Transmission Lock-Up Issue on 4BT swap (1 Viewer)

Jun 20, 2014
[Reposting this from 80 Series Tech as this seems a more appropriate section]

So, the transmission issue I mentioned my trip report last year (here) has reared its ugly head again. I’ll summarize the series of events chronologically.

Following a day of driving across Oklahoma (towards Utah) with a particularly strong headwind, lock-up in the 4L80E started to get fickle. It would continuously slip in and out of lock-up so I had to ease back on the speed to minimize the heat that was undoubtedly being generated in the torque converter.

All of the driving around Utah was relatively low speed so I never needed lock-up, but the drive home was a miserable 50-55mph for 2,175 miles.

A while after I got home, I got the Cruiser in to a see a good transmission guy (aka Wizard). Lo-and-behold, he could not recreate the problem. I figured I’d have to settle with the embarrassment that, during his diagnostics in which he checked the electrical connectors for moisture and shifted through the gears (including a low gear that I didn’t realize was there), he knocked a valve loose or some other inexplicable change that made lock-up work again. Automatic transmissions are built on sorcery.

I then drove it down to Ocracoke, where lock-up functioned wherever I needed it over the course of the ~800 mile trip.

Most recently, I drove to KY and back and on the way to KY, after about 300 miles, lock-up began acting erratically again. By the time I was return to VA, lock-up was just as unreliable as it was coming home from UT.

I have the programmer for the transmission and it’s currently set to shift at certain speeds:

1st -> 2nd @ 10mph

2nd -> 3rd @ 20mph

3rd -> 4th @ 30mph

4th -> LU @ 55mph

I understand the ideal scenario is to have the transmission shift at set RPMs, but as of now, my tach doesn’t work.

Poking around on various forums, I’ve found the following possible culprits

1. The brake switch is bad and the 4L80E is supposed to drop out of LU whenever you hit the brakes. However, I’m not sure that this transmission even has a brake switch (I hit brakes while in LU, and it didn’t instantly fall out of LU). That’s on my diagnostics-to-do-list.

2. A heat sensor. I think the 4L80E has a sensor that will keep it from locking up if the transmission gets above a certain temp.

3. Sensitivity to dropping out of LU is programmed incorrectly. I believe there’s a setting in the programmer that lets you determine what criteria will cause the transmission to drop out of LU (RPM, speed, etc.)

The above are all things I need to investigate further. I wanted to pose the following questions though:

1. If I get the tach working, given the 4BT -> 4L80E combo, does anyone have any suggestions on shift points?

2. Can anyone think of any other causes for erratic LU behavior?
Jun 12, 2007
Portland OR
What kind of tranny controller are you using? What sensors is that tranny controller using to determine shift points? TPS, speed sensor, etc?

If your tranny guy checked the components and wiring I'd say it's a controller setting or a failing sensor if the Lock up used to work fine before. There is a lock up solenoid in the tranny that I'm hoping your tranny guy checked and cleared as good or that could be part of your problem. Was the torque converter new when the 4bt was installed? Do you know what stall your TQ is set at?

On my system I use a speed sensor and a TPS. The speed sensor is on my output on my Tcase (custom) and my TPS (also custom) is mounted to my throttle arm on my injection pump. Using those two inputs my controller can shift my tranny according to the set point's I've tuned on my controller. I'm running a US shift controller (used to be called Optishift).

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