91 Suburban R2500: Tow/Haul Mode?

Greetings,
I've just picked up a 91 Suburban R2500 with a 6.2 diesel and a 4L80E 4 speed overdrive transmission.
I understand that the 4L80E has a tow/haul mode, but I can't
find any switch to engage this in my truck.
I called GM, and they say there was no tow/haul mode in 91.
Not that I don't trust GM ;-) but is this really true?
I'm wondering if my transmission does have this, but it just isn't hooked up, and whether I can wire it up myself?
TIA for any and all replies/pointer/suggestions,
--Duncan
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Tow/haul is not part of the transmission, its programming in the PCM and I'm sure a 91 has no chance of having tow/haul

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X-No-Archive: yes

nope, none, nada... In the past when I was going to tow a long distance I have unhooked the battery and left it for a day or so. In this time frame the computer will "forget" all of its settings and will revert back to default settings. Then when you drive while pulling the trailer it will relearn the proper shift points and so on based on towing the trailer. So this is about as close as you get to tow/haul. But don't expect too much this trans (IMHO) the trans is set to down shift too soon...but if you have a trans cooler and change the oil often it will last a long time. Also remember the engine is only about 150hp so the truck was built for fuel economy not to tow a huge load. So don't go overboard.... On second though you can get a black box that plugs in between the trans and the computer. The box will change the shift points and increase the line pressure but it is an aftermarket item. Any trailer or towing store will have the black box I can't remember its name but they will know what you want by description.
good luck, mark
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