KZJ78 diagnosis issue ATF/4WD (1 Viewer)

[spartanglory71]

Hi all. I’m at a loss for ideas on diagnosing my KZJ78 intermittently refusing to shift into 4th gear. It’s an auto with the 4speed A343F.

Basically, I was driving down the highway, and as I was passing a semi, the 4wd started to flash on the dash. After that, it refused to shift into 4th gear and continued to climb in revs to 3500-4000 in 3rd before I backed off and pulled over.

The following troubleshooting occurred:
• ATF was dirty and overfilled, but did not smell burnt.
•went though a couple of ATF changes. Has completely clean ATF now at correct level.
•replaced the 4th gear solenoid(did some rabbit hole digging here from a thread from 2022 where someone explained the exact same symptoms)
•Cracked open the trans ECU. Nothing leaking and no melted terminals/connections. It had been replaced before, as it was a Dorman branded ECU

After all this, I’ve done some other troubleshooting. I’ve found slight improvement after the above. To include, the trans will shift into 4th when driving on flat roads. However, when I drove up some of the canyons here near SLC, it seems like the increased load on the engine/trans going up a sustained incline produces more heat(?)Or whatever and will refuse to shift into 4th under this increased load.

During all this the 4wd will still blink when refusing to shift into 4th gear. The car will shift into 4 low and back to 4 hi without issue. So it leads me to believe that the trans thinks it’s in 4wd when it’s not, disallowing it to shift into 4th.

So, what gives? I’m out of ideas other than troubleshooting the 4wd solenoids and actuators for issues. After that I’ll be lost for ideas.

 

[Tanner H]

I also own a KZJ78 and I would not expect the 4wd light to flash due to a transmission problem. I would assume the Trans light near the shifter would light up, or the OD off light would flash near the steering wheel.


Just thinking out loud here:

-4wd light comes on when the transfer case button is pressed for 4x4. Light goes off when back to just rear driveshaft. flashing tells us its not fully engaged or something is wrong/binding.

....Is it possible something is going on inside the transfer case...causing more load applied to the transmission and its not locking up into 4th because of the extra load?


front driveshaft should not spin while driving. I would have someone drive the car forward slowly and you can look under neath to see if the front driveshaft is spinning. if its spinning...you can try removing the drive driveshaft and drive around to see if your transmission behaves normally.


Not sure if this will get you any closer to your resolution but its free to check and doesnt hurt.

 

[AVL KZJ78]

Hi all. I’m at a loss for ideas on diagnosing my KZJ78 intermittently refusing to shift into 4th gear. It’s an auto with the 4speed A343F.

Basically, I was driving down the highway, and as I was passing a semi, the 4wd started to flash on the dash. After that, it refused to shift into 4th gear and continued to climb in revs to 3500-4000 in 3rd before I backed off and pulled over.

The following troubleshooting occurred:
• ATF was dirty and overfilled, but did not smell burnt.
•went though a couple of ATF changes. Has completely clean ATF now at correct level.
•replaced the 4th gear solenoid(did some rabbit hole digging here from a thread from 2022 where someone explained the exact same symptoms)
•Cracked open the trans ECU. Nothing leaking and no melted terminals/connections. It had been replaced before, as it was a Dorman branded ECU

After all this, I’ve done some other troubleshooting. I’ve found slight improvement after the above. To include, the trans will shift into 4th when driving on flat roads. However, when I drove up some of the canyons here near SLC, it seems like the increased load on the engine/trans going up a sustained incline produces more heat(?)Or whatever and will refuse to shift into 4th under this increased load.

During all this the 4wd will still blink when refusing to shift into 4th gear. The car will shift into 4 low and back to 4 hi without issue. So it leads me to believe that the trans thinks it’s in 4wd when it’s not, disallowing it to shift into 4th.

So, what gives? I’m out of ideas other than troubleshooting the 4wd solenoids and actuators for issues. After that I’ll be lost for ideas.

May be a stretch, but I had an issue with lockup not happening, or happening intermittently. It was the thermostat stuck halfway open, the ECU will only allow lockup above a certain temperature, which in my case was not consistently reached. Sometimes an unlocked TC can feel like a lower gear.

 

[Tanner H]

@AVL KZJ78 yeah i thought torque converter at first too...but the 4wd flashing makes me think t-case related first and could be having a domino effect into the transmission shifting issue.


Intersted in how this pans out just in case my 78 has the same issue in the future.

 

[GTSSportCoupe]

Engine ECU can have leaking capacitors and cause weird transmission issues. Do some searching. There are some techs who can repair the damage on these. It's more than just capacitors, as usually traces on the PCB need to be reconstructed.

 

[ONIONSLAYER]

Hey guys, this post is about my KZJ78, OP is a good friend.

I've taken a look at the ECU and it's in perfect condition. Not saying it's not broken, just that it looks good.

Some further information on the issue. When it has the issue it also tries to engage 4wd at freeway speeds. I can hear the shift collar in the transfer case grind. I have since unplugged both the 4wd control module and the vacuum line so it does not grind anymore. However even with the control module unplugged, the VSV is still switching to engage the 4wd when this happens.

I just tested on my way to work this morning to see if it would happen on normal freeway driving instead of climbing a hill or under load and it did still happen. After about 20 minutes of driving, 10 minutes surface streets and 10 minutes freeway driving at 70mph and ~2000rpm the torque converter starts to unlock and lock repeatedly like it's trying to downshift then the first chance it gets it drops out of O/D.

After this happens it will also hang onto gears much longer, it won't shift from 1-2 until ~2500rpms and same with 2-3. While the 4wd light is not flashing anymore because the 4wd control module is unplugged the VSV had still switched as if it was trying to engage 4wd.

After a complete cool down it will drive again as normal until I try driving at freeway speeds or under load again and the cycle repeats.

Things I've done so far besides checking the ECU include replacing all of the transmission solenoids along with fresh fluid as OP stated. Disconnected the cooler lines as well and they both have fresh fluid leading me to believe the trans cooler isn't clogged. I opened on the 4wd control module and it's in good condition as well. After it has gone into the weird mode I've tried going through the gears in low range then back to high range to see if the ECU somehow thinks it's in low range. I read somewhere it won't go into O/D in low range and thought that could be a possibility. I've also tried disconnecting the battery after it enters the weird mode and trying to start the truck to drain capacitors and such with no change. It only goes back to normal after a complete cool down.

I'm nearing a total loss here. Please let me know if there is anything I can clarify or check on. I'll update as I find more info.

 

[ONIONSLAYER]

May be a stretch, but I had an issue with lockup not happening, or happening intermittently. It was the thermostat stuck halfway open, the ECU will only allow lockup above a certain temperature, which in my case was not consistently reached. Sometimes an unlocked TC can feel like a lower gear.

This could be a possibility on the freeway however climbing hills the temp gauge is well above halfway when it happens.

 

[ONIONSLAYER]

Maybe I'm confused here and it's not dropping a gear but the torque converter isn't locking? Any troubleshooting I can do for that?

 

[AVL KZJ78]

Maybe I'm confused here and it's not dropping a gear but the torque converter isn't locking? Any troubleshooting I can do for that?

I would still check the thermostat, unless you have an accurate gauge reading from the upper return neck or hose. The gauge in the dash displays “normal” over a wide range of actual coolant temps.