FS5W71B Failure Analysis - 1985 720 4x4

[Seven Two Oh No]

I have a stock '85 720 4x4 with a FS5W71B and have had three transmission failures over the past two years. This 71B has the larger 62 mm countershaft bearing and none of the issues are due to that bearing failing. The general symptom is that the transmission will briefly lock up (with a clunk) directly after a shift, and then there will be no power to the wheels, despite the shifter being in gear. I can then pull the shifter back to neutral (with some effort), put it back into gear (still no power to wheels) and rev the engine a little until whatever is binding loosens up and the transmission grabs.

 

This happened a few times with the original transmission (generally on surface streets at low speeds), so I took it apart, did not find any obvious issues, and rebuilt it with new bearings (except the needle bearings) and synchros. Problem solved for about 12 months (I only drive ~5-7k miles a year) but then I was on a four hour drive, cruising in 5th most of the time or in 4th for  uphills. I got off the interstate, downshifted and slowed, then dropped it in second at which time the rear end bucked when the transmission locked up. Pulled it apart once I got home and found the thrust washer behind 5th gear had snapped in half and that the input and tailshaft were bound together, so that turning the input spun the output in neutral. I had not changed the mainshaft needle bearings, so I suspected that may have been the cause. I then ordered a rebuilt unit from Midwest Trans, which worked great for 16 months (6,500 miles). Then, same failure - four hour drive (few uphills this time), mostly in 5th, came off the interstate, downshifted to 4th then into 3rd when it locked up. It still drove but was difficult to engage gears. Despite the low miles on the unit and only being 4 months past warranty, Midwest refused to cover any repairs (they would inspect for free but I would have to cover shipping, etc.). So I took it apart to inspect. Input and output shafts are binding in neutral (can turn separately with some effort). I pulled the main drive gear and found the needle bearing had been milled at the mainshaft face (see linked pic -having difficulty embedding), and there is pitting on the mainshaft where the bearing face sits. All gears with the exception of 5th are difficult to hand-turn when engaged, and reverse is almost impossible to hand-turn. I'm assuming the transmission locked up and compressed/distorted all of the needle bearings on the mainshaft but wanted to get the opinion of the forum. Particularly, how could the transmission be in gear with no power to the wheels? The clutch allowed me to shift into gear but everything locked up when I released the pedal. There is no damage to the input splines or the clutch so I don't think the problem is on that end, but the in-gear-no-power is very perplexing otherwise. Could be that the clutch is just slipping when the transmission is locked up, however. I would appreciate your advice.

 

https://www.dropbox.com/s/ax489j2ey98lvm2/01_Main Drive Gear Needle Bearing.jpeg?dl=0

https://www.dropbox.com/s/9vz9k5ub29nc0x3/02_Main Shaft Bearing Surface.JPG?dl=0

[docbainey]

That has the indication that the main shaft and input shaft where to close together. That could happen if the main shaft nut comes loose or a miss placed thrust washer. Yours being an 85 I would think it should already have the left hand nut fix.

[alexg89]

Had this issue happen to me with my fs5w71c .. came down to a bent shift fork that would slightly in-gauge and slip in and bind.. only happen in 1st and and 2nd

 

 

[docbainey]

If you have the larger counter shaft bearing then you have a 'C' transmission. Your damage is between 3rd & 4th. To trash 2 transmissions something else has to be going on?

[datzenmike]

It would be exceedingly unlikely that 3 transmissions would exhibit the same failures. So that leaves external things like your driving style which causes this, (no offense intended here) wrong or low transmission oil or the driveshaft to the trans case is (for what ever reason) too long and shoving forward on the mainshaft. Trans case mounts loose or broken engine mounts causing the driveshaft to be rammed into the case?

 

I think this is normal. The highest points would be the rollers and they would have wear on them. The

 

 

 

 

 

 

[datzenmike]

7 minutes ago, docbainey said:

If you have the larger counter shaft bearing then you have a 'C' transmission. Your damage is between 3rd & 4th. To trash 2 transmissions something else has to be going on?

 

All 71B switched to the larger 62mm bearing in '85 or late '84.

[docbainey]

Part of what you said Mike comes to mind. I never let anybody drive my cars unless they owned a foreign car. For some reason if they haven't owned a foreign, car they have visions of shifting a 396 Chevelle with a Muncie rock crusher transmission. That just doesn't work in a Datsun.

[datzenmike]

I don't shift any harder than I have to. If you have to pull or push too much you're maybe doing it wrong. A warmer transmission shifts faster than a cold one with the recommended 80w90 GL4 in it but I've had excellent shifting with GM Delco Friction Modified Synchromesh Oil. With the same effort as the thicker oil the thinner shifts faster. It downshifts without grinding now too. Anyway hard shifting mostly just wears the synchros out faster.

[Seven Two Oh No]

I appreciate the input, everyone, and agree that it is suspicious that I've had the same failure on two different transmissions. I do not drive or shift aggressively, however, and have always kept the trans topped off with Redline or Synchromesh. I've also considered the possibility that the transfer case is somehow involved, but the unit works fines in all ranges without noise/etc. I'll take a closer look at the motor/transfer case mounts though, and will pull the other gear clusters/shift forks to inspect (particularly 3/4).

[Seven Two Oh No]

Update - did not find any damage to the unit beyond the pilot bearing, although apparently Midwest Transmission did not install a countershaft bearing front shim or replace the (broken) oil gutter. Checked gear end play and O.D. gear was binding on thrust washer (thanks to Stoffregen Motorsports for pointing out this scenario in a different thread) - I loosened the main shaft nut and retightened until end play was in spec and now everything seems to be working properly. Still have not identified the cause of the initial lock up, but it's worth noting that I never checked end play after rebuilding the previous transmission (first time - lesson learned), so it's possible both units had too tight of clearance between the O.D. gear and thrust washer. That, combined with driving for a few hours may have closed the gap and bound the O.D. gear to the output shaft. Since I was driving in O.D. I didn't notice, but when I downshifted into third I was basically in two gears at once and the trans locked up, probably jamming the mainshaft into the input shaft. The shifter was in gear but there was no power to the wheels because the trans was locked and the clutch was slipping as a result. After I shifted into neutral, the input shaft was likely binding on the main shaft until the pilot bearing gave way and was slightly milled in the process (I heard a high-pitched whine "spinning out" when the trans loosened up). The O.D. gear and thrust washer were still tight together even after cooling down (possibly due to shaft locking up and distorting some things), which led to hard shifting and binding in all gears except O.D. This is very much a working theory, so I would appreciate any thoughts you guys might have. Going to order a few parts and put it back together, although I likely will never completely trust it again.