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Alternator wiring MPI swap

5K views 54 replies 5 participants last post by  Toms-CJ7 
#1 ·
Hi everyone! Got a 1989 4.2L with the 4.0 MPI swap out of a 1994 Cherokee, I was wondering if it's possible to wire the green and green/orange alternator execite wires to my 4.2L alternator somehow? I'd like to remove the code 41 check engine light from coming on every time I drive. Is this possible without having to completely buy a 1994 alternator?

I have the alternator "black box" from a 2.5L harness in a junkyard also
 
#2 ·
No. The alternator control circuit would need to be wired through a load in order for the light to go away. Swapping the alternator isn't that easy either because the bracket is different.
 
#4 ·
Yes, you can run a load across the alternator field wire. The right inductor/Coil should do it. I haven't looked at it to figure out what value.
 
#6 ·
If you connect a coil across the wire from the alternator the ecu will think it's functioning is good. The alternator that you are using is internally regulated so it doesn't need the ecu. Yes an inductor should work.
 
#13 ·
Unfortunately no. I have replacement gauges, including speedometer. The speedometer uses GPS for accurate speed but also uses a transmission adapter to give pulses, similar to the unit used by the MPI kit. The speedometer unit spun freely and broke the center pin and obviously stopped working. I was going to short cut it and share the same pulse output that goes to the ECM. I thought I checked everything out for voltage level et all, buttoned it up and started the jeep for a test drive. Jeep started ran for about a second or two the stopped permanently. Eventually hooked the scanner up and not had a multitude of sensor failures and intermittent communications with the ECM. Pretty much looked like a blown computer. (Basically I rushed the job since the wife was calling my day...basically stupidity)

Since I have not found a replacement house that will rework the unit I decided to open it up and check it out. When I started peeling off the conformal coating, I started to pick up the distinct odour of burnt circuitry. Once I got most of the coating off I found a burnt trace on the board, so bad the trace had peeled off the board. So at a minimum
I over currented a conductive path, probably took out a driver as well, though is could possible be a ground path and I just wired the speed sensor in wrong. If it is just burnt traces, I may be able to work around that and resurrect the controller, though that is going to take some time so my first effort is to get the donor 94 YJ ECM integrated. I will have to troubleshoot it blind as I have not found any circuit schematics for the ECMs. It will just be manual tracing and ohming out. Another option is to take the EEPROM out of the ECM and place in into another donor unit. Only issue with that is I don't know if MOPAR did any revisions to the various ECM models that would make the firmware incompatible, so that is a last resort option at this time. The first donor ECM has fired up the Jeep, so I am making forward progress. I just want to get a good power system setup without an ugly kludge.
 
#20 ·
Weird on the calibration unless its the California Mopar kit meeting CARB emissions requirements. Plenty of peeps have swapped in the fuel injection from the 91/95 to the 4.2L and I haven't heard about the premium fuel issue.


Back to the alternator. 10 ohm resistor should do it. You don't need a diode. unless your current setup requires one. Your field control for the self regulating should be switched ignition. The diode is used to stop the backfeeding through the battery to the coil. Your Jeep was running well before so what ever you had shouldn't be effected by just swapping the controller.
 
#21 ·
Weird on the calibration unless its the California Mopar kit meeting CARB emissions requirements. Plenty of peeps have swapped in the fuel injection from the 91/95 to the 4.2L and I haven't heard about the premium fuel issue.

Back to the alternator. 10 ohm resistor should do it. You don't need a diode. unless your current setup requires one. Your field control for the self regulating should be switched ignition. The diode is used to stop the backfeeding through the battery to the coil. Your Jeep was running well before so what ever you had shouldn't be effected by just swapping the controller.
All I know is the kit stated premium fuel and there are a lot of comments out in the web about heavy pinging both in and out of Calif. Might be the difference between junk yard kits and the commercial kits. dunno. Will find out when I get tested. If I fail for emissions mostly CO related I will either have to pull the PROM for the old unit and hope the program is compatible with whatever revisions may exist on my donor units or manage to fix the old computer. For now I'm going with the donor and crossing fingers.

I'm was guessing I need the diode in order to pass the field coil short circuit test. The fault code 41 write up indicates the ecm checks for both open circuit and short circuit. I was thinking the battery voltage would sneak back down the circuit test without placing in a blocking diode in the path. I still haven't studied the wiring to confirm the diode will do that, however that was my logic.
 
