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View Full Version : Engine: Just for fun, test your diagnostic skills . . .



Ted Preston
09-11-2012, 01:01 AM
(Can't give this post an honest prefix, or it will give away the part of the answer to this puzzle!)

I pulled up to a stop sign recently, slid her into neutral to wait for a gap in the traffic, and my 55 Champion coupe didn't want to go back into gear. I was stuck in neutral. (3 speed manual with a column shifter) The shifter just didn't want to go into first, so I tried reverse. No good. Second? Third? Well, after banging the selector around a bit, I finally got into first, but when I let out the clutch, the car acted like the parking brake was on. I killed the engine trying to take off in first, and the same in reverse. The engine started right back up again. I finally got the car to budge loose, and limped home very slowly. On the mile-long drive toward home, the engine sounded like it had a bad rod bearing: thump, thump, thump . . . very regular at idle in neutral, and the thumping sound sped up with the engine speed when moving. Oddly, the thumping went away entirely if I pushed the clutch in. With the clutch released, the engine at idle sounded smooth and trouble-free, and sounded just as smooth when revved. (The engine is a fresh rebuild with about two hundred miles.)

But going down the road, whether under power or coasting, or just sitting still and idling in neutral with the clutch engaged: thump, thump, thump.

So what are your guesses? I have torn down the offending part now, and I know the source of the problem, but, well, lets just say that my first hunch turned out to be incorrect . . . and my second. I'm hoping some of you will make the same good, but wrong, guesses I did at first, and make me feel better before I reveal what the problem was.:D

Nox
09-11-2012, 08:34 AM
I'd say that it probably is either the gearbox or the clutch but most probably the ingoing axle in the gearbox that's broke somewhere near the front bearing or if you're lucky only the bearing but yeah, it's all bad guessing done by me! (Usually some others has been here before & done some more professional thinking...)

Nox.

K-Hawk
09-11-2012, 08:37 AM
broken fingers on pressure plate, arm to throw out bearing bent and may have caused pressure plate problems.

Pat Dilling
09-11-2012, 09:42 AM
The clutch sounds like it is operating properly and the sound is only present when the clutch is engaged, thus turning the transmission input shaft. Based on your description I would look at the input shaft bearing. Next choice would be the mechanism in the transmission gear selectors that keeps the car from going into two gears at once. The second one would explain the car dying when trying to move initially, until your manipulations managed to move it out of two gears.

brngarage
09-11-2012, 11:06 AM
Stuck in two gears at the same time!

JEWELL
09-11-2012, 11:27 AM
A bad center support bearing?

RadioRoy
09-11-2012, 12:27 PM
I'll guess bad bearings in the cluster gear, but I cheated.

This is a most unusual and interesting post.

It's refreshing because we don't usually troubleshoot here. What usually happens is the original poster gives only a minimum amount of info, gets a wild hair or hears a wild guess from someone else, and decides what the problem must be, and then we tell him how to fix that.

The actual amount of diagnostic thinking is a very low percentage compared to the advice (usually good, BTW) on how to fix what the OP assumes the problem is - and we know what happens when we assume. :)

Hopefully we can increase the time we spend doing diagnostic thinking and cut down on the misguided (by the OP) speculation, but that won't happen until we can get the OP's to give us more complete information and less speculation right off the bat.

Ted Preston
09-11-2012, 03:36 PM
I'll guess bad bearings in the cluster gear, but I cheated.

This is a most unusual and interesting post.

It's refreshing because we don't usually troubleshoot here. . . . .

Hopefully we can increase the time we spend doing diagnostic thinking and cut down on the misguided (by the OP) speculation, but that won't happen until we can get the OP's to give us more complete information and less speculation right off the bat.

I agree. It's a sad fact of normal internet forum interaction that careful troubleshooting rarely takes place. It's a highly valuable skill, but in the end, most of us need to be taught how to troubleshoot. Maybe you and I can encourage more people to learn the skill. It sure would save a lot of amateur mechanics time and money if they would learn what their internet friends need to know to troubleshoot a problem, and then provide it. I'm glad to see that the people attracted to this thread all seem to know and use the skill!

I suspected someone might inadvertently "cheat" to find the answer to this question by reading my other recent thread. When I decided to post this challenge, I went back and deleted several parts of my other thread to avoid giving away the answer, but the two threads definitely share a clue, if only in their concurrent timing!

I'm glad to see a few guesses that are the same ones I made (damaged clutch, or input shaft problem of some sort) and a few others as well, equally good guesses. Ultimately the process of troubleshooting involves making educated guesses, then checking each in turn until we verify the source of the problem.

