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t vago · 09-03-2016, 03:20 PM

I might just try this.

My 2000 Dodge Dakota has a transmission computer that is physically separate from the engine computer. I have taken advantage of this fact to electronically convert my transmission from a 4 speed to a 5 speed. However, my truck's computer bus is incompatible with the 5-speed transmission computer's bus interface, so the transmission computer does not communicate with the rest of the truck network.

For the longest time, I've just lived with this, as I was able to get a 5th gear out of the deal. One of the side effects is that the torque converter would lock up at about 46-47 MPH, instead of the 45 MPH called out for in the factory service manual. The 5th gear, which is supposed to kick in at 55 MPH, actually kicks in at 57 MPH. Other shift points, transitions, etc, are similarly affected. It's like the 5-speed transmission computer was set for a tire size about 5% larger than what was on my truck.

Another reason I lived with this is that the actual speedo information is derived from the truck's ABS system, and is fed to both the engine computer and the transmission computer via a pulse stream. This is not a terribly complicated system, and the pulse stream is also feeding my electrohydralic power steering pump unit. This is the standard setup for the 3rd Gen Dodge Dakota and the 1st Gen Dodge Durango, as it allows a variety of different transmissions to be used (including 5-speed manual) without having to change the wiring.

Recently, though, I got this bright idea to temporarily hook up the 5-speed transmission computer to my 2001 Durango (which does have a compatible computer bus and the same transmission electronics), program the Durango to have a ridiculously large tire size, disconnect the transmission computer, reprogram the Durango back to its normal tire size, and then re-install the Durango's transmission computer.

For the programmer I would use to do the transmission computer tire size setting change, its user manual states that I could probably go up to a tire size of 42 inches. That presents some fascinating possibilities. For instance, torque converter lockup could occur at 31 MPH, instead of 45. The 5th gear would become active at 38 MPG, instead of 55.

I'm trying to think of some downsides to this, but can't think of any right now.

I'll post later on today with my initial findings.

09-03-2016, 03:20 PM	#1 (permalink)
t vago MPGuino Supporter Join Date: Oct 2010 Location: Hungary Posts: 1,807 iNXS - '10 Opel Zafira 111 Anniversary Suzi - '02 Suzuki Swift GL Thanks: 830 Thanked 708 Times in 456 Posts	Change Transmission Computer tire size setting for FE? I might just try this. My 2000 Dodge Dakota has a transmission computer that is physically separate from the engine computer. I have taken advantage of this fact to electronically convert my transmission from a 4 speed to a 5 speed. However, my truck's computer bus is incompatible with the 5-speed transmission computer's bus interface, so the transmission computer does not communicate with the rest of the truck network. For the longest time, I've just lived with this, as I was able to get a 5th gear out of the deal. One of the side effects is that the torque converter would lock up at about 46-47 MPH, instead of the 45 MPH called out for in the factory service manual. The 5th gear, which is supposed to kick in at 55 MPH, actually kicks in at 57 MPH. Other shift points, transitions, etc, are similarly affected. It's like the 5-speed transmission computer was set for a tire size about 5% larger than what was on my truck. Another reason I lived with this is that the actual speedo information is derived from the truck's ABS system, and is fed to both the engine computer and the transmission computer via a pulse stream. This is not a terribly complicated system, and the pulse stream is also feeding my electrohydralic power steering pump unit. This is the standard setup for the 3rd Gen Dodge Dakota and the 1st Gen Dodge Durango, as it allows a variety of different transmissions to be used (including 5-speed manual) without having to change the wiring. Recently, though, I got this bright idea to temporarily hook up the 5-speed transmission computer to my 2001 Durango (which does have a compatible computer bus and the same transmission electronics), program the Durango to have a ridiculously large tire size, disconnect the transmission computer, reprogram the Durango back to its normal tire size, and then re-install the Durango's transmission computer. For the programmer I would use to do the transmission computer tire size setting change, its user manual states that I could probably go up to a tire size of 42 inches. That presents some fascinating possibilities. For instance, torque converter lockup could occur at 31 MPH, instead of 45. The 5th gear would become active at 38 MPG, instead of 55. I'm trying to think of some downsides to this, but can't think of any right now. I'll post later on today with my initial findings.