Which one controls the speedo, the one in the trans or the one in the diff?

 

Depends on what year your truck is ... 97 back ... tranny gear. 98 up ... RWSS on the rear axle.

 

It's a 99 and has both, so if it uses the one in the diff then whats the other for?

 

The speed sensor is on the overdrive unit of the transmission, If the speed sensor goes out the transmission will still shift. The other sensor is for the ABS and will give you an open ABS circuit code.

 

But what if my ABS light is on and the speedo doesn't work until 25mph?

 

I'm betting it's the one in your rear diff. My '99 Ram did the same thing a couple of months ago. Changing the sensor in the rear diff. took care of my problem, got the speedometer working, and the ABS/Brake lights to go away.

HTH

--Kevin

 

Kevin is correct.

 

I'll give it a try.
Thanks