#26 ·
what i did was to use a 136amp alternator for a doge or jeep grand cherokee

you have to flip the alternator upside down and do a little grinding to make it fit.
my yj is a 1990 with 4.2L block with a 4.0L head and 99 intake obd1
Thanks, If I can't get around the generator field fault test I will have to resort to converting to an alternator without internal regulator.

Do you have to grind down the entire rear mounting ear or just shorten it a bit?
 
#29 ·
Toms-CJ7 Do you have to grind down the entire rear mounting ear or just shorten it a bit?[/QUOTE said:
If you look in the first photo you can see a boss on the alt. It is the rear boss I don't have a rear bracket on it.

Its good to hear you got code 41 cleared. The iac is discreetly control d by the pcm I think it is a 4 step stepper.
 
#30 · (Edited)
Thanks, I'll keep in in mind if I ever decide to go the YJ style alternator. Since I got the code 41 cleared. At the moment I don't need to change out the alternator.

As for the IAC, I only had the very high idle what seemed coincident with playing around with the gen field circuit. However I ended blowing out my original ECM by doing the stupid sequence of events described earlier whith what looks like sending 12V down a sensor ground line. It is possible that could have damaged my IAC - I don't know yet. Will have to wait a few start cycles and see if the high idle reoccurs. If so then the first course will be to probably change out the IAC. For the moment I'm in a wait and see.

Thanks for the alternator recommendations though.
 
#35 ·
Diode

By the way,

I installed the MPI kit onto my 83 Scrambler many years ago. I used the '83 style alternator, with 2-wires. The kit included a diode, as one of the installation steps. Was the diode supposed to be installed if using an '83 2-wire style alternator? Or was it for later model alternators?
 
#36 ·
By the way,

I installed the MPI kit onto my 83 Scrambler many years ago. I used the '83 style alternator, with 2-wires. The kit included a diode, as one of the installation steps. Was the diode supposed to be installed if using an '83 2-wire style alternator? Or was it for later model alternators?
The diode stops the feed from the secondary backfeeding the primary and powering up the ignition feed. It depends on the alternator if it has it built in. It could also be set up on its own circuit but that seems like it's more work than it's worth.
 
#39 ·
BTW, the resistor I'm using is like a 50Watt resistor. So even if the power system was running at 15VDC, the resistor would still only be dropping around 4.5 watts. So I don't have any issues of the 50ohm overheating in any weather condition. I'm just wasting 4.5 watts of power vs. 144 milliwats of power if I was using a 1K.

So the biggest difference in performance of the 50ohm vs. the 1K would be the slightly extra amount of gas I consume generating the extra 4.4 watts. I doubt it is really measurable using standard miles vs. gallons consumed at the pump.
 
#42 ·
Toms-CJ7, question:

I want to run the resistor between pin 20 and 57. But I'm not sure how to do that..

On my setup, pin 57 is a orange/green wire that appears to go to the ignition coil. Does that sound right? (I've seen in this thread that it's been referred to as an ASD wire or a voltage sense wire.) If this is the right wire, where do you recommend I tie into it? If this is not the right wire... what do I do now?

Also, pin 20 has no wire coming out of the harness connector. I don't have a donor harness either. So I need a female pin-connector to pop into the main harness connector. Are these individual pin-connectors only available through Mopar, or are they a standard thing I can find elsewhere?
 
#43 ·
I'll have to check the YJ or XJ schematics to confirm the wire color. I can't remember on mine. Will take a few days as I'm debugging some computer problems at the moment.

