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P0740 question

U

UNCC Grad

NAXJA Forum User
Location
Salisbury, NC
99 XJ, 4.0, AW4 and I'm getting a p0700 and p0740 code and the TC will lock but only when cold. Once warm the TC stays unlocked. I've adjusted and cleaned the NSS, swapped in a spare Trans control unit and the problem still persists. Fluid level is good and doesn't smell or look burnt. I tried testing the TCC solenoid for resistance but I'm having issues with my tester not getting consistent results.

Just wondering here, if my cruise worked intermittently, could it possibly be a brake switch issue? I have a spare from the junkyard to try but I just want to understand how my cruise could work sometimes (it's finicky) and it still be an issue with the brake switch.

I'm really thinking it may be the solenoid but I want to try the cheap fixes before dropping the pan and replacing the solenoid and filter/fluid.
 
If you want, you can do the tests of the lock out solenoid (and others)outlined in the images below. The resistance could be within spec but the solenoid pintle may be sticking. Use a quality manual digital Ohmmeter. I do not recommend auto ranging meters.

Wouldn't hurt to do the voltage checks while you have it open.

Visio-tcmpinout.jpg


TCMChecks.jpg
 
I did these tests today and got 0 resistance on the output speed sensor. I've been having an intermittent issue with rpms dropping from 1500 to 1200 and back when coasting in D, like when you are going to turn or stop.

Could a bad output sensor cause the TC to lock and unlock when coasting? I would think I would be throwing a code for and output sensor failure but I'm not. I plan to disconnect and clean the input and output speed sensors and see if it makes a difference.
 
UNCC Grad said:
I did these tests today and got 0 resistance on the output speed sensor. I've been having an intermittent issue with rpms dropping from 1500 to 1200 and back when coasting in D, like when you are going to turn or stop.

Could a bad output sensor cause the TC to lock and unlock when coasting? I would think I would be throwing a code for and output sensor failure but I'm not. I plan to disconnect and clean the input and output speed sensors and see if it makes a difference.
Click to expand...

What resistance did you see on the Input Speed Sensor?

What resistance did you see on the tree solenoids?

The output Speed Sensor can cause lots of problems. Here's some speed sensor data I put together some time ago. It may help you in understanding the ISS and OSS functions.

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.

P0720-OUTPUT SHAFT SPEED SENSOR ERROR

When Monitored: Continuously with the vehicle speed over 6 MPH (over the CCD bus) and the transmission is in a forward gear (1st, 2nd , 3rd or OD).
Set Condition: No signal from the Output Speed Sensor for 100 seconds.

Possible Causes:

INTERMITTENT WIRING AND/OR CONNECTORS
OUTPUT SPEED SENSOR GROUND CIRCUIT OPEN
OUTPUT SPEED SENSOR SIGNAL CIRCUIT OPEN
OSS SIGNAL CIRCUIT SHORTED TO OSS GROUND CIRCUIT
OUTPUT SPEED SENSOR GROUND CIRCUIT SHORT TO GND
OUTPUT SPEED SENSOR SIGNAL CIRCUIT SHORT TO GROUND
OUTPUT SPEED SENSOR GROUND CIRCUIT SHORT TO VOLTAGE
OUTPUT SPEED SENSOR SIGNAL CIRCUIT SHORT TO VOLTAGE
OUTPUT SPEED SENSOR (OUT OF RANGE)
TCM-OUTPUT SPEED SENSOR

NOTE: Low fluid level can be the cause of many transmission problems. If the fluid level is low locate and repair the leak then check and adjust the fluid level per the service information.

NOTE: The Input Speed Sensor resistance will vary depending on temperature.
NOTE: The resistance specification is 620.0 ohms at 20°Celsius 68°Fahrenheit.

A resistance of between 500.0 and 700.0 ohms is okay per the diagnostics manual.

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 hiding in there.

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

Visio-AW4COMPONENTS.jpg


Here's what the speed sensor looks like.

AW4SpeedSensor.jpg


The part number of both the input and output speed sensors are the same: 4897734. Rockauto has them. They are mistakenly referred to as Vehicle Speed Sensors.

