Diff swap gone wrong

[browndog]

okay, so the diff in my BA ute had gone, was making a whistle like noise around 80, so i replaced it with one from an XR8, and now the Speedo wont work it moves up and down, then goes back to zero and stays there, the trans wont stay in one gear, then goes into limp mode

plz help me

[Falc'man]

Hi buddy and welcome.
Did you or your mechanic fit it?
Its weird. I was going to say maybe there's a difference in ratio with your new diff, but that shouldn't cause such a problem. Maybe get your ABS sensors checked out since that's what the speedo works off.

[browndog]

well i think it is a larger ratio, as for the ABS sensors, they shouldnt be a problem as they worked before hand, and i tried swapping between the two sets from the old and new diff

[Paxton]

The gearbox gets its speed information from the instrument cluster, which is why you're having strange shifts.

I'm not familiar with how the speed sensors work on the later Falcons (my EF has a sensor in the gearbox), but I'd be checking the ABS first, like what Youssef said.

[GavL]

I believe the ABS sensors in the B-Series cars tell the car what speed it is doing

[Paxton]

I believe the ABS sensors in the B-Series cars tell the car what speed it is doing

All they do is tell the Speedo and ABS what's going on. You have a BA ute, which uses the 4 Speed Auto transmission. The 4 Speed Auto takes its speed information from the instrument cluster, which is why, as you have no speedo, you have an erratic gearbox that has no idea what is going on. If the sensors are malfunctioning, so will the speedo, so will the gearbox.