I had my 5spd rebuilt because of noisy bearings, especially the input shaft bearing. They were slowly getting louder, making a whining, rattly, gravely sound that was awful.
The mechanic thinks this failure was caused by a bad ground connection between engine and chassis, which forced current to instead flow from the engine to transmission through the bearings themselves. The arching then caused pitting and damage to the roller surfaces as you can see.
Supporting this theory too is the fact that multiple bearings failed at the same time, with low miles, and all bearings are a very high-quality brand that should not do this.
I personally had never heard of the phenomena before. It sound plausible except I would think there should be numerous other electrical paths between engine and chassis besides the bearings? The block and trans case bolt directly together, so wouldn’t current flow directly between them? And through the bolts and other stuff.
I’m wondering if anyone has experienced something like this and what are your thoughts? Do you think the bad ground is a likely cause?
I think that, that mechanic would do wisely by quitting thinking.
That kind of thinking is gonna hurt somebody sometime.
While I would say that a m/t normally will last longer than an a/t, it is not unheard of that the bearings will break down prematurely and 123k miles sounds about right for that to happen when they actually do go bad prematurely.
You’ve got them changed, now You can motor on for next many, many miles without noise
I can’t imagine why current would go through the bearings instead of the highly-conductive block and tranny case. Much more likely the lubricant broke down or got low.
I don’t think this is caused by a bad ground. I would think one bearing has worn through its case hardening and shed particles that destroyed the others.
I would think low oil level too but I haven’t seen any other evidence of that. It has never leaked and was always full, also had never been serviced to repair a leak in the past afaik.
Another thought - wouldn’t there be signs of extreme heat like blueing if there had been a lubrication failure?
Bluing wouldn’t occur except in extreme cases in a roller or ball bearing. I have replaced a few input shaft bearings that looked like your bearings … just not that bad… because I heard a noise like a throwout bearing and I removed the trans to find a perfect throwout and a worn input bearing.
Both the transmission and engine are isolated from the chassis by rubber or similar bushings. This is exactly why there is a ground strap. Electrical current will need to find a viable path in order for the system to function. I have seen issues caused by improper grounding but those bearings do not appear to have failed from such a cause to me. They look degraded due to mechanical issues.
True, a ground must exist. But if it’s not the engine strap to chassis, what is it?
There may be several alternate paths that exist and they may not be straight connections from the body of the assembly to the chassis, or more appropriately, battery negative post.
I actually do seem to have a grounding problem. I am getting 30ma of current with the engine idling measured between the block and the battery negative, and the strap connections are pretty dirty.
I have melted both the throttle cabel and the clutch cabel in one go when I forgot to fit the ground cabel after (if IIRC) a clutch exchange.
At least I knew who was to blame.
I can’t say for sure whether this is a factor, but when a manual transmission is mated to a 190 hp 3.0 L V-6 engine on a 3,000 lb. car, it’s easy to put a lot of strain on the transmission. I bet it’s a fun car to drive though.
I agree that it seems unlikely. The engine and transmission have many, many points of contact between them, and the transmission input shaft is certainly not the easiest way for electricity to flow from one to the other. The bolts should take care of the current easily. With 125K on the car, while curiosity is a powerful force, the more probable answer is some years of hard use, possible episodes of low levels of lubrication and just plain wear.
A doctor told me that when he was in med school he was taught - when you hear hoof beats, look for horses first, not zebras.
I wouldn’t have guessed that would happen to the transmission input shaft bearings, but CV joints can get damaged by ground currents when the engine isn’t properly grounded to the chassis. The path for the starter motor current (over 100 A) for example is supposed to be from the battery pos to the starter, to the starter case (which is bolted to the engine/transmission), then to the chassis via a grounding cable, then back to the battery neg. When that ground cable is missing the path is from the transmission through the cv joints and axle shafts to the steering knuckle, and through the suspension components to the chassis. The ignition system current, fewer amps but continuous when the engine is running, would follow the same ground path presumably.
One of the reports we heard here iirc was the poster – who was having trouble w/failed CV joints — discovered the CV joints were getting really hot, and the rubber boots were deforming. Traced to a faulty ground ground cable between the engine and the chassis.
It would seem to me that lack of oil would cause the bearing to discolor. At least that’s been my experience with manuals.
While it will never be known at this point I wonder if the transmission mainshaft end play was set up too tight.
There’s always a number of checks that should be performed without simply throwing bearings and syncrhonizer rings in it.
I would think if that ground strap theory was true (and I don’t) then all of the bearings and internal gears would suffer damage. It wouldn’t just beat up one bearing.
I don’t think it’s true in this case either but been chasing grounding problems in electrical designs for many decades. Mostly due to poor design but sometimes failures of the primary intended ground. Electrons will always take the path of least resistance. When one path cannot provide the full current, alternate paths will then become the next preferred route. It is not uncommon to find multiple paths branching off after one primary path. It all depends on the resistance presented by the various pathways. So I don’t find it all that implausible that one bearing could be taking the brunt of the current and then it finds multiple branches after that, thereby causing the majority of the damage to one element and none noticeable elsewhere as those paths are small enough current to not do any damage. Someone would have to look very carefully at the grounding paths in this particular case to establish plausibility. Not worth it unless you just can’t stand not knowing. Just fix the bearing and the ground strap and move on.
