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JK 4BT Swap - Tach/Alternator Issues

13K views 37 replies 6 participants last post by  Mudwasp 
#1 ·
Hi everyone,

I am helping a friend do a 4BT swap in his JK, and we're having trouble getting the tachometer and alternator to work.

Original Vehicle:
-2008 JK Rubicon, Manual Trans
Project Details:
-4BT, NV4500 Trans, Atlas T Case.
-Engine harness and ECU flash done by Bruiser.
-Bruiser Tone ring welded on crank pulley and stock JK CPS being used
-No cam sensor (the pigtail was removed by Bruiser)
-Alternator from 2001 Dodge Ram Cummins
-Original gauge cluster
-In general, this project was done with the goal of keeping the factory JK ECU functionality.

The Jeep starts, runs, and drives. With the exception of the battery and engine lights, no dashboard lights are on. (sorry, I don't know what the code is at this time)

We can measure 0 or 5 volts as the teeth of the tone ring pass the CPS.

Is it true that the ECU uses only the CPS to determine that the engine is running before supplying voltage to the Field Source wire on the alternator?

I understand that the actual level of the field voltage is regulated by the ECU considering system load, but are there any other inputs or prerequisites (like another sensor) that we are missing?

What does the Field Sense wire on the alternator do? Is this for the idiot light only, or is it also used by the ECU to regulate the Field Source voltage?

What voltage should I measure on both the Field Source and Field Sense wires?

Bruiser does not respond to contact, otherwise it would be ideal to talk to them.

I appreciate any advice you may have.
 
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#2 ·
Yes your jeep ecu will need to see a valid crank signal before it will run the tach and alternator. You don't need a cam signal to make this happen and I would make sure that they arn't trying to supply a cam signal as this will sometimes cause the ecu to not sync up. Make sure the reluctor ring on the balancer is installed correctly, the ecu is expecting a certain count and if the ring was installed backwards it will never sync.
 
#3 · (Edited)
Thanks for the response.

I didn't install the reluctor ring, but I know improper install has been considered. I will double check this.

What do you mean by "make sure that they aren't trying to supply a cam signal"? The Bruiser kit was purchased, and they removed the cam sensor plug from the harness and flashed the computer. They do not supply a cam sensor reluctor ring. The cam sensor was intentionally removed, and I would expect that the flash was done accordingly.

But maybe a mistake was made. Without a scan tool, could I reasonably determine whether a cam signal is needed? When a sensor is removed for a custom job like this, is it typical to remove excitation voltage in the software? Or is it all handled on the feedback side?

In theory, if I located the cam sensor excitation pin on the ECU/ECU plug and measured 5V, would that be meaningful? I suppose if I measured 0V, I could assume the flash was done with the cam signal in mind.
 
#4 ·
tach for 4bt

Thanks for the response.

I didn't install the reluctor ring, but I know improper install has been considered. I will double check this.

What do you mean by "make sure that they aren't trying to supply a cam signal"? The Bruiser kit was purchased, and they removed the cam sensor plug from the harness and flashed the computer. They do not supply a cam sensor reluctor ring. The cam sensor was intentionally removed, and I would expect that the flash was does accordingly.

But maybe a mistake was made. Without a scan tool, could I reasonably determine whether a cam signal is needed? When a sensor is removed for a custom job like this, is it typical to remove excitation voltage in the software? Or is it all handled on the feedback side?

In theory, if I located the cam sensor excitation pin on the ECU/ECU plug andhi, I am doing the same conversion and having the same issues. I spoke with Shawn at jd jeeps and he told me sometimes they have to have the cam sensor and sometimes they dont. i ordered a cam sesnor and i will be here next week so i will keep you posted. I was never able to get ahold of anyone at bruiser so I went with jd jeeps...guys are pretty helpful. i did not have my ecm flashed yet and am getting all the dash lights i wired mine myself and eliminated everything the gas engine used with the exception of the crank and tach wires and. oil and temp wires....i get 5v on obne wire and 3v on the other from both the tack and cam sensor circuit....my temp guage works but the computer does not turn on the fan either...from what i understand the computer controls all of the functions including the abs, alternator, fan, and tach so for me im pretty sure my ecm id not getting "excited" by the tach/cam sensorsmeasured 5V, would that be meaningful? I suppose if I measured 0V, I could assume the flash was done with the cam signal in mind.[/QUOTEhi, I am doing the same conversion and having the same issues. I spoke with Shawn at jd jeeps and he told me sometimes they have to have the cam sensor and sometimes they dont. i ordered a cam sesnor and i will be here next week so i will keep you posted. I was never able to get ahold of anyone at bruiser so I went with jd jeeps...guys are pretty helpful. i did not have my ecm flashed yet and am getting all the dash lights i wired mine myself and eliminated everything the gas engine used with the exception of the crank and tach wires and. oil and temp wires....i get 5v on obne wire and 3v on the other from both the tack and cam sensor circuit....my temp guage works but the computer does not turn on the fan either...from what i understand the computer controls all of the functions including the abs, alternator, fan, and tach so for me im pretty sure my ecm id not getting "excited" by the tach/cam sensors
 
