I really hope someone here has some help. I am ready to shoot myself. I have a 98 P30 (school bus) with 6.5 and 4L80. Here is my problem: replaced 5:13 gears with 4:88 (looking for lower RPM on HWY) shifting problems started almost immediately. Rebuilt trans $1200.00 (thought I burnt 2nd gear) shop took my $$ and now claims the problem is electrical not their fault. Someone has just mentioned that on these (newer trucks) the speed sensor on the transmission will confuse the ECM and trans will not shift at the correct RPM. this happen if you run a different size tire or taller/lower ratios than the truck was equipped with. Also as a result of the confusion in the computer, the ABS light will come on (it did!) so does the new gear ratio really affect the transmission at a computer level? If so how do I fix it without reverting to the original low gearing? the Speed sensor is only available in one part number from the dealer so how do lighter trucks deal with this issue?

I really hope someone here has some help. I am ready to shoot myself. I have a 98 P30 (school bus) with 6.5 and 4L80. Here is my problem: replaced 5:13 gears with 4:88 (looking for lower RPM on HWY) shifting problems started almost immediately. Rebuilt trans $1200.00 (thought I burnt 2nd gear) shop took my $$ and now claims the problem is electrical not their fault. Someone has just mentioned that on these (newer trucks) the speed sensor on the transmission will confuse the ECM and trans will not shift at the correct RPM. this happen if you run a different size tire or taller/lower ratios than the truck was equipped with. Also as a result of the confusion in the computer, the ABS light will come on (it did!) so does the new gear ratio really affect the transmission at a computer level? If so how do I fix it without reverting to the original low gearing? the Speed sensor is only available in one part number from the dealer so how do lighter trucks deal with this issue?


The smaller trucks get the ECM programmed with the different gear ratio...

The transmission speed sensor is probably getting confused with the wheel speed sensors...