March 22, 2011

Don't Get Shook Up

Dan Tucker

Let's face it; we make our living solving problems. Troubleshooting is the backbone of our business success. We are a typical shop that experiences problem vehicles almost weekly, just like many of you do. You know what I am talking about, the type of problems which already have been worked on and the original complaints still exist. Actually, sometimes previous work creates new, "friendly-fire" concerns. It seems I am amazed almost weekly with some incredible "new" solution that is found to a common type of problem or concern.

Here is a trouble vehicle suffering from a problem whose unique cause was difficult for us to find: a 1999 Chevrolet 4WD Silverado extended cab pickup. The customer's complaint was that the transmission to transfer case adaptor had repeatedly cracked. I don't know the exact count, but by the time we were blessed with this job, I think the count was three broken adaptors. Figure 1 is an example of one of the broken adaptors that was cracked near the transfer case mounting bolts.

We all know this is common, and most of the time the cause is a bad U-joint, bent driveshaft, out-of-balance driveshaft or something that is inducing vibration into the adaptor. Because we could see the vehicle had a new U-joint, we immediately suspected a damaged driveshaft. Those aluminum shafts are fragile and easy to distort. There are a couple of quick techniques we use as a preliminary test to confirm a vibration and isolate its cause.

First, we use the vehicle for a lathe, start the truck up and put it in manual low. This allows the driveshaft to rotate slowly, and you can run an indicator up and down the shaft to see if it is bent. We lifted the Chevy up and went from the front of the shaft to the back; it was out less than .015".

Our next test is to run the vehicle on the lift up to about 70-80 mph and then use a hand to feel the vibration in the transfer case's back housing, or even feeling of the adaptor. I know this takes a little experience or "feel skill," but if you get a known good vehicle and compare it against a bad, it doesn't take long for you to know if the driveshaft is out of balance enough to cause something to break. We tested the Chevy and it felt great. "Oh boy, what have we got a hold of," I was thinking. Could the customer be hitting something and physically damaging the adaptors?

I told the guys to go ahead and pull both driveshafts so we could take a closer look at them. This unit had a 246 (NP8) auto transfer case which spins the front shaft all the time, even in 2WD. That was why we needed to inspect both shafts on this unit. As the shafts were being removed, we got a break. The technician, Don Williams (with the keen hearing of a 25-year-old), thought he heard something moving inside the rear driveshaft. He said it sort of sounded like a liquid. We looked at the aluminum tube, and near the front U-joint we could see that the little black rubber plug usually found on the shaft was missing. Figure 2 shows the shaft with the rubber plug missing, and Figure 3 is a different shaft that has the rubber plug in place.

We stood the driveshaft up and let it sit for about 5 minutes with the hole down. Sure enough, the water inside ran out, leaving a puddle on the floor. We got to thinking about how the shaft was made. As luck would have it, we still had another, similar driveshaft on hand from a previous job so we took a Sawzall® and cut it open to see how it was made. As you can see in Figure 4, these driveshafts are foam filled.

The cause of the Chevy's problem was becoming clear. Water would soak into the driveshaft foam, settling in bottom of the shaft when the vehicle was sitting still. If the shaft was allowed to sit outside in 20-degree weather and freeze, the ice stuck in one side of the foam would create a terrible imbalance issue. To make the problem even more difficult to diagnose, the imbalance would solve itself as soon as the ice thawed. Running the Chevy in manual low at the beginning of our test distributed the water outward in the foam equally enough to correct any imbalance issue. I feel this is what allowed the driveshaft to pass our 80 mph test on the lift.

Given this information, we figured out what was happening to our customer. The guy would get into the Chevy on a cold morning and, before the ice melted, he was on the expressway doing 80 mph and shaking the vehicle apart…literally. Then, by the time the truck got to a shop, the mystery balance condition had healed itself. I feel that water by itself soaking one side of the foam could cause enough balance problems even in areas which don't see freezing weather that often.

We considered drilling holes and trying to remove all the water, but because of the foam and with no way to know if all the water was out, we choose to replace the driveshaft. Once the shaft was replaced, it cured the repeat adaptor failures.

Sonnax recently released replacement weld yokes for aluminum shafts which allow you to cut the end off and replace them without having to worry about losing overall length on the shaft. While I am sure these weld yokes were made to repair the damaged shafts in the U-joint clip area, I guess it might have helped get the water out. Here is a Sonnax weld yoke in Figure 5.

As a final note, how did the water get in the shaft? With more than 80" of rain in 2009 in Arkansas, water seemed to be everywhere. Of course, getting stuck and sitting with the driveshaft submerged would fill it up with that plug missing. The thing that made it difficult to diagnose was not considering what was in the shaft. We got lucky having a driveshaft we could do an autopsy on and discover that the foam was inside. It amazes me how critical something like a rubber plug can be. Great care should be taken to not damage these plugs during U-joint replacement.

Dan Tucker is owner of Tucker's Transmission in Pine Bluff, Ark., and a member of the TASC Force® (Technical Automotive Specialties Committee), a group of recognized industry technical specialists, transmission rebuilders and Sonnax Industries Inc. technicians.

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