89-91, all Cummins with automatics came with the 727 and 3.07-geared Dana 71 rear/61 front (4wd) axles, no intercooler.
91.5-93, A727 dropped in favor of the A518 transmission, essentially a 727 with OD bolted in place of a tailhousing. 3.07 differential gears were dropped for 3.54's and 4.10's, but kept on as an option for 5-speed trucks.
Also included an intercooler behind the new-for-91 grill, and smaller fuel injectors. The KSB (timing advance/cold start aid) changed in how it functioned. Otherwise the pumps are the same VE44 Bosch. Very tuna-able for fuel mileage.
The transmission is capable of towing, adjust the bands, increase the line pressure, and put a Goerend or ATS converter in it. There's a guy out of Minnesota that tows a large 5th wheel camper and a boat behind that. Goerend built him a torque converter that's tight enough he can engine brake through it.
My short list of mods if I was buying one to tow:
366 (3200 RPM) governor spring
M&H timing spacer (more advance at top end without advancing timing at low RPM)
fuel pressure gauge (doesn't need to be seen from inside, can stay underhood)
trans temp gauge
torque converter (AVOID DTT!!!!!!! LIKE THE PLAGUE!!!!)
headlights on relays
trailer marker lights on a relay (all power to lights goes through the headlight switch in stock form and has caused fires over the years)
Many engine parts can be sourced through Case.
Go through MOPAR for the fan clutch and Cummins for oil pressure sending unit, temp sending unit, water pump, gaskets, etc.
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