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July 2002 4x4 Truck Repair Questions - Techline

Posted in How To: Tech Qa on July 1, 2002
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Lift It, Break It
Question: I have a 1977 F-150 with a modified 351ci engine, 8 inches of lift, and 36-inch tires. I keep breaking my rear U-joint where the driveshaft connects to the axle. I have to replace it every few weeks.

I've been told to shim the axle, extend the driveshaft, get stronger springs, or change the flange. What advice do you have for me? I am running a typical Ford 9-inch rear axle in back with 3.73 gears. I've been told I have axlewrap.

I've been breaking U-joints during highway use and stop light to stop light. I have a four-speed transmission in the truck and the breakage seems to happen when I change gears.
Brett Roberds
Mesa, Arizona

Answer: I can understand that you'd be breaking U-joints with 8 inches of lift and a Ford 9-inch axle. Do you think that most likely the problem has to do with driveshaft angularity? I do. I'd be willing to bet that there is not a whole lot of downward axle movement before the U-joint goes into a bind. Just for curiosity's sake, block the front wheels to keep the truck from moving and lift the truck by its frame so the back wheels are off the ground. With the transmission in neutral, try to rotate the driveshaft. My guess is that the shaft won't turn because the U-joint is hitting the yoke. Even if it isn't binding, the angle probably will far exceed the recommended 3-degree factory setting for U-joint operation.

Here are a couple of potential solutions. The first one may or may not completely solve the problem. Order a high-angle driveshaft with a CV joint at the transfer case end from one of our advertisers. Before you send them the shaft-length measurements, you'll want to rotate, or index, the differential so that the pinion is pointing directly at the transfer case. That is, you'll want the new driveshaft to be directly in line with the pinion shaft with no angle or perhaps 1 to 2 degrees of down angle. You could do this with shims between the spring pack and the axle pad, but it would be much better to cut the pads free, rotate the axle, and weld the pads back on. This will place the pinion at a pretty steep angle and it may not get enough lubrication, so I suggest you overfill the differential and use a synthetic lubricant.

A better solution, however, while considerably more costly, is to order a new high-pinion carrier that uses the 8.8-inch ring-gear setup. The Currie-designed carrier, available from several of our advertisers, places the pinion at the upper section of the carrier instead of at the lower section. This new carrier will greatly reduce driveline angularity-but I still suggest using a CV joint in the shaft.

You may also need to find some way to address the springwrap/axlehop problem. Several of our advertisers sell traction bars that can help.

The Transmission Hunt
Question: I just rebuilt the motor in my '93 3/4-ton Chevy pickup, and while it runs great, my transmission shifts erratically. It goes from First to Second then back to First before shifting to Third, and sometimes will do the same thing between Third and Fourth. The local transmission shop has no idea why it's doing this; the guy there just says it needs to be rebuilt. I didn't have any problems until I put the new motor in.
John Logan
Salt Lake City, Utah

Answer: Automatic transmission problems are difficult enough to diagnose with the vehicle at hand, let alone through the mail. But in this instance, I have a couple of clues. First, you had the engine out, and second, it shifted fine before the engine removal.

Before I send you to the shop to have the trans rebuilt I suggest you check a couple of wires. Most likely your transmission is a 4L80-E. The "E" at the end means it is electronically-controlled. My guess is that you've either hooked its control wires to the wrong location or that they are disconnected.

On the left side of the transmission about in the middle of the case you'll find the input speed sensor. It should have two wires connected to it-one that's dark blue with a white stripe, and one that's gray with a red stripe. Toward the rear of the transmission case is the output speed sensor. This also should have two wires connected to it-one that's light green with a black stripe, and one that's purple with a white stripe. Your truck, being a '93, will have a third speed sensor on the transfer case. This will have a light blue wire and a dark green with a yellow stripe.

While it's just a guess, I would almost bet these wires were damaged or removed and reinstalled incorrectly during the engine swap.

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