Bearings and thrust washers destroyed by electricity?

A great friend of mine, Phil, reminded me of those golden years of the HydraMatic 350 transmission, used from about 1969 through 1984. It was typically paired with small block V6 and V8 engines. Although it was destined to become one of the greatest automatic transmissions ever built, we were having issues with the 350’s burning up back in the day.

I recall the first one I ever disassembled. Almost every thrust washer and Torrington bearing were wiped out. The old-timers out there, like my friend Phil, know exactly where I’m going with this. On the 350’s, it was due to a bad ground. Those pesky electrons were traveling through the shift cable, which was also failing, and transmission parts through the driveshaft and the rear axle.

The repair was installing two braided ground cables--one from the cowl (firewall) to the wing nut at the carburetor and the other was from the bottom of the cowl to a bolt on the bell housing securing the transmission to the engine.

As I explained to John and Greg here in Shepherdstown, some thrust bearing premature failures have been attributed by poor engine grounding. When the starter motor is engaged, electrons need to go to ground and if the engine isn’t properly grounded, current can run through the crankshaft and directly into the thrust bearing’s steel backing. In time, the thrust bearing faces can rapidly eat away, behaving as if the thrust surface on the crankshaft isn’t finished properly.

It’s simple and necessary to check for too much voltage in the drivetrain. You simply connect the negative lead of your DMM to the negative post of the battery, and the positive lead to the transmission. You should see no more than 100mv on your meter while the starter is cranking for about 5 seconds. Although the most current flow usually occurs while the starter is cranking, current in the drivetrain can happen while accessories are operating. That’s why you should perform this voltage drop test with the ignition on and as many accessories operating as possible. Again, the maximum is 100mv. If the voltage is too high, check or replace the negative battery cable, or add ground straps from the engine to the frame; or from the transmission to the frame. Some systems may reach 300mv briefly without causing a problem. For added assurance, improve the ground with a larger battery cable or additional ground straps.

“Is it possible that there is too much latent current flowing through the drivetrain, but without measurable voltage?” Greg asked.

“Sure,” I said, remembering one I dealt with about a year ago. “If you have a ground issue in the chassis, but the transmission and engine grounds are fine, the vehicle could pass the test, because the driveshaft and suspension components become the ground path. Saturn Vues going through wheel bearings can be the result of this.”

“So, how would you check that?” John asked.

“I pull the driveshaft and then measure the voltage drop. Now the frame and drive train must pass the 100mv test. Then you run a ground strap from the engine or transmission to the frame.”

“Remember the rear brake caliper slide pins freezing on W body cars years ago?” My friend Phil reminded me. “The engineers finally determined it was due to the static electricity created be the friction of the pads against the rotors looking for a ground through those mounting pins.”

John and Greg are great technicians who take pride in their work and I was glad to have an opportunity to help them out.