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Weird Engine Behavior

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On Saturday we went back to the MJ located in the New Orleans Pull a Part and among other items we were able to get the speedometer gear out of the transmission. Yesterday we tried to install it in my son's MJ but were not successful - it would not engage the worm gear (even though the tooth count and all other dimensions appeared the same). We test drove it several times to confirm the speedometer was not operating with the replacement gear, but then re-installed the origional gear.


When done, we noted strange engine behavior after the truck warmed up. When in idle, everything seemed nornal. When driving the truck would travel about 45 seconds (at any speed) and then the RPM's would suddenly start surging up and down, the engine would bog down an appear to run extremely rich, and then after 10 seconds of this behavior it engine would die. None of this was happening before we fooled with the speedo gear.


The truck would re-start immediately, behave for about 45 seconds and then repeat the process. If I shifted into neutral when it started it would sometimes calm down and idle normally at around 700-900 RPM. If the brake was held (no forward movement) and the truck was placed into gear it would wait the 45 seconds and then go nuts again.


I believe this is a sensor input error to the ECM, but please correct me if I am wrong. My question is, could this be a result of the truck being driven without the correct gear? and, (if previous question is true) does the TCU need to flashed somehow or could it be toast?


In the searches I ran on this I found no posts but did noted some other folks had 'flashed' thier ECM by removing the positive wire and touching it briefly to the negative post. Could I have confused the ECM?


Thanks in advance.



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is this on a renix or high output?



if it's renix, the speedo has nothing to do with anything but itself, so I'd hazard to say you accidently bumped a sensor in the engine bay or something.


if it's high output, again...it's not gonna cause running issues.




btw....the speedo gears must be clocked appropriately to work. the mount/sensor that they go in labels which tooth count goes where.

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