I'm not sure just cleaning the OSS will help. There may be other damage to it, like shorted wires or corrosion in the connector. An OSS is expensive so I'm reluctant to tell to buy a new one before attempting to clean/inspect your OSS. You may want to see if you can get one off of a junk yard AW4 ('99 - '01 XJ).
 
The solenoids tested at 12.6-12.7 for all 3. I couldn't get a reading at all for resistance of the OSS. ISS was around 670 if I recall...it did check within specs. Temp was around 70* during the tests.

Strangely I got no voltage when testing the TPS with the Jeep running (pins 16&17). The TPS was just replaced with an auto store unit, thinking it may be causing the rpm changes but it had no effect. The throttle body was cleaned at the same time.
 
UNCC Grad said:
The solenoids tested at 12.6-12.7 for all 3. I couldn't get a reading at all for resistance of the OSS. ISS was around 670 if I recall...it did check within specs. Temp was around 70* during the tests.

Solenoids and ISS look okay. OSS needs attention.

Strangely I got no voltage when testing the TPS with the Jeep running (pins 16&17). The TPS was just replaced with an auto store unit, thinking it may be causing the rpm changes but it had no effect. The throttle body was cleaned at the same time.
Click to expand...

You don't need the Jeep running to do the voltage checks, just turn the key to RUN.

Sometimes it's advisable to put safety pins in the connector pin cavities for the meter hook up to free up your hands while depressing the gas pedal.

You should get voltage readings for the TPS. If the TCM is not getting TPS signals there will be problems.

Pic of safety pins you can get at a fabric store or hardware store. #1 or #2 should be used for connector pin cavities. #3 to be used for back probing.

Probesjpg.jpg


Example of using safety pins.

CavityProbing2pins.jpg
 
I figured it needed to be running to measure the resistance across the TPS. Will have to try that one again. I'm trying to source a some sensors to at least swap out the OSS.

Thanks for the help so far, very much appreciated.
 
UNCC Grad said:
I figured it needed to be running to measure the resistance across the TPS. Will have to try that one again. I'm trying to source a some sensors to at least swap out the OSS.

Thanks for the help so far, very much appreciated.
Click to expand...

You should be doing voltage checks on the TPS, not resistance checks.

You should not be doing resistance checks with power on the bus. Always disconnect the battery when doing these checks. Obviously, the battery needs to be connected when doing voltage checks.

I thought I covered that on the check list.
 
That was a typo on my part, yes voltage check on the TPS. I did that but with the engine running...I still should have been reading something but got nothing regardless of throttle position.
 
Sidewinder CC said:
s
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 hiding in there.
Click to expand...


Well a trip to the local scrapyard yielded the ISS, OSS and all three shift solenoids. I didn't have my ohmmeter so I couldn't test before removing them. The trans was from a '00 2wd so the OSS was easy to get to. On the 4x4 models, do you have remove the transfer case bracket to get access to replace the OSS? I didn't spend much time today looking at my Jeep but it seemed like it would. Be really hard to replace with the bracket on there.
 
UNCC Grad said:
Well a trip to the local scrapyard yielded the ISS, OSS and all three shift solenoids. I didn't have my ohmmeter so I couldn't test before removing them. The trans was from a '00 2wd so the OSS was easy to get to. On the 4x4 models, do you have remove the transfer case bracket to get access to replace the OSS? I didn't spend much time today looking at my Jeep but it seemed like it would. Be really hard to replace with the bracket on there.
Click to expand...

Remove the bracket.
 
OP, were you able to fix your problem? I have a '98 having the same issue. I have read a lot of different posts and people never seem to post their "fix." I also have changed all 3 solenoids with quality replacements, changed my TPS with a MOPAR, fluid looks to be in good shape, and besides occasionally not shifting into OD once warm, the jeep is running great. It is so weird that it happens intermittently and never consistently. I do not trust transmission shops and would rather try to isolate and identify the problem myself. Someone please advise.
 
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