AW4 Troubleshooting

AW4 Troubleshooting
Diagnosis of faults in the Jeep AW4 transmission.

The automatic transmission used in the Jeep XJ Cherokee is nearly bullet proof. Seldom does it ever give trouble. Maybe that is why it is so hard to find good information on how to fix it when it occasionally does act up. The same transmission is used in 600hp turbo Supras so it can handle a lot of abuse.

To test if the trouble is electric or mechanical, pull the fuse to the TCU or disconnect the wiring harness. 1-2 will give first gear only, 3 will give third gear. D will give forth gear. The torque converter will not lock manually. You can drive the Jeep just fine this way until you figure out what is wrong.

If the transmission does not shift as described manually, then you have a serious problem and you need to get another transmission from the junkyard. Don’t buy a rebuilt one. If you have yours rebuilt make sure the shops knows how to handle the AW4. If you choose to rebuild it yourself, the FSM has lots of nice pictures.

Most of the time the trouble will be associated with the Transmission Control Unit or TCU. It is handy to have a spare for trouble shooting although the computer itself rarely goes bad.

A transmission control unit located under the passenger side panel under the glove box controls the AW4. The panel is one piece all the way across and there are more control boxes on the driver side so be careful when you remove it.

There are two controllers that I know about. The 1990 and earlier use a different one from the 1991 and up. Somewhere along the way they combined the TCU and the ECU but I don’t know what year that was. The 1991 and up unit has a green connector. It is not interchangeable with the earlier unit.

The TCU gets inputs from a speed sensor, the Throttle Position Sensor, the brake pedal, the battery and the ignition switch. Some models also have a power / comfort switch.

The only outputs control three solenoids. One controls the torque converter lockup and the other two select the four gears in a binary fashion.

First Gear S1 on S2 off
Second Gear S1 on S2 on
Third Gear S1 off S2 on
Forth Gear S1 off S2 off

As you can see, if the TCU has no power, you will get forth gear in D position. Also with the TCU off, 3 will give you third gear and 1-2 will give 1st gear. You can’t get 2nd manually.

The wires on the 1991 and up TCU are connected as follows:

C3 Speed Sensor Tan
C8 NSS 1-2 Light blue
C9 NSS D Green
C10 Brake switch Light Blue/ yellow
C11 Power switch Tan
C14 Torque Converter White
C15 Solenoid 2 Violet
C16 Solenoid 1 Blue
D7 Ground Black
D14 Battery Red
D16 Ignition Yellow
D1 TPS+ Gray
D3 TPS- Tan / orange

Note that the wire colors and traces may be different on different years. Check your FSM wiring diagrams for the right colors for your Jeep. The pin position is the same on all the TCUs however.

If the speed sensor signal is bad, the controller will force the transmission to stay in first gear. It will not shift at all. Unplugging the controller will give manual operation. This speed sensor is mounted in the side of the transmission under the transfer case shift linkage if you have 4wd. Don’t confuse it with the speedometer sensor in the transfer case.

The brake input is used to unlock the torque converter. The 1990 and earlier used +12 to lock and zero to unlock. The 1991 and up is the opposite.

The TPS voltage should read about .5 volts with the throttle closed and 4.5 volts with it open. It should transition smoothly between the two numbers. It seems to work fine even if the voltage is a little off.

The solenoids are located inside the transmission. They get power from the TCU. You can monitor the voltage to see if they are getting power at the right times. You can also add a power wire and switch to juice up solenoid 2 to get second gear when in the 1-2 position. This gives you control of 2nd gear which is hard to come by any other way.

You can check the solenoids by testing the resistance across the coils. Attach one side of your meter to the battery ground. Good grounds are hard to come by inside the Jeep. Connect the other side to each wire going to the solenoid. They should read around 15 ohms give or take a couple. They seem to read different warm than cold but will be in the teens if good.

The solenoids are expensive and require dropping the pan to change. Make sure you have eliminated other possibilities before swapping one.

Contrary to most wiring diagrams I have seen, the solenoids do not ground through the black wire. They ground to the transmission case. So if you are having trouble with a solenoid energizing, make sure it is properly grounded to the case and the case is properly grounded to the battery before swapping it out.

The speed sensor uses the black wire. If either the black wire or the white wire are damaged in the transmission wiring harness, the transmission may hang in first gear. The harness passes right behind the transfer case shift linkage and can be damaged by the linkage if it is moved around by a transfer case drop or something like that.

Normally, the transmission with switch to 2nd at 20 to 25 mph depending on the TPS signal. It gets its speed reference from the internal speed sensor so if you are in low range, the speed will be different.

In the 1-2 position, the torque converter can lock up in first or second gear. In 3 or D it will only lock in 3 or four. The normal sequence is to shift to second, and then third, then forth and then the torque converter will lock up giving an engine speed of about 2700 RPM at 55 MPH.

Armed with this information, you could be able to sort through nearly any fault in the AW4. The FSM is most unhelpful in that it calls for a DRBII scanner that seems to be unavailable at any price these days. Even if you had one the TCU only gives a few codes that are really not very helpful. You will probably still have to sort through each of the inputs and outputs to find out what is really wrong and get it working again.

