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Old 09-24-2019, 11:05 PM
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Have i got one for you guys!!!

Its going to be a little long winded, but bear with me. I have a 2004 Rubi with 120k miles on it, auto, stock gears, 35's. I will be regearing to 5.38's after i hear from Mr. Bransford if he still likes his setup. Anyways, heres the back story... I had a sub in the back of the jeep that was powered from an amp under the drivers seat. Everything was working properly and I decided to ditch that setup and just do an all in one powered sub that fit under the drivers seat. Since all the wiring for the amp was already there it was a slam dunk install. I hooked it up and all was well. That is until i came back out to the garage to mess with the sound of it a little. Out of nowhere it starts making a very loud sub harmonic feedback noise. Since I got it from Amazon, it was going back immediately. As i went to unscrew the mounting screws that were mounted into the jeep tub, I watched as they sparked as i unscrewed them. As soon as I got all the screws out and the speaker was no longer in contact with metal, it worked fine. Obviously some kind of internal grounding situation was happening. I pulled the thing out of the jeep and boxed it up to return it. Now for the real issue.
The next day i was leaving first thing in the morning to go wheeling. As i left my driveway I got about 1/4 mile and out of nowhere the jeep starts lurching and sputtering around 2k rpm. My first thought was fuel pump since mine started whining after i put the savvy gas tank skid on. No i didn't replace it when I had the tank down ( rookie move). So i ordered the fuel pump and through it in. Still same issues. The next thought was tps since it was happening around the 2k mark and i know that sensor starts really doing its thing around that rpm. Through that in. No change. However, when doing that test drive its then i noticed that the speedometer was reading about 90 when i was really doing 25. Well that explained the fuel cutting out.
From here is where things get interesting. First thought was speed sensor. Well, the great folks at Chrysler stopped producing that sensor in April of last year. After scouring the internet for the part without any success, I remembered that the jb conversions short sye kit used a sensor that looked an awful lot like the one in my jeep. so i emailed them. To all who may need a tj rubicon speed sensor, the awesome folks at JB will sell you one for $40. Its the exact same part. That wasn't my problem though. Next, I read about the tone ring breaking the key piece and freewheeling in the t-case. So i pulled off the back to inspect and see that everything is as it should be. I needed to change the fluid anyways so not a total waste of time.
My next attack was on the cluster itself. I have an extra one kicking around so i swapped it out to no avail. same issue.
So as im working towards the front of the jeep, i also read where some people have fried diodes in the alternator and thus allowing some low level ac power to feed back through the ground side and mess with the sensors. Que the new alternator and battery. Nope.
As a last ditch effort I put my tail between my legs and took it to the dealership. If Waterdog were still around he would have slapped me . I told them I wanted them to reflash the computer and see if it would set things straight. They did that and set the computer to now read that I'm running 35's but it didn't fix the speedo issue.
Heres the crazy part... I do have a speedohealer on my jeep. Before all this happened I had it set to a positive 6.6. I now have it set to a negative 65.4 and the speedometer reads to within on mph of gps. How is it possible that the computer is reading consistently wrong? Like it somehow has some arbitrary positive setting stuck in it. The jeep seems to run great with the speedohealer as the band aid fix, just curious as to the real issue. Btw, when the dealer said they thought the problem was in the differential that's when I stepped in to rip it back out of their hands. Sorry for the long explanation, just a really weird set of circumstances. Also, are 5.38's still the way to go on 35's with the auto? Thanks brothers and sisters

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Old 09-25-2019, 12:15 AM   #2
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Boy that is going to be a real hard one to De-Bug. Good luck and keep us informed so the next guy might not have to go through hell like you are. AND, I am running 37's and everything I have read says that 5:13's would get the power back to near stock. If I thought I might be towing anything, I would then consider the 5:38's. Remember, the higher you go, the worse the MPG gets. If it's a DD, that might make a difference which ones you go with.

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Old 09-25-2019, 10:03 AM   #3
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If the ECU is programmed for the correct size tire do you still need the Speedohealer?
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Old 09-25-2019, 11:01 AM   #4
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Quote:
Originally Posted by jjandascog View Post
If the ECU is programmed for the correct size tire do you still need the Speedohealer?
shouldn't...but the fact that he needs the healer and it's set to -65% correction indicates that something is definitely jacked.
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Old 09-25-2019, 11:14 AM   #5
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Remember, the higher you go, the worse the MPG gets. If it's a DD, that might make a difference which ones you go with.
not necessarily. I'm averaging +2mpg since I went from 3.73 to 4.56 on my 5 speed with 32s. If the gearing is too low, the engine is below its powerband and ends up requiring more fuel to deliver the required torque, despite being at a lower rpm; or you end up losing OD and driving in a lower gear, putting the rpm at a higher than optimal point which also uses more fuel.

When it comes to gearing, I would suggest believing anything Jerry says. I'm an engineer and refuse to take anything at face value, so I calculated and analyzed every which way I could and it always came out to what he suggested. I went so far as using an OBD2 reader to gather average engine load on the same 10 mile drive at 3 different speeds at the same rpm to generate a wheel torque-vs-speed curve (to overcome aerodynamic and mechanical drag), which I could then apply against the 4.0's torque curve to determine what engine load is required to maintain a specific speed at any desired combination of transmission and axle ratios. Put that against rpm to predict mpg (not necessarily accurate, but directionally correct and probably within 2mpg). Jerry's thought process of targeting an rpm @ speed ends up with the same ideal axle ratio every time, with a lot less work.
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Old 09-25-2019, 11:23 AM   #6
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Originally Posted by jjandascog View Post
If the ECU is programmed for the correct size tire do you still need the Speedohealer?
reprogramming the ECU rarely works. i've been running the speedo healer for ~6 years and have zero complaints. very easy to install and adjustments are simple
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Old 09-25-2019, 12:18 PM   #7
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Is the speedo jumping around?
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Old 09-25-2019, 12:26 PM
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My jeep is used strictly for rock crawling and not a daily driver at all. My only concern with going to 5.38’s is when I’m wheeling with the 4:1 transfer case. Is that going to be an issue with the transmission. Is it going to be so low that I’m wheeling in 3rd gear? Would that even be an issue?
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Old 09-25-2019, 12:31 PM
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The speedo is not jumping around at all. It’s consistent all the way through. Which is why I believe the speedo healer can correct for it. When the dealer recalibrated it for the 35’s I did notice that the needle wasn’t pegging out but was still into the 90’s. So what they did changed something. Just not back far enough?

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