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P0720 Speed Sensor Output

W

wheelin79cj

NAXJA Forum User
Location
Florida Panhandle
I am trying to decide if I want to start "throwing parts at the problem" or if there i smore diagnostic work needed or maybe I caused something.

I just spent a few days installing a 4.5" lift w/TnT Customs Y-Link Upgrade and an Advance Adapters Slip Yoke Eliminator Kit. Got it all back together yesterday and took it for a drive. Right away the check engine light comes on. Pull codes and the only one that is has is P0720 "Speed sensor Output incorrect". Initially I assume that this is the speed sensor in the tail housing of the transfer case, especially since I messed with it, but the speedometer worked. Further research leads me to the output sensor on the transmission. My only question is I looked everything over good and it appears to be visually ok, but from my research the transmission should be shifting weird right? Everything seems to work as normal with the exception of the check engine light. Should I just replace that sensor and go from there? It's hard to believe that it is just a coincidence that it went bad when it did, but I figure most likely it is just that. Let me know what you guys think.
-Aaron
 
Ok, new question, same problem. I went and swapped the output (rear) speed sensor with the input (front) speed sensor, cleared codes and went for a drive. It took a bit longer, but the same code (P0720) still came back. Will the code be the same for the input and/or the output sensor if it goes bad? I need to make some sort of progress on this thing and don't really want to just start throwing parts at it, especially when they are almost $100 a piece. Thanks for any help/advice you guys can provide.
-Aaron
 
This code sounds like an OBDII Trouble Code ...
I am guessing that your Jeep is a 97-01 XJ 4.0L with a AW4 and NP2__???

I have a FSM at home for my '98 XJ which may help you to troubleshoot the problem; if your Jeep is a 97-01 XJ.
 
Ah, crap. Sorry. I guess I haven't filled out any info on here yet. I will get that done in just a second. You are however correct. It is a 1999, 4.0L, AW-4, NP231. I have a FSM for a 2000 in PDF, but it appears to be incomplete since the index refers to pages that are after the last Chapter that I have. I believe it may have come from two books and I only have book 1 or something. Either way, If you don't mind sending me the steps for that code that should be a start in the right direction.

I'm just ready to get this thing back on the road again. I'm not used to having to share a vehicle with my wife, not to mention, it looks great with the new lift, I want to get out and test it out!

-Aaron
 
wheelin79cj said:
Ah, crap. Sorry. I guess I haven't filled out any info on here yet. I will get that done in just a second. You are however correct. It is a 1999, 4.0L, AW-4, NP231. I have a FSM for a 2000 in PDF, but it appears to be incomplete since the index refers to pages that are after the last Chapter that I have. I believe it may have come from two books and I only have book 1 or something. Either way, If you don't mind sending me the steps for that code that should be a start in the right direction.

I'm just ready to get this thing back on the road again. I'm not used to having to share a vehicle with my wife, not to mention, it looks great with the new lift, I want to get out and test it out!

-Aaron
Click to expand...

At work right now..., but will be home in about 2 hours...
I will look thru the FSM for the information.... and send you what I find tonight.

Charles
 
CharlesS said:
At work right now..., but will be home in about 2 hours...
I will look thru the FSM for the information.... and send you what I find tonight.

Charles
Click to expand...

I appreciate it. No huge hurry, you are doing me the favor. I'm just not real sure where to start with this problem and was hoping someone had either dealt with it before, or had more intricate knowledge than I do. Thanks for taking the time to look for me.
-Aaron
 
Well, I gave in and ordered a sensor. Now the next question is which one to replace? I guess I will replace the front (input) one that USED to be in the rear since it is easier to get to and see if it fixes it. If not, then I will swap them again to see if that fixes it. If that still doesn't do it, I will be back on here begging for help again. lol
-Aaron
 
Ok, I guess I will pass on what the problem was in case anyone else searches and finds this thread. Today right before I was going to get the speed sensor the transmission quit communicating with the computer and the transmission would only go into 1st gear. I looked everything over underneath and decided to dig around and find the plugs at the computer as I was under the impression that despite being OBDII that the TCU was seperate from the engine computer. Regardless, I found a wire bundle that some &%$$#@# rodent decided to chew through. Good news is it's in an easy place to get to. It completely chewed through the speed sensor wire and got the insulation on 3 other wires on the same plug. Fixed that up and I am good to go. I guess it's kind of good though. I was going to buy a hundred dollar part and instead it only costed me a few butt connectors and some shrink wrap/electrical tape.
-Aaron
 
Re: P0720 Speed Sensor Output - TEMPORARY THREAD HIJACK

Aaron, I have a question that's sorta related, sorta not. First off, though, congrats on your low-buck fix! Would that all of them were so easy, or so cheap.

