1985 F-150 Differential Failure?
Asked by Mr. Jackson Mar 06, 2018 at 09:40 AM about the 1985 Ford F-150 XLT Lariat Standard Cab 4WD LB
Question type: Maintenance & Repair
I was driving down the road a few months ago in my '85 F-150 and all of the
sudden, on a flat road, no incline or anything, my speedometer dropped from
~55mph to 0mph, while I was still going, I lost all acceleration and slowed to
a stop, engine still running. I just couldn't move anymore. Wouldn't go into
gear, wouldn't move when I pushed in on the gas. After I got it towed home, I
had my roommate put it in gear while I was looking under the truck and
noticed that my driveshaft spun when he let off the clutch, but it still wouldn't
move. Did a bearing go bad, a joint fail or did my whole diff give out?
5 Answers
If the rear driveshaft turns while the tires are on the ground and the vehicle doesn't move then the rear differential has a serious problem. It could be a broken axle, bad ring and pinion or bad spider gears in the differential itself. Since removing the axles is part of the disassembly process, you will easily find out if it is an axle. I would remove the differential cover first to see if the unit has metal fragments in the oil.
If the problem is anything other than a broken axle, and broken NOT inside the housing, you will not be able to repair it if you are not familiar with the procedure. And will be a total pain in the butt if you try to do it without removing the axle from the vehicle and placing it on a bench. This involves removing all the brakes, brake lines etc. Shop labor alone will be expensive, not counting parts.
If, after removing the cover, you see damage is extensive requiring a removal and rebuild of the entire assembly, you may want to check the net for remanufactured axle assembly. Probably around a thousand bucks. Something an average DIY'er can do with a little help, jack stands, regular tools, and a lot of time.
