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3rd to 4th shift gear grind


Icehouse

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I picked up a 93 D21 4x4 with the VG30 it shifts great except from 3rd to 4th no matter how fast or slow I shift it grinds.  Although if I double clutch it and shift slightly fast it doesn't grind.  That makes me think the clutch isn't fully disengaging.  Maybe the second pump is allowing it to disengage slightly more...    A friend of mine has a first year 300z twin turbo and he runs diesel through the trans once a year because it has a second gear sincro issue.  I was thinking about trying that.    

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Why the diesel ?

 

Bleed the hydraulics?

 

 

I wouldn't put diesel in it but you could change the oil every year. Have you tried the GM Delco Synchromesh Transmission oil that I crow about?

Friction modified or regular GM Synchromesh ?

 

 

 

If money to burn I would look at BG Synchroshift II. Right now I’m using factory Improved formula Honda MTL in my 2003 Acura CL type-s 6spd manual trans w/LSD. I have faith in it but Acura wants 30k interval changes (260hp/232tq). This particular trans is known for being notchy sometimes.

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My 91 Pathfinder does that occasionally going into third.  The synchro is damaged.

 

Also note that all 86-95 FS5R30A 4X4 transmissions had a design flaw where the fill hole was a inch too low, causing the transmissions to be underfilled by a little more than 1.5qts.  It's pretty rare to find one that had the transmission case replaced by the proper one.   I filled mine by jacking one side up on ramps to tilt it to get more oil in.  I have 297,000 miles on it, so it seems to have worked.  Certainly made it quieter.

 

http://www.nissanpathfinders.net/files/NTB97-009a%20-%20FS5R30A%20TRANSMISSION%20WHINE.pdf

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I have yet to try the oil Mike talks about, but I do know that with the right oil, the shifting will improve, sometimes to the point of fixing a grinding shift.

 

I use Redline MT90 myself, but give Mike's suggestion a try. If it works, it's a cheap fix.

 

Years ago, a customer of mine complained of a grinding shift, and wanted his box rebuilt. I tore the cases off the box and found nothing wrong, so instead of rebuilding it, I put it back together and filled it with MT90. He thanked me for the great job on the rebuild, and when I told him I didn't touch it, he was baffled. Needless to say, I didn't charge him for the rebuild, and he was happy with the improved shifting.

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If clutch was the problem, it would grind into reverse.

 

This is a good idea. Place in neutral with the clutch up, or engaged. As soon as the transmission is disconnected by the clutch the counter shaft with all the gears slows suddenly from friction being submerged in a bath of 90w oil. Step on the clutch and try to place in reverse normally. If it grinds the engine is still turning the input shaft.

 

Could be....

 

Air in hydraulics

Flywheel face or pressure plate face damaged.

Bent diaphragm finger

Damaged clutch disc... had one of those little coil springs around the disc fall out and jam things.

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Agreed on guessing a damaged synchro.

 

GM FM synchromesh is good stuff. I like redline as well (I’ve used it a bit more).

 

The car I picked up. I looked at the height and build (tall guy) of the original owner. I watched his relaxed seated position while he was driving. He rested his hand on 4th gear whilst driving around and his elbow on the center console. Ironically , that is the gear that I have a SLIGHT small buzz when shifting into. Tranny is in very good shape. I have been told by my auto shop teacher and the tranny rebuilding shop I use to never rest your hand on the manual trAns shifter handle whilst in gear and driving.

 

I’ve seen people mess up linkage and blame synchros to

 

Tranny shop around the corner told my grandfather to engage 2nd (while stopped) then engage reverse on his Nissan hardbody.

 

 

I really don’t know...just some thoughts

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I suspect that there is only one supplier and they just add different color and label to the containers.

 

 

I've heard this also but once in gear the synchro is locked in place relative to everything around it and not rubbing anything. The shift fork will be rubbing the synchro hub though.

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I suspect that there is only one supplier and they just add different color and label to the containers.

 

 

I've heard this also but once in gear the synchro is locked in place relative to everything around it and not rubbing anything. The shift fork will be rubbing the synchro hub though.

Yes sir !

 

Definetly. I thought it was fascinating to see the theory that this “could” ( or not ) come into play for 4th gear of the car... from the 1st owner. I’ll never know , just had a glimpse.

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