You mentioned ordering a VSS for your rig... may I ask where? The local parts shops don't list them, the local dealership says it's discontinued, and the online sources don't list it either. So where can I go to get one?

Ours is an '89 with the 4.0 NON-HO, AW4, NP232, bone stock. In the last year it's started jumping out of gear at highway speed, usually when we've been driving a while, and particularly if we've driven and stopped for a bit, then continued on, like when shopping. It's the usual thing, it'll stay in 1st or 2nd if you rev the hell out of it, but nothing beyond that. I can disconnect the TCU and manual shift it, but that's getting old.

I'm not 100% certain it's the VSS and not the TCU, but I know it's not the solenoids and it's not the trans side of the TPS, as I've tested those. I don't have a meter that can keep track of the VSS output (curse me for throwing away my analog multimeters!), so I don't know if this thing is going intermittent or not.

Any suggestions?
 
Re: P0720 Speed Sensor Output - TEMPORARY THREAD HIJACK

n9viw said:
Aaron, I have a question that's sorta related, sorta not. First off, though, congrats on your low-buck fix! Would that all of them were so easy, or so cheap.

You mentioned ordering a VSS for your rig... may I ask where? The local parts shops don't list them, the local dealership says it's discontinued, and the online sources don't list it either. So where can I go to get one?

Ours is an '89 with the 4.0 NON-HO, AW4, NP232, bone stock. In the last year it's started jumping out of gear at highway speed, usually when we've been driving a while, and particularly if we've driven and stopped for a bit, then continued on, like when shopping. It's the usual thing, it'll stay in 1st or 2nd if you rev the hell out of it, but nothing beyond that. I can disconnect the TCU and manual shift it, but that's getting old.

I'm not 100% certain it's the VSS and not the TCU, but I know it's not the solenoids and it's not the trans side of the TPS, as I've tested those. I don't have a meter that can keep track of the VSS output (curse me for throwing away my analog multimeters!), so I don't know if this thing is going intermittent or not.

Any suggestions?
Click to expand...

I'm not really familiar with the transmissions in these, but I tried to look up a VSS for an 1989 where I ordered mine and found nothing. I thought the sensors were the same in the earlier ones and the older ones though, there was just one less on the older ones. I guess not. If you find out that it is the same and the listings are incorrect, I ordered mine for a 1999 at Advance Auto Parts ( http://shop.advanceautoparts.com/we...or-BWD_18481125-P_1179_R|GRPSENSAMS_521829388___ ) Sorry I couldn't be of more help. Good luck with your troubles.
-Aaron
 
Re: P0720 Speed Sensor Output - TEMPORARY THREAD HIJACK

wheelin79cj said:
I was under the impression that despite being OBDII that the TCU was seperate from the engine computer.
Click to expand...

You are correct. It did move over to the drivers side above the gas pedal though.


n9viw said:
Ours is an '89 with the 4.0 NON-HO, AW4, NP232, bone stock. In the last year it's started jumping out of gear at highway speed, usually when we've been driving a while, and particularly if we've driven and stopped for a bit, then continued on, like when shopping. It's the usual thing, it'll stay in 1st or 2nd if you rev the hell out of it, but nothing beyond that. I can disconnect the TCU and manual shift it, but that's getting old.

I'm not 100% certain it's the VSS and not the TCU, but I know it's not the solenoids and it's not the trans side of the TPS, as I've tested those. I don't have a meter that can keep track of the VSS output (curse me for throwing away my analog multimeters!), so I don't know if this thing is going intermittent or not.

Any suggestions?
Click to expand...

The sensor in the tranny is not called the VSS. The rear output speed sensor is usually either good or bad and not intermittent. On an 89, the sensor is basically a magnetic switch that closes once per revolution. You could measure the voltage across it with the digital meter when its acting up (xfer case in neutral if you don;t feel like moving the vehicle). Being a square wave, it might be jumping on the volts scale, but it shouldn't be sitting at 12volts or 0 volts. Have you measured the solenoid resistance, as your symptoms do sound like a marginal solenoid.

Can you clarify what you mean by "jumping out of gear". It is suddenly slipping like its in neutral, or downshifting unexpectantly. Note that in 4th gear neither shift solenoid is powered, so a bad solenoid won't cause an unexpected 4->3 downshift.
 
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