[b3lha]

Originally Posted by b3lha

I know that notionally VSS1 reports the rear wheel speed. But it can't count rear axle rotations because it is located in the gearbox, not the rear diff. To report rear wheel speed, the TCU would have to count rotations of the prop shaft and divide this by the diff ratio to work out the rear wheel speed. In this case, the TCU must know the diff ratio.

If this is how it works then VSS1 would read incorrectly on cars with a swapped diff ratio while VSS2 in the front diff would read correctly. So there would be a discrepancy between the front and rear speed sensors.

I figured out another piece of the puzzle. The TCU counts pulses/time from VSS1 and multiplies them by a constant to work out the speed in km/h.

On a USDM TCU, the constant is located at address 0xC019.

I *think* the formula for the constant is 470 x 1/ratio.

For a stock 3.545 rear diff it is set to 133
For a 3.700 rear diff it should be 127
For a 3.900 it should be 121
For a 4.111 it should be 114
For a 4.444 it should be 106

Changing this value in the TCU should correct the VSS1 reading after a diff swap and may cure the solenoid C problem that Harvey mentioned.