I recently changed the differential in my 1989 F350 2WD dually. Now the speedometer doesn't work correctly. I also changed back to the ZF transmission, but I don't know the original configuration. How can I get the speedometer corrected (cheaply)?
Also, I want to know how to drain the oil. There is no plug. The differential I put in is a limited slip, and it needs more of the conditioning oil they told me.
'Thanks.

Jim

 

the haynes manual says to loosen the screws and pry the cover off. the fluid will then just slop out.

 

The 'conditioning fluid' is called friction modifier, and is available at the Ford dealer in a little bottle that is enough to do one axle, or you can buy synthetic gear lube that already has the friction modifier in it, since you seem to be planning to change the oil anways. I don't know if your F350 is like mine, but my speedometer reads through a sensor on the top of the axle housing that reads from the 'tone ring' which is on the ring gear inside the axle. Maybe something got knocked out of alignment when you were in changing the differential?

 

Well you change two things that affect the speedo. The final drive ratio of the ZF and the rear axle. My guess is the rear axle has a different ratio than before. Any local parts store should have new drive gears which is the gear located on the end of the speedo cable that goes to the trans. Changing this gear will make the adjustment you need. You just need to determine which way you are off, speedo is slower or faster than before? Once you have this do a search for speedo calculators to find a chart that will poit you in the right direction.

 

If your speedometer reads off of the tone ring in the rear end, you can get an aftermarket product that intercepts the signal and has adjustment pots for endless compensation changes depending on your gear/tire configuration. Superlift makes one for the Ford trucks. I believe there is another company making them also. I have one installed in my F150 and it works fine.

 

89 trucks have the cable.