16 Replies to “AW4 Troubleshooting”

  1. Great blog! Sorry to get off subject, but I’m new to Nashville and I’m looking for a great Nashville auto repair company,so I can get my oil changed. Have you read any recent buzz? There’s a new auto repair shop called Veterans Auto Services, but I’ve only seen a few reviews. Here’s the address of this new Nashville Auto Repair, Veterans Auto Services 2404 CruzenSt Nashville, TN 37211 (615) 712-9777. Thoughts? Thanks!

  2. Hi. I’ve had a AW4 Jeep xj 1991 that “Hangs” only from 1st to 2nd with really revs high with no power..but then shifts… when I let off the the throttle pedal, (feather it a bit). I’ve replaced the TCU, no diff but it did shift manually just fine as you described. I’ve done the TPS , cables NNS, replaced filter and all 3 solenoids. It’s clean and fluid looks good. I think maybe I once replaced the speed sensor also. Anyway, I’m starting to think it’s the white or black wires on the harness you mentioned in the write up. I wanted to ask that when you used the word “Hangs” That you also meant delays?? Meaning it would eventually shift. “””The speed sensor uses the black wire. If either the black wire or the white wire are damaged in the transmission wiring harness, the transmission may hang in first gear.”””” In any event I’ll replace the sensor again check the wires or run piggy back wires to the TCU. I’ve lived with this shifting like this for over 10 years.

    Nice write up and thank you for posting. Scott

  3. Mine would only shift when the wires made contact. Once I replaced the switch with and wires it shifted fine again.

  4. Hi! Question . aw4 1999 xj , I replaced pump , torque converter , input seal , new fluid and reinstalled , started up fine pressure in cooler line return was 45-55 until warm and settled to 17-18 at idle . I checked fluid level nd was just below max. Line on dipstick. Went for drive shifted fine for several miles than it was starting in 1st shifted to 2 nd ,3 rd and than hesitated and I lost all power. Pulled over and shifted thru every gear ! I have no R-1-2-3-4 th and have 0 pressure on cooler return line!

  5. hi guys i have a question i have a 92 Cherokee it wasn’t moving either way so i decided to change the trans but its doing the same the the car is lifted no a realized when in park its not locking my question is could the problem be the transfer case? any suggestion is welcome

  6. Hi, I have a 96 Cherokee that has same problem(Just 1st and reverse), no codes show up and replacing 10 fuse allows manual shifting, at a loss what to do cause the shops around here are trying to rip me off wanting to replace/rebuild transmission, have been told could be anything from relay switch, vacuums, valves, solenoids, TCU, wiring, unclean parts, faulty wires etc…

    Pretty exhausted over this as can’t get right answer and Jeep not worth putting much into it, has almost 300K miles, debating pushing it off cliff…

    96 was a change year and some parts don’t match up, would appreciate any help, thank you…

  7. Sounds like you have lost the speed reference sensor. This is different from the one that drives the speedometer. The wires goes behind the transfer case shift linkage and can be easily damaged if the transmission mount is loose.

  8. Hi Mike, I have just build a AW4 transmission in my 97’TJ. Line up perfectly 3mm play on the drive hub and the oil pump. TCM not yet connected. From starting the engine I get revers and the forwards no vibrations or funny sound. a Couple of minutes later the talk converter hub drive snaps of at the base of the neck at idling speed both times.
    Any suggestions on what can course this to happen.

  9. Ok im lost i can only manually shift. Checked tcu power replaced tps cleaned nss checked the brake switch it seems to be working. Idk what else it could be. Unless its the tcu its self and info? Also the other day after using riping out the carpet and bed lining the floor board. Then useing 4×4 to get out of buddys drive way it started shifting on the way home. Thats the only time its done it. I have no clue what is going on here

  10. Hi Mike, 94 cherokee had a service on the transmission, fluid change, new gasket and filter kit. Since then the trans temp will rise to extreme when climbing hilly winding roads. Any suggestion please before i return to the service provider.

  11. 92 jeep Cherokee 4×4 aw4 &np231
    Shifts fine but in 3rd goes 1-2-3-and immediately locks the tourque converter
    And in 4th goes 1-2-3-4-and immediately locks the torque converter. Can’t figure out why the converter is instantly energized after reaching top gear. Help. Any ideas?

  12. Hey Guru! My 96 xj will lock up in 1st gear after about 5 minutes of operation . I can unplug TCU and it shifts fine manually. After cooling I can repeat. Also, I’ve had the transmission rebuilt, new NSS, new TPS, and new speed sensor. Any suggestions? I’m losing my mind!

  13. 1997 XJ 4.0L / AW4 2WD. 32″ tires – 3.55 rear gears – 189K miles
    Services done at scheduled time or before – no prior tranny issues.
    I recently had the AW4 rebuilt after loosing all forward and reverse. The tranny shop indicated significant damage to the 3/4 clutch drum, the sun gear and some related small parts as well as the standard rebuild parts. I had been using the AW4 as a 3 speed rather than using the OD position of the shifter to keep engine RPM more in range of where it would be with the OEM 27″ tires. The P1898 code sets within a short distance of shifting into OD, and it never set a code using the 3rd gear position. The tranny shop is contributing the cause for the failure to using the tranny as a 3 speed rather than allowing the TCM to control shifting to OD when conditions are met. The Jeep needs a axle gear ratio change. Having a ratio that allows the speed at RPM to be closer to what it was with the 27″ tires should solve the P1898 code when using OD. Hopefully that will prevent future issues. Until then, is using the AW4 as a 3 speed causing damage that could be avoided if OD is used?

Comments are closed.