i searched a bit and found some good info. more on cable speedo's than electronic.

but anyways, my speedo jumps around after about 30 mph. i am loooking into replacing the sensor on the rear diff, and cheching the tone wheel in the rear end.

there is no ABS light but when i lock the brakes up on snow i have no ABS engagement.

so my game plan is, pull the sensor out and make sure its not damaged, and rotate the rear end to check the tone wheel before i pull the diff cover.

anything else i should check before i pull the cover and look for internal problems? i am planning on pulling the cluster out to put a few new bulbs in it so i will check the connection back there too.

thanks in advance. specs are 96 bronco 302 5spd 129k on the clock

you might as well buy the sensor in the diff, your going to break it pulling it out. Ive..never had one come out in one piece that was orignal.

plus there what 35 bucks.

Check the wiring from the sensor all the way to the cluster, look for breaks in the line or places where it could have rub-through. (vibration short)

As far as looking down in the hole where the abs/speed sensor is. I couldnt really get my head in there to look at it. so if you want to check it out, it doesnt hurt to change your gear oil in there while you got it out.

The gasket cant be more than 5 bucks and another 10 on oil. and you just serviced your rear end.

As far as the ABS goes. I had my truck since it had 85k miles on it. Ive never had the abs do anything more than lock up. Except that one time i blew a caliper up on the highway and it started flashing at me when there was no brakes what-so-ever. I have 163k miles on it now. I did hav ethe abs shudder once in the rain.

I have a 1996 f150 302 aod-e 163k

Edit
I dont know if this helps, but i just diagnosed a abs/speed sensor (in the diff) on a dakota a few months back. Its issues where check engine light on, abs light on. and speedo wouldnt work until 30 mph. just to give you an idea.

My guess you'll find some carnage in the rear end..........at the very least lots of metal flakes in the gear oil. Even more likely if you've got the factory limited grip. If you're lucky a new sensor will fix it. If it re-occurs quickly you'll need to figure out what's getting eaten up so quickly - possibly worn pinion bearings, pinion nut backed off.......crush sleeve gone awry in some manner, or a wheel bearing eating itself. Lots of possibilities - it'll be much clearer once you've taken a good look.

If you have a meter run a test on the sensor, should be 800-1,400 ohms most of the sensors I see go bad read open. Used to see a lot of the sensors go bad.