Change Transmission Computer tire size setting for FE?

[t vago]

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.

[LittleBlackDuck]

Main downside I can see is lugging the engine at low RPM. There was a thread on this elsewhere in the forum and I will edit in a link if I can find it.

Simon

[t vago]

Running the numbers, I am going to try for an apparent tire size of 37 inches in diameter. That should give lockup in 4th at a bit over 35 MPH at an engine speed of about 1100 RPM, and 5th gear at 43 MPH at an engine speed of about 1190 RPM.

The 4.7L engine in my Dakota has a fairly respectable torque curve. The 4.7L in the Durango, being a next-gen Corsair with higher compression and two spark plugs per cylinder, would likely have an even better torque curve. At least, that's how I'd interpret my Durango's better response and performance over my Dakota's engine.

[t vago]

So, for a test of this idea, I changed the tire size setting on the Durango. First, I set the tire size to a 32 inch diameter. Then, I pulled the transmission computer ignition power fuse, and verified that the programmer could not read the transmission computer. Then, I reset the tire size back to normal. Finally, I reinstalled the transmission computer ignition power fuse.

I took the Durango out for a quick test drive, and verified that the new lockup and 5th gear set points changed at 40 and at 45 MPH. So, the idea works...

Kind of. You don't need to worry about lugging the engine at low speeds. Once the Durango encountered any sort of hill, it disengaged the torque converter lockup and shifted out of 5th, if it had gone into 5th gear.

Lucky for me, the transmission computer has a wire signal input for the throttle position sensor. I might be able to come up with something that will fool the transmission computer into thinking the throttle is less open than it really is.

[t vago]

I did some further research into this idea. I took the Dakota's TCM (Transmission Control Module, also known as the transmission computer) out of the Dakota, and temporarily installed it into the Durango in order to change the tire size setting.

Along the way, I also inserted a thin washer between the throttle stop and the throttle sector, so that the TPS would read slightly more open at idle than previously. As the engine would be stopped during the time I would be changing the tire size setting, this was not seen as having any problems. I did this because I considered that the TCM might reset its minimum TPS voltage when it received the new tire size setting.

The programmer used was my trusty old Snap-On MT2500. It allows me to enter in the revolutions-per-mile setting manually, as opposed to changing the tire diameter in 0.25 inch increments that my SuperChips programmer is limited to. I set the tire revolutions-per-mile setting to 624, which corresponds to a 32.3 inch diameter wheel.

I then re-installed the Dakota's TCM back into the Dakota, and restored the Durango. (this included removing that washer - don't want to have loose hardware rattling around)

This morning, on the way to work, at speeds above 45 MPH, the Dakota managed to keep its torque converter lock-up on 5 out of the 6 upward inclines I normally encounter. The lone incline, that saw the Dakota go out of torque converter lock, is rather steep, anyhow. The Dakota also engaged 5th gear at 51 MPH, which is much better than the 57 MPH it had used in the past.

It appears that this idea does work pretty well. It also appears that using that thin washer to temporarily increase the TPS reading, during the procedure to change the TCM tire size setting, also works. I'll do this to the Durango tonight.