As far as a donor harness, I didn't have one either. You most liekly won't be able to get the pins from MOPAR as they have pretty much discontinued parts for these vintage parts. I had to get a few donor ECU's since I fried my original MPI kit ECU. One of the ones I ended up purchasing had the harness connector and a short pigtail on it that allowed me to retrofit the wires needed to make the resistor bridge. I have seen on ebay, people offering similar connector stubs in the $40-$70 dollar range. I have also seen an ebay sellar that appears to have the bare contacts, however he is charging a lot for them. I suspect if you search around in the online used parts you will be able to track something down here are a few to get you started...https://www.ebay.com/itm/90-95-Chrysler-Dodge-Jeep-60-way-ecu-connector-wire-harness-pigtail/173030472091?hash=item28496b799b:g:DXMAAOSwqbxaMOKG&vxp=mtr and https://www.ebay.com/itm/10-CHRYSLER-DODGE-JEEP-PCM-ECM-ECU-FEMALE-PIN-TERMINAL-3-1447221-3-SUPERSEAL-USA/311954191194?hash=item48a1eb335a:g:UwIAAOSwI-BWMWS6&vxp=mtr

When I track down the wiring, I'll post a follow-up reply. You might chek the further up in this post, as I think one of the guys that guided me on this posted something on the wiring - I'll check as well.
 
#46 ·
Yeah, that took me a few minutes to figure out myself. I don't have my pigtail handy at the moment, however from what I remember I used a thin flat blade screw driver or needle nose pliers and you carefully pull out the red insert that wraps around the contacts on the front of the connector. That red insert holds in the pins, when you remove the insert I think you can use the wire hanging off the back side and fairly easily wiggle the contact out the front of the connector. Don't force the red piece unless you don't care about the donor connector, otherwise, just work it back and forth from each end and it should slowly come out.

If you run into problems let me know and I will go try to find my connector and confirm the process.
 
#49 ·
Yes, almost there! Today I got the pin into my connector (pin 20). Got the resistor soldered inline in the wire. Now where to connect on the pin 57 circuit. My pin 57 wire is Green/Orange. The only other place I see a green orange is on of the two wires going into the coil connector (although I haven't ripped the wire out of the loom / harness sleeve). It seems I could also connect it at the ASD relay? What would you recommend?

thanks!
 
#50 ·
According to the YJ Jeep wiring diagram, the sinse lin pin 57 connects to the fuel injectors, the ASD, the ignition coil, and the oxygen sensor, (also pin Con the YJ alternator - not used in the MPI conversion).

I pulled the wrap off my harness loom and followed the dark-green/orange wire from pin 57 until I found a splice point in the harness and solderend onto that. You could also just get a parallel splice from an auto parts and just splice in anywhere it is convenient.

I grabbed a photo from the autozone for the general type of splice that allows you to tie into an eisting wire without breaking it. You will need to place a pigtail on the end of the resistor that ties into this line, however this would make the job fairly quick.

https://www.autozone.com/miscellaneous-non-automotive/electrical-wire-connector/dorman-weatherproof-quick-splice-terminal/88098_0_0
 
#51 ·
Thanks!

I am aware of those parallel splices, but have been suspicious of them. Are they solid and reliable "permanent connections", or are they likely to cause a problem in years to come?

What is "pin Con the YJ alternator"? Anyway, I have a CJ alternator, internally regulated. I'm wondering, could I connect to the stud on the back of the alternator with a ring terminal? Or is that not the right place?
 
#52 ·
Thanks!

I am aware of those parallel splices, but have been suspicious of them. Are they solid and reliable "permanent connections", or are they likely to cause a problem in years to come?

What is "pin Con the YJ alternator"? Anyway, I have a CJ alternator, internally regulated. I'm wondering, could I connect to the stud on the Jback of the alternator with a ring terminal? Or is that not the right place?
Was ment to be pin c on the you jeep alternator. Not relevant in our situation.

I have used those splices. If the contact pulls enough current, it will most likely keep oxidation clear. Could be problematic over long haul with what you are doing or could be ok. If you are concerned, you should solder your connections. That leaves you to tracing your harness and looking for an existing splice or just making your own new splice. Or you could just shave off some insulation on the wire and solder on a police where convenient and just tape it up good.

Connecting up to your alternator output is not where the sense circuit ties into. I don't know if it would clear the fault. It should not hurt anything, it just may not work on clearing the CEL.
 
#54 ·
Yes! I finally got out there today and finished it up. After the resistor (added to wire 20), I doubled the wire back toward the ecm connector, and spliced it into wire 57. I chose to make a soldered splice rather than using one of those splice connectors that cuts through the insulation. Maybe overkill, but I felt like doing something that would probably never fail. It was AWESOME to see the Gen Field code gone, and the Check Engine light NOT on!!

Thanks for all the help!! I'm gonna jump in my Jeep now and go get some tacos!
 
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