1998 xj classic 4.0l 4wd cant pass smog because obdll fails , so fixed NNS then tps and then cps then batt volt low sensor and now code P0720 is coming up whats next i just need this enging light off, so i guess i have the toyota tranny. what do i fix first ive read that theres two speed sensors on the tranny do i do the front one or the back one to get the engin light off an stay off reserched other post but most lead to grand cherokee stuff not xj and the others dont finish an tell if they've fixed it an how . so any help before i spend more $$$ plz thanks

 

"so any help before i spend more $$$ plz thanks"

Check the wiring.

 

P0720 is OUTPUT SHAFT SPEED SENSOR ERROR (Back one)

The OSS is mounted on the left side of the transmission transfer case adaptor, and is a bugger to get to. When you get a chance crawl under your XJ and look around, and behind, the TC shift linkage. It's the blue devil hinding in there.

Here's a pic of the AW4 with the input and output speed sensor locations.



Here's what the speed sensor looks like.



The part number of both the input and output speed sensors are the same: 4897734AA. They are available from Rockauto .com for around $95.00.

If/when you get to it, detach the connector from the OSS and measure the resistance of the sensor from pin-to-pin using a digital Ohmmeter.

The resistance specification is 620.0 ohms at 20° Celsius 68° Fahrenheit.
Between 500.0 and 700.0 ohms is good. If the resistance is too far out of range, replace the OSS. If the resistance is in range, further troubleshooting of the wiring between the OSS and the Transmission Control Module will be required.

Let us know what you find.

Low fluid level can be the cause of many transmission problems. If the fluid level is low adjust the fluid level per the service manual then clear the code and drive the Jeep to see if P0720 reappears. It will only take 100 seconds to come back if the problem is still there.

 

Any time you get a P07XX code......

Clean your NSS before anything else.

 

i cleaned original nss cracked it taking it out got new one from junk yard cleaned an replaced thats when other codes started popping up now im at this code and will check wires tomarrow an to get to the hard to get sensor in the morning , any idea what a new sensor is going to run me

 

When you replaced the NSS (Transmission Range Sensor) did you disturb any of the other connectors at the transmission oil dipstick tube?

Pic below shows the connectors. The '98 may have a Black NSS connector instead of it being Gray. Of the three connectors at the dipstick, the large connector with the red lock is C107. It does not carry transmission data.



Below is a pic of the transmission connector(s) pin outs. Note that the Transmission Control Assembly connector (next to the NSS connector)carries the ISS and OSS signals and grounds.

 

there are some wires that are all screwed up at that conectin point by the dip stick looks like a nest of wires and some do look like there not conected to each other like they should be i will fix that first thing this morning and see what happens , thanks for the info its weird cause the damn jeep runs an shifts so good even the tranny shop said its weird cause it runs so good and that the light is on and all my gages work right

 

That area can be a clusterf**k and is often neglected. Clean all the connectors and wiring off using electronic cleaner before beginning wire repairs so you can see the wire color codes clearly. CRC makes an acceptable electronics cleaner. Note that the wire color codes coming into one half of a connector may be different than the wire color codes going out of the mating connector. Just make sure any wire repair matches colors.

The butchered wiring in that area may be causing 'intermittant' faults which would trigger a CEL. When they are making good contact the transmission would operate normally (?).

Here's some data you can tuck away about the ISS and OSS.

INPUT SPEED SENSOR (ISS)

The TCM uses the Input Speed Sensor (ISS) to detect transmission solenoid functional faults (P0751 solenoid A functional fault, P0756 solenoid B functional fault, TCC solenoid C functional fault). The ISS is a variable reluctance sensor. Changes in the reluctance of a magnetic circuit caused by the passing of the rotor lobes on the overdrive clutch drum result in the ISS outputting an AC periodic voltage wave form. The frequency and voltage of the wave form are proportional to the transmission input speed.

NOTE: SINCE THE OD/CLUTCH DRUM IS STATIONARY IN 4TH GEAR OR WHEN THE VEHICLE IS IN GEAR, BUT NOT MOVING. THERE WILL BE NO ISS SENSOR SIGNAL

OUTPUT SPEED SENSOR (OSS)

The Transmission Control Module (TCM) uses the Output Speed Sensor (OSS) to determine shift points and TCC engagement points. The OSS is a variable reluctance sensor. Changes in the reluctance of a magnetic circuit caused by the passing of the rotor lobes on the output shaft result in the OSS outputting an AC periodic voltage wave form. The frequency and voltage of the wave form are proportional to the transmission output shaft speed.

 

I should add that if fixing the wires doesn't fix your problem you could read out the OSS from the TCM connector instead of trying to do it at the OSS. Refer to the drawing above that shows the Transmission Control Module pinout.

You would access the TCM under the dash on the drivers side, to the right of the steering column, by removing the steering column trim panel (three screws and snaps), and the kick panel under it (two screws).

If you have trouble detaching the connector from the TCM you can remove its two attachment fasteners and drop it down.

Set your Ohmmeter to 2K (2,000) Ohms scale. It will show a three decimal place readout, e.g., 620 Ohms would read .620.

You will read from pin cavity 3 to pin cavity 4 on the TCM connector. This will be reading from the TCM, to the Transmission Control Assembly connector combo at the tranny dipstick, to the OSS. The maximum allowable resstance in the wiring is 5.0 Ohms so this may show up as part of the OSS resistance reading you see. The spec on the OSS is 620 Ohms, but if you see between 500 (.500) and 700 (.700) Ohms the OSS, and its circuit, can be considered serviceable. If you don't see this wiggle the wires at the dipstick and see if it changes.

 

not sure if these pics show up dont know how to upload right but anyways found under the hood by the dip stick at the cluster connector a real ugly sight wires were byassed cause the were to short to splice back in right one or two not even connected in there splice job possibly came apart when i fixed the nss an diconnected it , so went to junk yard and the only cherokee there ive been scavageing from for stuff had a grey connector not a black one like my jeep has , so now can i buy the connectors at my local auto parts store or am i gonna have to hit up a junk yard out of town

 

pics wont come up i tryed but dont know how to copy them from my computer to this thread

 

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