#5 ·
hi, I am doing the same conversion and having the same issues. I spoke with Shawn at jd jeeps and he told me sometimes they have to have the cam sensor and sometimes they dont. i ordered a cam sesnor and i will be here next week so i will keep you posted. I was never able to get ahold of anyone at bruiser so I went with jd jeeps...guys are pretty helpful. i did not have my ecm flashed yet and am getting all the dash lights i wired mine myself and eliminated everything the gas engine used with the exception of the crank and tach wires and. oil and temp wires....i get 5v on obne wire and 3v on the other from both the tack and cam sensor circuit....my temp guage works but the computer does not turn on the fan either...from what i understand the computer controls all of the functions including the abs, alternator, fan, and tach so for me im pretty sure my ecm id not getting "excited" by the tach/cam sensors
 
#6 ·
Your 08' JK uses a single separate cam and crank sensor. Since the K cars Chrysler has "synched" the cam and crank signals for various reasons. One reason with T Belt engines was to shut the engine down in the event of a T-belt breakage.

The JK uses the cam signal for several purposes, one being cylinder misfire detection. But more to the point the cam sensor is used for one touch start on automatic JK's which is also part of remote start. This means when you crank the engine the PCM receives a crank pattern and a cam pattern. These two patterns are compared by the PCM and synched up, at that point you will have proper starting, spark, fuel delivery, etc....

If the cam signal is corrupt or missing the PCM will continue to look for it for up to 15 seconds, during this time your starter may stay engaged and grind. After a timeout period the PCM will ignore the cam signal and operate off the crank signal alone. Once a valid crank signal is had and the tach works this will allow the alternator, cooling system and AC to come alive, even without a cam signal.

If an improper or corrupt cam signal were to be supplied it is worse than having no cam signal at all because it can cause the PCM to reject the crank signal and refuse to run inhibiting your AC, cooling fan, charging, etc.....

The first JK swap I did in 2009 did not use a cam signal so it took about 5 seconds to get the crank signal alive, we used a timed relay output on the starter relay which was not ideal but worked. On cold days it would not crank long enough and on hot days it might crank too long.

Then we went into the PCM and limited the amount of time the PCM would look for the cam signal and reduced it to milliseconds above the start rpm, this worked okay but again was not ideal and if we went too far the PCM would reject the crank signal and you were left with no tach signal at all. At one point we found a solution by modding the TIPM but it again was not the solution.

We have come a long way and currently and our electronics supply a perfect cam and crank signal on both the 3.8 and 3.6 swaps that are synched, the 3.6 swaps requiring 4 cam inputs. This assures perfect starting every time with no overrun, perfect remote start operation and tach function.

Check pin 34 of the C2 PCM connector and see if any wire is there? I don't know how Jeff does it, it may be through PCM programming; however, if you eliminate the cam signal altogether you should get a crank signal and charging function after about 10 seconds? If running a crank trigger wheel it is not symmetrical and if installed backwards you will not get a signal.
 
#8 ·
Thanks for the information on how the cam and crank signals work together. That is an insightful explanation.

The engine has been started maybe 30-40 times with all of the electronics plugged in, and it has been driven about 10 miles on separate occasions. Many of the starts have been long due to unbled injectors or just a cold engine. The tach has not worked on any occasion.

Does the PCM only search for the cam signal while the starter is cranking or does this search continue after the key is released?

I guess there's no real telling what Bruiser did to the computer to make up for the removal of the cam sensor.

I will check pin 34 of the C2 connector and let you know if there's anything in there.
 
#9 ·
Hi
I am running the tone ring from Jd jeeps and using the factory crank sensor. The ring I have appeared to only be able to be installed one way. I have started and ran mine several times and never get any tach signal or the other functions to work...I do get voltage to both sensors and ran a jumper on the fan relay to keep it cool while attempting to figure out why the computer would not wake up. I changed spacing on the sensor from the tone ring, checked all the circuits and could not figure out why it will not work and thats when i called JD to see if they had any insight.I just left my cam wiring intact and extended my crank sensor wires to be long enough. I removed all of the fuel injector, throttle body, egr, map and mass airflow sensors from the harness....could I have removed something the computer is looking for in conjuction with the crank? Thank you for all your help....im new to the can bus system and need all I can get!
 
