What a Tow/Haul tune actually changes — and what it should leave alone

Press the Tow/Haul button on a stock truck and three things happen: upshifts are delayed a little, shifts are slightly firmer, and the transmission downshifts on its own when you brake downhill. All of it is a step in the right direction and none of it goes far enough. The converter still unlocks on every grade, the truck still hunts at 60 with a trailer behind it, and on a long descent the grade braking kicks down later than you would like. A Tow/Haul tune finishes what the factory started, and does it in the Tow/Haul schedule so the empty truck keeps its manners.

Two schedules, two jobs

Every platform we tune carries Tow/Haul as a separate set of tables — its own shift points, its own converter strategy, often its own pressure offsets. That is the whole opportunity. Normal mode can stay comfortable for the commute and the grocery run, and Tow/Haul can be as businesslike as a loaded truck needs. When a customer tells us the truck is hooked up most of the time, we move some of the towing behavior into Normal as well; when it tows twice a year, we keep the split clean. The order form has a place to say which.

Converter lockup under load

The single most important change for a towing truck is what the converter does on a hill. The factory releases the clutch when engine torque crosses a threshold, partly to protect the clutch and partly because a surging engine is "smoother" to a test driver than a held gear. With 10,000 pounds behind you that threshold is crossed on every overpass: the converter unlocks, the engine jumps 500 rpm, transmission temperature climbs, and the truck feels like it is fighting you. In Tow/Haul we raise the release threshold so the clutch stays applied under load, lock earlier and in lower gears so it is coupled before the grade rather than after, and set the apply pressure so the lockup holds without slipping. A locked converter under load makes no heat; a slipping one makes a lot. On the Allison and the 68RFE, where the stock strategy also holds a controlled slip, this change alone is why towing transmission temperatures drop after a tune.

Gear holds and the highway hunt

A trailer turns a mild 5-6 hunt into a constant one, because the load pushes the truck across the upshift and downshift lines on every rise. In Tow/Haul we hold gears longer before the upshift — the engine stays in the part of its torque curve that is actually pulling — and we space the downshift lines well below the upshifts so a rolling grade does not cross both. On the ten-speeds this also means fewer skip-shifts: a loaded truck wants to go 5-6-7, not 5-7, because the double step drops it out of its torque band exactly when it needs it. The 10R80 guide has more on skip-shift logic.

Downshifts and grade braking

Tow/Haul grade braking downshifts the transmission when you touch the brakes on a descent so the engine helps hold the load back. We make the downshift schedule more willing in Tow/Haul — it kicks down sooner and will take the next gear down when the first one is not enough — and we set the downshift speeds so the engine never lands above its rated rpm. On Cummins trucks the exhaust brake request works together with these downshifts: the brake needs a low enough gear to have anything to push against, and a more willing downshift schedule gives it one. On diesels with a factory exhaust brake we leave the brake's own logic alone and give it a transmission that cooperates.

Pressure under load

A towing shift carries more torque through the clutch than an empty one, and the stock pressure offset for Tow/Haul is modest. We raise shift pressure in Tow/Haul so the clutch packs are applied firmly under the heavier torque, which is the difference between a shift that holds and one that slips a little on every grade and eventually fails. This is the part of towing tuning with real durability consequences, and it is why we keep the numbers inside what the pump and valve body of each unit can hold rather than chasing the hardest shift possible.

Platform notes

• Ram 68RFE (6.7 Cummins): EMCC slip replaced with full lockup, earlier lockup in Tow/Haul, raised release torque, longer gear holds, downshifts that work with the exhaust brake. Platform page.
• Allison 1000 / 10L1000 (Duramax): lockup in third and up at full pressure, Tow/Haul holds and pressure set for the Duramax's torque, with the TCM's slip detection kept honest so it never calls limp mode on a tune. Platform page.
• GM 6L90 and 10L90 HD gas trucks: firmer Tow/Haul shifts, lockup in the cruising gears, hunt removed at highway speed. 6L80/6L90, 10L80/10L90.
• Ford 10R140 and 6R140 Super Duty: lockup that stays put under load, tow/haul scheduling, and tire and gear corrections after a lift so the shift points land where they should. 10R140, 6R140.

What to tell us

How much the truck weighs loaded, how often it is loaded, and what it pulls — a 14,000-pound fifth wheel and a 4,000-pound boat want different Tow/Haul schedules. Gear ratio and tire size if either is not stock. And whether the truck has an aftermarket converter, because stall speed changes every lockup number we would otherwise use. A short VCM Scanner log of a loaded hill, if you can get one, is worth more than a paragraph of description.