P0720 problem I'm stumped on.

I'm working on a 2002 Grand Cherokee for a friend and I'm having a problem with a P0720 code. When he brought it to me, something got caught? underneath and ripped the wires from the output speed sensor all the way to where it joins into the main harness on top of the trans! I knew there was no way I'd be able to splice wires to that to fix it so I ran new wires from the TCM to the sensor thinking this was a pretty easy fix. Erased the code and as soon as I started it the code came back? I don't even have to drive it for it to happen so I'm assuming it's a wiring issue but since I ran new wires, I don't understand how it could be? I did feel where the break was in the harness to make sure the wires weren't touching and separated them but the code still pops up. Is there a chance that if they were touching that that could've fried the circuit in the TCM? Any tricks I can try before trying another TCM? Thanks for any suggestions.

 

welcome to the forum - hope we can help...

I don't want to guess - 4.7L or a 4.0L ? BUT sounds like a 4.7L with the 45RFE trans?
Any chance you replaced the OSS?
And you replaced both wires OSS to PCM?

 

Welcome @dniksich ...any issues with trans shifting or speedo? Inspect the wire harness further back from the damaged wires. Check the resistance and/or damage to the OSS and replace

 

Yes, forgot to include 4.7/45RFE trans. Changed both wires, cut the signal wire (Gr/Wht) completely and replaced it and since there were like 3 wires for the sensor ground, I just spliced into one with the new wire since they're the same color (Blue/Blk) and I didn't know which was the broken one. Speedo works but it shifted a little late/hard. Haven't tried it since running the wires since the light pops back on as soon as I start it. I did change the sensor but the old one checked out good. I'm lost as to what it could be now with new wires run from the TCM down? Can't get any further as far as the wires cause the break was way up where they run into the main/larger harness.

 

Gonna try to figure out which blue/blk wire is for the wire that's broke and just snip it. It's acting like there's a short somewhere causing the light to immediately come back on and since I already cut the signal wire, I guess the ground is the only thing that can be bad? And I figured this was gonna be easy when I found the broken wires....

 

How much voltage should the signal wire have with the key on/off?

I just checked for voltage and with the key on but engine off, I only get 1.5v with the sensor unplugged and with it plugged in, it drops to like .1volt?

 

look at diagram - did you add the splice at S113 ?


do you have a SCANNER?

 

I spliced at about 6" out of the TCM. #13-14 I have a small scanner and HP Tuners.

 

Gonna try the diagnostic sheet you posted now, hopefully that will narrow it down at least.

I'll post the results as soon as I get through it.

Thanks for the help!

 

Failed step 7 There's no resistance at all between ground and the signal wire. Now it says to repair the output signal circuit for a short to ground? If the wires not grounded, why would there be resistance? This is with the TCM disconnected.

 

Failed 8 also but passed the others after.

 

ok I am confused *


again TCM unplugged and OSS unplugged -
failed 7 - there is voltage at wire? this is DB/BK wire?
failed #8 - voltage on LG/WT wire.

* Sorry I just got time to reply, Sunday is Church BBQ Bourbon and football - paying attention is low and unreliable. But will try.

 

No problem with the reply, I'm just glad your trying to help. With both unplugged, I get no voltage or resistance at either wire. I only got voltage with the TCM plugged in.

 

its SUNDAY. BUT I just pulled up the p0720 trouble tree -- AND there are two different paths....way to much to deal with when sampling Bourbon sorry - brain is now OUT OF SERVICE today.

 

lol, no problem. Whenever you're available is fine. Enjoy your day!

 

don't want to leave you hanging ---the conflict will need a sober mind - know any?

both from 2002WJ, troubletrees -

 

haha, mine'll be sober till I figure this out....then not so much. Will check out your attachments, thanks

 

The thing that's bothering me is the code comes back immediately after erasing it and I start it? I don't even have to drive it or even let it run. I'd think it would have to be moving for it to set unless it's a wiring issue? And with me replacing both wires, I'd think the wires would be ok now? I hate electrical issues!

 

Ok, disregard the failed 7&8 tests. I feel like a real idiot. I was disconnecting the harness at the TCM and not the transmission! No wonder it failed both tests and had me scratching my head as to why it was with new wires ran down there. I disconnected it at the trans and now it passed all tests up to the replace the TCM part. Guess for 35 bucks, I'll head to pick and pull and grab one. That's all I can figure would be wrong now. Hopefully this is the problem.

 

As of right now, without driving the light is off! TCM must've been bad. Gonna run the new wires permanently and take it for a drive and see what happens....but the fact that it went off when I started it is promising!