#12 ·
Do any of your other functions of computer work?
I didn't really answer that question. Yes, some other functions of the computer work: speedo, fuel gauge, engine temp, oil pressure. The ABS, TCS, and SRS lights are not on.

However - from what I gather - none of the engine rpm dependent functions are working. Correct me if I'm wrong, that list is: Alternator, AC, and cooling fan.
 
#11 ·
Which CPS are you using? On JDJeeps it states that JK's require a TJ style Crank Position Sensor with their reluctor ring:

JD Jeeps - Diesel Conversions - crankshaft

However, that doesn't mean a whole lot on its own because the 03-06 automatic TJ uses the same CPS as the 07-11 manual JK. Altogether, there were 4 different CPS's used on the TJ. Just something to look into.

The cooling fan has never kicked on, and we have not tried the AC. That would be a good thing for us to check.

Because Bruiser modified the harness, the plugs for the fuel injectors, egr, MAP, O2's (and probably more I'm not thinking of) are all removed. We do have the throttle body plugged in because Bruiser left that plug on the harness and soft of mentioned something about it in the "instructions". It wasn't clear, but it appears to be required for their flash.
 
#13 ·
I am using the stock CPS from the jk wich was a factory manual transmission jeep. JD said it should work fine....my oil light goes off and temp guage works but never tuns the fan on I used a jumper wire on the fan relay so i could run the engine to test...the relay for the coolig fan on my jeep is over behind the drivers side headlight. It has two wires feeding signall voltage one has 12v and the other nothing so I am assuming the pcm is switching it to ground to energize the circuit....it was the only 12v ignition source I found under the hood so I tied into it for the fuel shut off solenoid. I am running a p-pumped engine so i have to have a relay for the hold voltage and a relay for the start voltage. Since i havent flashed my ecm im not sure what all it controls but my abs light and the red lightning bolt light is on i think the lightning bolt is the throttle system. Shawn said they are working on an 08 now and if I recall are having the same issue. I have not driven it to see if my speedo works yet I am still fabing up the air intake. What engine mounts did you use? How is the vibration at idle?
 
#14 ·
I am using a dodge alternator also and just ran the two feild wires over to it and connected them...i am running a sanden compressor and ran the black wire from the jeep harness to ground and ran the blue/green wire to the compressor. Thats really the only wires i messed with other than removing all the others from the harness. There is a chip you can install in these alternators that regulates the voltage with just one 12v ignition source i may put in to eliminate the computer controlling the voltage too but since 12v ignition power is scarce under the hood i just figured i would try it as it was. I have an 87 k30 i put a cummins in years ago and i just ran an external regulator from a first gen dodge and it works fine too. If i do that it would leave my computer only controlling my fan tach and abs.
 
#16 ·
And my fuel guage works too so your computer may not be working. Sorry for the spotty responces I am working nights this week and am in need of a nap! I will check back here when I wake up this afternoon and see if you had any luck. I wish this forum would send me a notification when somebody responds, I am new to the forum world and probly dont have it set up right or am doing something wrong!
 
#17 ·
I believe the engine mounts were purchased from Bruiser. Here's their part list:

BruiserConversions

This is what they look like:

1205 4wd 06+diesel Bruiser Jk Cummins Conversions+custom Hydraulic Motor Mounts - Photo 35558630 - Bruiser Jeep Wrangler JK Cummins Diesel Conversions

I'd say the vibrations are fine, but I haven't had a lot of seat time or paid particular attention. I suppose if the vibs were bad, I'd know it.

It doesn't surprise me that your speedo worked while being towed. Apparently the speedo works without the PCM registering rpm because that is our scenario (but I suppose something in the PCM code could have been changed).

I would plug the throttle body in and then all of the other sensor too if that doesn't work.

We also have a 12V ignition circuit constant going to the injector pump. I'm not sure where it comes from. Bruiser added this wire.

Not sure I understand your fuel gauge comment. Our gauge works and the computer seems to work for everything that doesn't have rpm as a prerequisite.
 
#18 ·
Sorry for the confusion, and I may be way off here but I will explain my thoughts and please correct me if I am wrong I am still learning about these things. From the little I have read about the can bus system basically the guage cluster is a computer itself that controls certain functions and communicates with the PCM but works independantly of the PCM so some functions like the fuel guuage and speed odometer may be contolled by the guage cluster and not the PCM? and that would be the reason for these things working while others dont? On the engine mounts I was stuck between the hydraulic mounts or the solid mounts from a first gen dodge and ultimately went with the first gens becayse I heard different opinions about the strength and durability of the hydraulic mounts....when I have run my jeep I have noticed some vibration at idle in the steering wheel...my keys rattle a lil on the column...do you have this? I fabbed my own mounts and may really consider redoing them if the hydraulics eliminate that vibration so any info on that would be greatly appreciated too!
 
