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Unread 11-03-2008, 05:28 PM   #1
Jim0395
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1998 ZJ 
 
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Replaced output speed sensor, still check engine light

Help! Check engine light due to code for output speed sensor. I've just replaced the sensor, and reset the check engine light and computer stored codes. But as soon as I reach above 15mph, the check engine light comes on again. This makes me thinks it's truly a new error each time and not something saved in the computer. it shifts and runs fine, but here in Oregon the emissions testing to relicense your vehicle fails due to the output speed sensor code, P0720. What's next, can't license my jeep and can't just throw hundreds at a repair place. Thanks!

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Unread 11-03-2008, 07:11 PM   #2
mtcox
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Did you run an OHM test on the old speed sensor? Compare that to your new speed sensor. It may not be the problem. The throttle positioning sensor can give you symptoms your describing, too.
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Unread 11-06-2008, 05:09 PM   #3
98jgc
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I was just where you are, Code 720 - came up again replaced the other sensor that cured its the one that threads in with a 1" deep socket. No issues so far.
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Unread 12-16-2008, 11:03 PM   #4
JGC1998
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which sensor?

I am in the same boat both of you were. I just replaced the speed sensor, i didnt replace the wires and plug which attaches to it, yet. I doubt it is that part though. You mentioned you replaced another sensor can you be more specific? I am not sure which one to go for next.

BTW funny story, I about had a mechanic do this for me figuring it would be difficult he was gonna charge me a whole load of money to unplug one sensor and plug in another, what a joke. Between that sensor, brake pads, and a new rotor, I saved myself 200 in labor alone.
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Unread 12-17-2008, 10:21 AM   #5
rm2001wj
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You guys have two "speed" sensors. One is the vehicle speed sensor, which is not the problem that code 720 refers to. The second is the transmission output speed sensor which you need to deal with. You might gets some help from my post (it's #56) to this thread:

http://www.jeepforum.com/forum/f310/replacing-transmission-speed-sensors-287793/
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Unread 12-17-2008, 04:29 PM   #6
JGC1998
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rm2001wj-
You are exactly right.

Alright I fixed it everyone!
Here is what happened to me and what I did, perhaps it may help someone else save a little money a little time, and a lot of frustration.
My JGC 1998 (4.0 I6) had the check engine light on. Code coming from it was P0720. It is: Low Output Speed is less than 60 rpm with vehicle speed above 15mph. I was told this was the VSS vehicle speed sensor, which I replaced ($43). I unconnected the battery to reset my light and it stayed off until I was about 15mph at which point it turned back on. Thanks to "98jgc"s comment he mentioned a sensor requiring a 1 inch socket to remove. I climbed under the jeep to take another look and there it was attached to the transmission. So I went back to autozone, and they looked up "speed sensor" for my jeep, there came up the VSS and the transmission speed sensor. I got a TSS transmission speed sensor($20) and some electronics spray cleaner to clean the plugs. I cleaned both plugs and put in a new TSS. Reconnected my battery once again. The light stayed off, and is still off. Passed the computer emissions test here in Utah today. Problem Solved.

Last edited by JGC1998; 01-01-2009 at 02:33 PM..
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Unread 09-07-2009, 10:46 AM   #7
rsvector10
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Problem Fixed!

I have a Haynes Repair Manual for 1993 thru 2000 Jeep Grand Cherokee. It mentions nothing about the Transmission Speed Sensor (TSS) . I have a 1998 Grand Cherokee 4.0L. However thanks to you guys I did pick up a TSS at Auto Zone and it fixed the problem. It does indeed use a 1 inch socket. Do not confuse TSS with the Vehicle Speed Sensor! They are different. I did lose maybe 1/2 QT of transmission fluid but it was worth it. Auto Zone even reset the the Check Engine Light. Interestingly Auto Zone was the only place to have a TSS and it was the last one on the shelf.
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Unread 04-04-2010, 05:06 PM   #8
vg3dck
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I just bought my 98 GC in Feb and was having shifting problems around 15mph, diagnostic read Output speed sensor, after reading all the posts here I went for the TSS sensor and my Jeep runs awesome. Now can someone tell me how to disconnect the alarm permanently, since mine goes off at random when the doors are locked.
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Unread 04-05-2010, 12:40 AM   #9
JGC1998
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vg3dck,

Try to unhook your rear door sensor. I think it is actually the rear window sensor. There is a plug in the rear door that all you have to do is unhook the plug. Once I did that to my 98 GC, the random alarm going off stopped completely. It has been fixed probably a year now, no problems. The only small side effect is that when you open just the back window your dome lights may not come on in your jeep... who cares.
There should be more information on the site about how to do what I have described briefly.
Good Luck.
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Unread 08-15-2010, 05:46 PM   #10
netmonky
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Hi all, Is the TSS the output sensor?
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Unread 10-19-2010, 04:17 PM   #11
scodo68
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is the tss the output and the vss the input?
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Unread 10-19-2010, 05:01 PM   #12
Candymancan
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Yea like stated above that code is the transmission speed sensor. $17 part. My mom's 4.0 had the same problem check engine light was on for so long and we ignored it. Now that the Government has strickter rules we couldnt pass inspection without it being replaced. I found it was the sensor on the transmission..

Btw i bet you a million dollars after you replaced this sensor your Jeep was shifting alot better huh ? See after awhile you get used to the Jeep shifting and you "think" its fine when its not. After i replaced this sensor the Jeep shifts were so much more smooth and alot sooner
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Unread 10-19-2010, 05:38 PM   #13
scodo68
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im going to replace both trans sencors and the back o2 sencor tomarrow.
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Unread 10-19-2010, 07:58 PM   #14
scodo68
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Quote:
Originally Posted by rm2001wj View Post
You guys have two "speed" sensors. One is the vehicle speed sensor, which is not the problem that code 720 refers to. The second is the transmission output speed sensor which you need to deal with. You might gets some help from my post (it's #56) to this thread:

Replacing the Transmission Speed Sensors - JeepForum.com
So me having a 98 gc larado with the i6 4.0 i have TWO sensor's? One the trans speed and the other the vehicle speed?
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Unread 10-20-2010, 12:21 PM   #15
scodo68
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Just replaced my output speed sencor while the battery was disconeccted and the cel is no off.
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