How to add a Tow / Haul switch to your 91-up GM truck equipped with an electronic auto (4L60-E or 4L80-E) transmission
Starting around 1999-2000 model year, GM introduced the Tow / Haul feature on some of their trucks. When activated, this feature altered the shifting strategies of the auto transmission to aid towing and hauling of heavy loads. Earlier trucks (and some later) were not equipped with this feature.
Most GM trucks equipped with electronic auto transmissions dating back to as early as the 1991 model year had separate transmission shift tables in the computer programming which were only used when the cruise control system was engaged. The computer determined the cruise control was engaged when it received a ground signal on a certain input wire from the cruise control module. What we have discovered (on our own applications) is we can reroute this circuit to a simple toggle switch mounted on the dash to force the computer into using these shift tables in the programming regardless of what speed the vehicle is traveling at or if the cruise control is truly engaged or not. Cruise control function is NOT altered at all with this modification.
We can then reprogram these "cruise" shift tables in the computer with delayed upshifts to help the engine pull heavy loads in around town driving situations and prevent moderate throttle kickdown shift points at highway speeds (this prevents the transmission from kicking down so easily when towing a load going up a slight hill and making the engine rev). We have found this particularly useful when the cruise control is activated on the highway. The transmission will still kickdown a gear if you apply full throttle at highway speeds, it just won't do it at the lesser throttle openings that the cruise control usually operates at. This can help save gas and wear and tear on your engine.
We are offering this service at the lower priced "off the shelf" tune price and will also supply you with instructions on how to wire up the toggle switch. This modification only works with 1991-up Gasoline Trucks equipped with the 4L80-E transmission and 1993-up Gasoline Trucks equipped with the 4L60-E transmission. It will NOT work with manual transmission and non-electronic auto transmission (such as the 700R4 / 4L60) applications.
If your truck already has a Tow / Haul mode button, we can make changes to those specific shift tables if you prefer.
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