONDUCTOR Man Z88Z's Avatar
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    Keep us in the loop. I'm anxious to see how this works out.

    My 93 build YellowJacket was all about cleaning up the engine compartment.

    I made most of the Efan wiring invisible, but if I could eliminate the hose sensors and their wiring it would be that much cleaner and that much less wiring in the harnesses. (I could have eliminated a lot more if I did away with various VIC inputs but I didn't find out about the VIC delete cubby until I was mostly done with the harness)

    As it is, if what you're saying about the AC switch is right - my Efan might run on low speed a little more than it needs to, but that's no huge deal I guess.


    This is the only visible portion of my Efan harness now.

    This, and a small portion you can see going out to the hose sensors, but even that's mostly hidden.












  21. #21
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    Incredible Mr. Z!

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    Forum CONDUCTOR Man Z88Z's Avatar
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    Quote Originally Posted by MexiJeeper View Post
    Incredible Mr. Z!
    Thanks Tony!

    I've been meaning to do the build writeup since like, forever! LOL

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    Quote Originally Posted by Z88Z View Post
    I've been meaning to do the build writeup since like, forever! LOL
    What did you do with your evap canister? I need to move mine to fit my intercooler heat exchanger.

  24. #24
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    Well, I now have the extra PCM connectors I got off ebay, so I just have to get off my lazy butt, swap 2 of the pins / wires into my connectors and wire up the e-fan stuff to the PCM so I can turn on the e-fan stuff in the PCM and test it out. I'll probably get to it one evening this week.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  25. #25
    Forum CONDUCTOR Man Z88Z's Avatar
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    Quote Originally Posted by comptiger5000 View Post
    Well, I now have the extra PCM connectors I got off ebay, so I just have to get off my lazy butt, swap 2 of the pins / wires into my connectors and wire up the e-fan stuff to the PCM so I can turn on the e-fan stuff in the PCM and test it out. I'll probably get to it one evening this week.

    You're smart Rob, so you probably already knew this but.....


    To remove/add pins into the PCM connector all you need to do is slide that light blue tab over and very firmly pull the pins out from the back/harness side. (That's after you take that taped-on 90 degree angled shell off the connector, and you can only see/move the blue tab with the connector removed from the PCM. It only moves a tad, which is all you need. Maybe like 1/32" - if that)

    First time I tried to remove a PCM pin I disassembled the connector Wish I'd figured out that trick back then - friggin nightmare trying to get it back together! LOL!


    Also - see the EVAP resistor thing mentioned below. Now might be a good time to do it if it interests you.




    Quote Originally Posted by carlmon View Post
    What did you do with your evap canister? I need to move mine to fit my intercooler heat exchanger.

    ummm - I tossed it?


    It wasn't about performance as our emissions controls don't really affect that much (with the possible exception of that stock catalytic converter robbing it of a bit of power) and it wasn't really even about weight as that stuff altogether weighs maybe 3 to 5 lbs or a tad more.

    I just did it to have less plumbing and wiring in the engine bay.

    I removed all the EVAP stuff and it's plumbing, plus the canister, plus 6 to 8 feet of wire - times 5 wires!

    Those wires come all the way over from the PCM/PDC area, along the harness, across the bumper support, and up the left side harness. FYI, all 5 of those wires are there on stock ZJs - even FED spec ZJs still have the 3 wires for the LDP/Leak Detection Pump that's only used on CALI spec ZJs.

    To fool the PCM into not giving a check engine light I used the 100 ohm resistor trick that Ross posted. I put the resistor right into the harness across the Orange and Pink/Black Duty Cycle/Purge Solenoid wires about 8 or 10 inches away from the PCM. Then pulled the 2 pins for the Leak Detection Pump out of the PCM C3 connector and yanked all the extra wire out of the harness.

    FYI, that resistor trick only works on FED ZJs that only have the DC/PS. It won't work on CALI PCMs that are looking for their LDP. I believe Ryan said he can convert CALI PCMs to FED specs while he's doing the tuning though.

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