#19 ·
I see what you're saying about the gauge cluster vs the PCM. I haven't done any reading on that, but it's a good thought. The engine temp and oil pressure sensor wires pass through the PCM. Not saying that validates it as working correctly, but it is doing something.

I don't own the Jeep or live near it so some of my feedback on these issues will be slow. I don't want to give bad advice about those engine mounts. All I can say is that I haven't noticed any vibrations, but I've been concentrating on other things while it's running. I've not read any complaints about vibrations by those who have the Bruiser swap. I say worry about the mounts after you figure everything else out.
 
#20 ·
My Jeep is a factory manual trans with cruise control, I do not seem to have the starter running problem, although there does seem to be a slight delay between the key and actual starter motor running.....could the manual trans/cruise combo be the culprit in needing both signals to energize the computer or is that just not possible?
 
#21 ·
Yeah that was my plan on the mounts I just figured I would ask while we were talking, I have had a hard time finding anyone that has the same set up as me so its exciting to be able to see what results you have had!!....I will let you know what happens with the cam sensor....Im hoping it works out so I can finally get this thing on the road!
 
#22 ·
I have confirmed the orientation of our reluctor ring. The ring rotates clockwise as viewed from the front of the engine. This means that the crank position sensor sees this pattern: 1 Big Tooth, 16 Small Teeth, 1 Big Gap, 15 Small Teeth.

Can anyone please confirm whether this is correct?

I can't seem to find pictures of other reluctor rings mounted or what the pattern is supposed to be.
 
#23 ·
On the top is our trigger wheel from 2009, we used it exclusively on truck engines. As time went on and we started to support the high revving LS3 engines this wheel was too heavy. This wheel worked great but was made from 1/4" steel, water jet cut and had a lot of mass. These wheels were not recommended for 430 or 480 HP engines but guys ran them and some had issues with them over 6,000 rpm.

On the bottom is the latest trigger wheel, it is 1/4th the weight of the earlier wheel and when combined with our large diameter stands it worked great up to 7,000 rpm; this wheel is 3/16" but we also used 1/8". We also had trigger wheels for TJ/LJ's, Pentstars and others. We eliminated the trigger wheel years so I had to dig through old stock for the photo.



In a stock JK the ECT signal is a hard sensor on the engine that sends a analog signal to the ECM/PCM, the ECM/PCM then broadcasts that signal on the bus to be used by other modules like the TIPM for fan control and the CAB module(cluster) for temp reading. This signal can be replicated and transmitted on the bus in a swap but I do not believe Bruiser does that.

There is an odd behavior in Chrysler vehicles where they need to see a valid tach signal before enabling functions, like fan control, AC, and even charging. I guess the logic is the engine is not running why would you enable these things. So a valid rpm signal is required to get AC, fan control and charging. Like everything else there are ways around it but just get a tach signal and and your life will be easier.
 
#24 ·
Ok, so I got the cam sensor installed today and still no tach? I a have 5v on one wire and with the key on 3.75 to the other wire. With the engine running the 5v doesnt change but the 3.75 changes 2.39 to 2.49 and seems to rotate between the teeth of the excitor ring so I think my cam sensor is working....Talked to Dan. At JD jeeps and he thinks it is tied into my throttle body wiring and is getting with one his guys in the morning and will get back with me tommorow.....so I will keep you posted!
 
#27 ·
I'm owner of this crazy project that started as a "Bruiser Conversion".

At the cable pedal, the plate I got from the kit put the pedal so far over that your pinky toe was directly over the pedal. So I cut the bottom of their plate off, flipped it and welded it over. I just had to drill a new hole for the cable in the plate too. It's in a much better location now.

Many of the Bruiser parts in the kit I had to modify due poor design or I scrapped all together. I was able to return some parts and I actually got money back from them.

I still have the plate bracket that the JK stock AC and alternator mount to. I'll probably sell that at some point along with the 8 groove pulley on the JK alternator and the 8 groove clutch kit on the JK AC pump. Once I know all my stuff works I'll post all the extra parts I ended up with.

For the AC and Alternator I went with Ram parts and mounted it to this bracket system which worked well with my header and turbo choice for packaging. I still had to cut my firewall back a bit to fit the 3.5" downpipe. Bracket from Duiser. Cummins 4BT and 6BT Accessory Mount

I am happy to share any details with anyone. Either my partner in crime 08JK4BT or myself will try to help anyone we can.
 
