4L60-E "Event" Has Me Stumped So Far

[Joe In Montana]

Riding in buddy's '98 Yukon, 5.7V, Push button 4WD/2WD/Auto system, 65 MPH, stock tires/rims, heater ON, leval ground, gently twisting sweeper turns, nothing really demanding of the vehicle at that time.........

All of a sudden it drops out of OD/Lockup, and goes into Neutral.

I had him drop the shifter into "3rd" and it immediately connected with the engine and was running along just fine.

We had another 2 miles to go to our destination so I had him slow down a bit and just cruise the rest of the way in 3rd. Like I said, it was hooked up in 3rd OK.

Made a stop sign, and the transmission shifted from 1 - to 2 - and then to 3rd (direct) and worked just fine.

Since we were over 200 miles from home, I told him we'd look at it soon enough, but for the time being, just run in 3rd for the rest of the trip - or at least until we could get to a town of some sort - there was no cellphone coverage where we were at this time either.

We got our business done 'way out almost to Canada, and got back to Kalispell where I felt good enough to drive it myself. (I'm on crutches from knee replacement surgery, so I'm not totally up to speed yet and I certainly can't walk out of the woods for much more than a few dozen yards at best).

Well, I manually shifted up to OD/TCC and although the 1-2 shift was a little too firm, everything seemed normal.

I checked the fluid level - it was right where it should be and was still clean, not burnt and no sparkles in it.

This seems to be a one-time occurrence. But I don't believe in transmissions repairing themselves.

Anyone with any ideas?

I'm thinking valve body - but I've never had a unit just drop out of gear spontaneously - especially when it's in a cruise mode and no shifting has just taken place nor is one wanted - we were cruising at 70 MPH and it just neutralized all by itself - so I don't get it.

 

[Justin V]

I had my S-10 act up in a similar way once. It had about 155K on it, and I was on a long drive, in cruise, sweeping curves and rolling hills/almost mountainous terrian. I don't really have any answers for you. But it did the same thing. I let it cool off upon reaching my destination, and never had any more trouble with it until a couple years later (like 180K) when I was parallel parking one day and went from drive to reverse (at a full stop), and it slammed into gear. A day later it slammed into reverse again and I pulled it and gave it to my transmission guy. He said the clutches were gone, but nothing else was wrong. He said it was one of the easiest overhauls he has ever done. It's been roughly 14K since he overhauled it. No problems.

 

[Blue85]

It's possible there's nothing wrong internally but there is (was) an intermittent connection with one of the sensors or one of the solenoids. I had a car that would freak me out about once a year by downshifting or going into neutral or something - only while using the cruise control. Would never repeat it.

Going into Neutral makes me wonder about the transfer case. I've had an NP208 drop out of gear after hard stops because the shift forks were done. Had a GM "Auto" transfer case pop into neutral on the highway at seemingly random times. It was the sensor on the T-case actuator - would lose signal from vibration and shift. Stop the truck, turn it off and it was fine again (for a while).

 

[Justin V]

I forgot to mention that my transmission guy said that he performed an update to the TCC circuit. If I remember correctly (which it is possible that I don't), he said something along the lines of different TCC lockup solenoid or something. He said GM used a variable control unit factory, something that would roll into lockup, gradually applying pressure or only partially engaging it, but GM released a TSB saying to change it to a full on/full off type control.

 

[Blue85]

You can swap a valve in the valve body to disable the PWM. From what I've read, the PWM control is fine, as long as you have a later style torque converter. The '00s and '10s trucks run PWM a lot, almost like having a variable transmission.

 

[Joe In Montana]

About PWM .....

As I understand it, PWM convertor use a Kevlar matrix friction material only. Since this unit has never been apart I ca n only assume it is non-PWM and non-Kevlar as it is all original...... albeit old OEM internals.

Last service that t I did for my buddy, was clean. That was two years ago. I'd better check it again as soon as my knee heals a little better.

But please, keep the ideas coming. I hate diagnosing a non-replicable problem.

 

[Greg Ducato]

Could have been as simple as a sticking/leaking solenoid or could be the beginning of something worse. I would keep on trucking with it, if the trans is going away it will warn you a few times more before failure. You may also want to check for any codes stored that may lead you to a fault, one time or otherwise. Intermittent problems are the hardest to diagnose.

 

[Joe In Montana]

Fortunately I had my Snap-on scanner with me ..... we were looking to buy another Yukon on that trip.

There were no codes or failures since last code clearing. I know there was enough mileage on it to have qualified two or three OBD Drive Cycles.

I am still upset that I cannot say what truly happened. It was cruising at about 70MPH and it just dropped into neutral. At this point I'm not going to dump the Adaptive Strategy either, as that won't prove anything.