Manual operation of the torque converter lock-up clutch in 1992-1995 trucks with the 4L80E

In our opinion, the factory transmission programming is quite well sorted out and works very well in the majority of applications. This factory programming affords torque converter lock-up clutch function only in 3rd and 4th gears. When towing uphill, with vehicle speeds that are below the converter unlock point, a significant amount of torque converter-induced transmission heat will build. Being able to hold the converter clutch in lock-up mode will prevent this unwanted heat build-up.

An annoying characteristic of the program is that it 'unlocks' the lock up clutch as soon as the throttle is released. The engine returns to a near idle speed and the truck just coasts down, without the benefit of being slowed by the engine.

A simple two-position toggle switch (Part #HDP1053) can be installed on the dash panel to provide a full, manual control of the lock up clutch. This will allow the manual locking of the torque converter in 2nd, 3rd and 4th gears and this is beneficial when towing.

Under towing conditions when vehicle speed must be low due to traffic and or road conditions, the converter can now be held in the lock-up mode while the trans is held in 2nd or 3rd gear.

To accomplish this modification, you will need a two-position, three pole toggle switch. The one we use us a Cole-Hersee #5584.

You must locate the 'BROWN' wire coming out of the electrical connector at the transmission. This wire runs to the ECM and controls the lock-up function in the transmission.

You will interrupt this brown wire at some point conveniently away from the electrical connection on the transmission. Splice a wire to both the transmission-end of this brown wire as well as the ECM-end of it. These two spliced-on wires will run to the new dash mounted switch. We usually mount the toggle switch in one of the ‘block-off’ plates on the dash, however it can be mounted virtually anywhere you like.

The switch has three connector terminals. Connect the ECM-side wire to one of the end terminals on the switch. Connect the transmission-side wire to the center terminal on the switch. Connect the remaining terminal to a good ground hose.

In one position, the switch connects the ECM to the transmission to achieve factory control of the converter lock-up clutch. In the other position, the switch grounds the wire running to the transmission to force a lock-up of the clutch.

That's it—you are done with this installation.

You will find that the engine does not enjoy being held down below about 1500 rpm and that this manual type control will require a bit of getting used to. If, for instance, you forget to switch the manual control off before you stop the truck, you will find that upon leaving a stop, the shift to 2nd, 3rd and 4th gears will be rather abrupt, as the torque converter will be locked up.

We hope that you find this info helpful, however you must feel free to contact us with any questions you may have regarding your 6.5. We are dedicated to support of the 6.5 and are always happy to help.

Questions about this or any Heath Diesel product? Feel free to contact us anytime 1-877 894 6266 or sales@heathdiesel.com

- Bill Heath

Related Links:
GM Electronic Adjustable Transmission Control
GM 6.5 Diesel Parts