pjg

Read the whole document here: http://www.motor.com/article.asp?article_ID=1656

Poor Engine Grounding

This may sound like a fantasy at first, but it’s been noted that some thrust bearing failures have been caused, or at least promoted, by inadequate engine grounding. How can a poor ground result in trashing a thrust bearing? When the starter is engaged, the current flow wants to go somewhere. If the engine isn’t properly grounded, it’s entirely possible for the current to run through the crank, and potentially directly into the thrust bearing’s steel backing. If this occurs often enough, the thrust bearing faces can quickly erode, which then affects the thrust bearing. It’s like the thrust surface on the crankshaft isn’t finished properly.

How do you check for poor grounding? Following is an explanation of a test taken from a service bulletin written by ATRA’s Dennis Madden:

It’s easy to check for excessive voltage in the drivetrain: Connect the negative lead of your DVOM to the negative post of the battery, and the positive lead to the transmission. You should see no more than .1 volt on your meter while the starter is cranking.

For an accurate test, the starter must operate for at least 4 seconds. It may be necessary to disable the ignition system so the engine won’t start during the test.

If the voltage is excessive, 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 .3 volt momentarily without incurring a problem. For added assurance, improve the ground with a larger battery cable or additional ground straps.

Although the greatest current draw usually occurs while the starter is cranking, current in the drivetrain can occur 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 threshold is .1 volt.

One final problem that may occur is current though the drivetrain without measurable voltage. If the grounding problem is in the chassis but the engine and transmission grounds are okay (or vice versa), the vehicle may pass the test. What happens here is the ground circuit can be completed through the driveshaft and suspension.

To test this, measure the voltage drop with the driveshaft removed. Both the drivetrain and frame must pass the .1-volt test. This is where a ground strap from the engine or transmission to the frame does its best work.

Diagnosing a thrust bearing failure can sometimes be simple, but often is tricky. Chances are the problem is more likely to occur with an automatic transmission setup. Check torque converter freeplay; beyond this initial check, start checking transmission line pressure and look for kinked and/or restricted cooler lines.

Try to eliminate the transmission-related variables first. Simply rebuilding an engine (and naturally verifying clearances and assembly in the process) and reinstalling it won’t cure the problem if the source doesn’t lie in the bearing/ crankshaft. If the cause is transmission-related, the thrust bearing will act like a fuse and fail again. In other words, don’t simply repair the engine and send the vehicle out the door. The root cause of the thrust bearing failure must be determined and remedied.

blown 87

Guess it is possible, but not very likely.

123quattro

If this were to happen, I would expect the same wear on the main bearings as well. I think this is smoke and mirrors. If you have a poor ground it just doesn't start.

dr bob

The driveline alternate alternator- and starter-current path-to-ground in the 928 would be through the torque tube before the crankshaft. The crank floats in an oil film, and the driveshaft is somewhat isolated by the ball bearings and sleeves until it gets to the transmission. Torque tube connects the engine block directly to the other end of the driveline where presumably the current flow paths would otherwise be common through the trans to the chassis at the rear.

Not saying that the engine ground shouldn't be confirmed, just that the writer's theory doesn't consider a torque-tube car.

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You can test the voltage drop across the engine ground connection by putting a voltmeter in parallel. One leadon chassis ground, the other on the engine block. The highest current draw is while cranking, second highest would be with the alternator at highest possible load. In either case, insufficiencies in the engine ground wire will show up as voltage between the block and the chassis.

blown 87

Current will go to the path of least resistance, not to the path of most damage unless they are the same.

Landseer

Its awfully hard on these cars when they run with intermittent missing.

pjg

I thought there might be some correlation since 928's are known to have ground issues due to corrosion, age, etc.

dprantl

If this were true then there would be many more 5-speed 928's with thrust bearing failure...

Dan
'91 928GT S/C 475hp/460lb.ft

AO

I'm gonna have to call BS on this one. Plausible? Maybe.

FBIII

good point.

NIACAL4NIA

Another Porsche 928 rumor to make us paranoid. How about thunder storms and hail hitting your shark? This forum is becoming like FOX news.

Shark Attack

Ummm.. No

Chuck Z

..........at least its fair and balanced! Well, unless you don't post a picture!

SeanR

You should join us in Off Topic, you'd be a hoot.

Landseer

Here's another one: Fuel lines might burst and burn your shark to a crisp.


(The next picture is faked, using techniques from the '69 moonwalk)