I'm still very confused as the symptoms were the engine rpm dropping way down when it upshifts into 4th, which isn't slipping or a typical transmission issue.

 

To be honest, I’m equally confused by the symptom and that’s probably why I’m explaining it poorly.

The only time I’ve noticed the symptom is while shifting to 4th gear (around 45mph). At that speed or above, it feels like I’m letting off the gas pedal and pressing again in short bursts. When it shifts back to 3rd (or below 45), I regain control of throttle and rpm’s.

Now that I’ve shifted the transmission manually, I’ve noticed that when it shifts to 4th, the rpm’s only drop a small amount (when slowly climbing speeds). So I don’t even know if it’s actually shifting when the symptom occurs.

Last night I drove for an hour at 45-55mph with TCM disconnected and it drove perfectly. So today I’m planning to do the same with TCM connected and hopefully get some video to show what’s happening.

 

The small drop once it's in 4th is probably the torque converter locking up. If you tap the brake with your left foot, you should see it unlock and the rpm jump up a few hundred rpm. With the TCM disconnected, you won't get the torque converter lockup.

 

I was able to get it to do the RPM bounce a handful of times in the videos linked below.

Sometimes this symptom will occur endlessly while above 45mph, and days like today it only does it intermittently. Not only do the rpm’s jump back and forth, but I can actually feel the change and it kinda feels like running out of gas (slight head bobbing).

Shorter video:
Longer video:

 

Also, since this problem started occurring I’ve tried replacing TPS, CrankPS and cleaned entire throttle body (including IAC, MAF and linkage).

 

If I'm doing the math right, assuming you're on stock 215/75 tires with 3.55 gears, you should be at around 1400 rpm in 4th gear at 50 mph with the torque converter locked up. I think what you're seeing for the rpm drop is the torque converter intermittently locking up. Does the problem stop if you pull up on the brake pedal with your left foot?

 

My bad, I should’ve included that I’m riding a 3 inch lift with 265/75R16 (~31inch tires). I’ve tried lifting up on the brake pedal but nothing changes. I also tried tapping the brake with my left foot while this happens but I can’t tell if it’s helping since the rpm bounces are so short.

 

Did you regear or swap the speedo gear to compensate?

 

I’m like 95 percent sure the Speedo gear is stock. I bought the Jeep 2 years ago and lift was already on it so not sure how long it’s been like that.

I added SYE kit to the transfer case last winter and I feel like I remember checking that while I had it apart.

Could the wrong speedo gear be causing that bounce? Or are you using that info to determine something else?

 

Is there a way to tell if I have an aftermarket Speedo gear? I know it’s yellow but I’m not sure if that helps determine how many teeth it has. Or if it’s the right one.

 

Speedo gear won't cause this. I was just trying to figure out if the rpm and mph were correct if the t/c was locked up.

 

That makes sense. After doing some homework I’m starting to wonder if they did install the right Speedo gear after installing the lift and tires. Front diff has a tag that says 3.55 on it so I’m thinking a yellow 30 tooth Speedo gear is what it should have. I know it’s yellow but I never counted the teeth.

Either way, you’re thinking it looks like my TC is locking and unlocking while in 4th? And that’s what’s causing the bounce?

 

Found this in the FSM…

“The engagement and disengagement of the Torque Converter Clutch are automatic and controlled by the Transmission Control Module (TCM). Inputs that determine clutch engagement are: coolant temperature, vehicle speed and throttle position. Clutch engagement is controlled by transmission valve body solenoid number three and by the converter clutch relay valve. The solenoid channels line pressure to the clutch through the relay valve at clutch engagement speeds.”

Maybe I should perform resistance checks on the TCC and Coolant Temp sensor?

 

The TCM controls the locking of the torque converter using solenoid #3. There is no relay. The inputs used by the TCM are the throttle postion sensor (TPS), brake signal, and output speed of the transmission. The transmission has it's own output speed sensor. The input speed sensor is really only there for diagnostics, and as you and the mechanic noted, it doesn't even output a signal in 4th gear. The sensor in the transfer is only for the speedo, and I think the engine computer may use it..

Another reason I suspect you're seeing the t/c locking and unlocking is that you said the problem is not there when the TCM is disconnected and you're shifting by hand. The torque converter does not lock up if the TCM has no power to engage that solenoid.

 

What you’re saying makes a ton of sense. And the biggest problem I’ve been having is not knowing what my issue is, so this at least gives me some hope.

My trusted mechanic is usually a week or two out on appointments so maybe I’ll schedule a time for him to check it out. Do you think I should try piggy-backing the wire from TCM to solenoid 3 in the meantime?

Also, the only DRBIII I could find was at the dealer and they quoted $180 to hook it up and test. So god forbid if I can’t figure it out then maybe I’ll pay them to take a look.