#28 ·
Thanks for an informative thread. I have been slowly wrapping up a 4BT conversion in a 07 JK (2 years!) and was curious if you ever worked out the Tach signal issue? I finally gave up and installed an Autometer tach just so I'd know RPMs. I'm at a point now that I'd like to get A/C functional for this summer if possible. Also, if you have come up with a way to get A/C functioning without a tach signal I'd definitely like to hear about that work around.

Like you, I'm using a Jonesy/Duiser bracket setup and stock 6BT alternator, A/C compressor. On the alternator I just used a voltage regulator to wire it up and it is working great. On the fans I used a fan temp switch from a 90 Corvette and wired that into a relay to control fans. It works well enough for now.

I reached out to MoTech LS guys via their website and enquired about a crank trigger wheel as it sounds like they have it all worked out.
 
#29 ·
I have not made much progress in the past 6 mo. A new baby will do that.

I have a goal to get it driving before May so I can make it to Blessings Of the Jeeps in Mesick, MI. Hopefully soon my buddy and his roommate who worked on JK's for Jeep can help shed some light on why my tach is not reading(with the reprogramed ECU by Bruiser...supposedly. So far $800 wasted). My friends have a Chrysler Diagnostic tool and some other tricks to get into the ECU and troubleshoot. I hope this will happen by the end of the month. If I can't figure it out soon i will be doing the same bypasses you have done with V reg. and fan. The AC will be a bit trickier.

Another idea would be to modify/hard code the ECU to think there is rpm so everything is active. Might be my plan B. Not sure how doable that is. Maybe RPM Extreme could do something like that.

If you need some ECU mods you can talk to the guys at RMP Extreme. JKO user name 0lllllll0 RPMEXTREME. They are very knowledgeable and willing to talk to you folks with questions. Unlike BRUISER jokers.

You can send your ECU to them and they can make engine light not ever come on and maybe hard code the rpm. Home :: RPM Extreme

I'll keep you posted. Wish me luck, same to you!
 
#30 ·
Pretty simple, well maybe not. In the early days we ran a trigger wheel and the stock CKP sensor, it actually works very good if set up right; it is similar to what Jeff was doing when I wheeled with him in the diesel - but we ran a thicker wheel. I'm still driving my 09' JKUR with the trigger wheel and I have no reason to upgrade it after 80,000 miles. Our newer trigger wheels were lightened and beefed up to handle the 480-500 HP engines but it won't matter in a diesel.

Currently we can interface the CKP signal over the CAN network when using a GM ECM and we also have an RPM module that will accept an input signal then output the proper pattern for either a 3.8 or 3.6 JK. Currently we are only accepting signal inputs from various GM trigger wheels, i.e 24X, 58X. Our module is programmable though for other inputs and very fast and accurate. When we dumb down the GM 60-2 signal to the JK 36+2-2 pattern we have almost twice the resolution.
 
#31 ·
This sounds like a good plan B.

I have a tone ring on my crank that matches what was on the 3.8 flywheel for teeth, my stock sensor is reading 5v and 0v as I rotate the crank by hand. My ECU was flashed by Bruiser so it should just work right? So far no luck.

All Bruiser says is bring it to FL with $$$. Won't even answer simple questions over the phone. They say go to local offroad custom shops for help that have zero experience with the diesel swap.
 
#34 ·
The VR used was off an 80's Dodge. Oreillys PN R296 and pigtail is PT173. Make sure you have a good ground for the VR.

The temp switch was PN 60600 off of Summit and a Bosch 50 amp relay.



The trigger wheel from Bobbie @ Motech got my tach working but there are other issues now and the tach won't stay on for more that 3-4 minutes at a time. Just about ready to scrap the entire wiring system and get a Painless kit and wire everything direct.
 
#35 ·
The VR used was off an 80's Dodge. Oreillys PN R296 and pigtail is PT173. Make sure you have a good ground for the VR.

The temp switch was PN 60600 off of Summit and a Bosch 50 amp relay.

The trigger wheel from Bobbie @ Motech got my tach working but there are other issues now and the tach won't stay on for more that 3-4 minutes at a time. Just about ready to scrap the entire wiring system and get a Painless kit and wire everything direct.


Thank you very much! I'll keep ya posted. I am determined to have it running and driving for May 6th Blessings of the Jeeps in Mesick, MI.

That sucks about the inconsistent tach. I know how you feel. Just gut it all like MrACV24 did on Youtube. Check out his videos if you are interested in going that route. Still a lot of work.
 
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