JEWELL, I'll start by explaining why I did not suspect the center bearing of the driveshaft, or any other problem "behind" the input shaft. When I disengaged the clutch and the car was not moving, the thumping noise went away. It came back immediately when I engaged the clutch, whether the car was in gear and moving or not. Since the thumping existed when the car was NOT in gear or moving, that told me that the problem was located from the input shaft forward, through the clutch, into the engine. Since the noise went away when the clutch was disengaged (thus removing the spinning connection between the engine and the clutch plate/input shaft) I felt safe in assuming that the problem was limited to those portions of the clutch and input shaft that are spinning when the clutch is engaged. (or potentially a problem with something not spinning, but right NEXT to some part that is spinning which can touch the stationary part.) I immediately thought: input shaft or clutch (disk, pressure plate or flywheel). (Good guess, K-Hawk!)

My first guess was some kind of clutch damage. and I pulled the engine to check, but the clutch looked right and appeared to operate just fine. I bolted it back up with proper torque after checking also that the flywheel was properly attached to the back of the crankshaft and torqued. No problems either place, so I pulled the tranny and turned my attention to the input shaft. As it turns out, this guess was both right and wrong, in a way.

The input shaft's bearings, which were potential problems I suspected, were both operating just fine. The shaft itself was undamaged . . . except for a bit of metal jammed between two of the teeth. That little bit of metal may have been what killed the engine initially, and eventually caused the thumping sound when the clutch was engaged. That bit of metal would have thumped against the cluster gear on each revolution of the engine/input shaft.

But the little bit of metal jammed into the teeth on the input shaft did nothing to explain why I couldn't engage any gear when the problem initially started. (frankly, my clutch theory didn't really explain that problem either.) (Good guessing on this point, brngarage and Pat Dilling the safety interlock could well have been a trouble point!) So I took a closer look at things in the transmission after prying the foreign bit of metal out of the input shaft gear.

. . . and I saw something completely out of place: laying on a ledge in the tranny case, I saw a quarter-inch long piece of a broken needle bearing. I stuck my magnet into the lubricant in the bottom of the case and came up with half a dozen more little bits of broken needle bearings. So I disassembled the entire main case to get at the needle bearings that support each end of the "cluster gear" (sometimes called the countershaft) in the bottom of the main case. Each end of the cluster gear has a set of needle bearings inside, between the tubular gearset and the shaft it spins on. I had trouble disassembling the cluster gear because the thrust washers that keep it sitting straight on its shaft were apparently tweaked, allowing it to sit a bit cocked on its shaft.

After fighting with it a bit to remove it, I found that the bearings in both ends were pretty much disintegrated. So were the thrust washers that hold the bearings and the gearset in place on the shaft. I suspect, from the carnage in the photo shown below, that the bearings started to break down, turning to abrasive and wearing the inside surfaces of the thrust bearings. eventually, they wore the holes so large that the bits of bearing were able to work their way out of the cluster gear and into the lubricant in the case, where they began to be flung around.

I suspect now that the difficulty engaging any gear resulted from a bit of a broken bearing getting in the way of a sliding gear, or else the gears on the mainshaft could not engage the gears on the countershaft because the countershaft/cluster gear was cocked crooked.

When I started troubleshooting this problem, I didn't think of the countershaft in the transmission. That's because troubleshooting requires us to have a good understanding of which parts are engaged and operating in a given scenario. Sometimes we simply don't understand the internal workings, and this was one of those times for me. Now that I've torn down this tranny logic tells me that the countershaft is always spinning when the clutch is engaged. It is always engaged with the input shaft. Power gets transmitted to the output shaft through the always-spinning countershaft and into the first/second third/reverse gears on the mainshaft. That's not true for some of the motorcycle transmissions I've worked on in the past, and this is my first time inside a car's transmission, so I learned something new.

Next time I do some troubleshooting, I'll have more information and understanding, which is an important key to troubleshooting. That's the reason old time professional mechanics could so often tell you the source of a problem just by standing there listening: they fully understood what was going on inside, and could diagnose problems quickly based on that understanding. It's sad that troubleshooting is becoming a lost art, in these modern days of onboard diagnostics. Let's keep it alive for our Studebakers!

Here's the carnage that USED to be the needle bearings in the cluster gear, and the thrust washers at either end:

http://i3.photobucket.com/albums/y93/waterlaw99/003-7.jpg

Nox
09-11-2012, 05:14 PM
Off the thread a bit:

It's a fun thing this troubleshooting, I had to tow a friends -63 Dart back home 'bout a week ago & I said "if it starts we drive & you first".
It had died suddenly just before a tunnel (where the speed-limit is quite high & most folx drive faster there) & started & died again & then no more, at one o'clock in the night...
So we went there the next day & the car didn't make one single "poff", so it came towed back home.
Then I took one sparkplug out & NO spark.
& everything under the distributor-cap (+ the cap) was new!
Flimsy silicone wires, hmmm... NOT my favorite...
Took the really old copper-&-plastic center cable from my own engine & guess what...?
Yeah, c'mon guess...

Like a dream, it then runs sooo smoooth!

K-Hawk
09-11-2012, 07:03 PM
This was a good post
those needle bearings are toast