wro87

We have a 85' with a 4/3 overdrive unit. The Car has just 43k miles on it and has lived most of its life in a temperature controlled garage. It has been a pleasure to drive without any issues until last night while driving it all of a suddened it was hard to shift into 1st and second, it would grind and sometimes pop out of gear. The O/D still works in2,3,4 and 3/4 doesn't seem to have any problem. It will make noise(throw out bearing) when at a light only at a stop and if you rev it some will shift fine 1/2 and 3/4 is just easy as hell. What is the best guess? bearing failure on 1/2 and synco's?

Thanks and looking forward to your thought?

Thanks wro87

rocco16

Clutch.
Check fluid level first.

knikula

sounds like the clutch is not fully releasing...is that a hydraulic clutch system? either the master or slave system can leak by and give you those symptoms.

Just a thought from other vehicles that I've worked on... I'm a Corvette noob.

When they start to fail, they do so randomly, and can make diagnosing the problem more difficult.

if you have an external slave cylinder, replacing both will be much easier. If it's internal, then it's a tranny removal situation. I'm just guessing here, really.

c4cruiser

Check the fluid in the clutch master cylinder. Also check carefully for leaks all along the steel and flex portions of the line to the slave cylinder. Is the slave cylinder wet/soaked with fluid?

Other possibility may be a worn or maybe broken 1-2 shift fork. Check the linkage at the side cover for any excess movement or loose attaching nut for the linkage arm.

wro87

Thanks for the reply's. We did check the fluid and it is real dark, not fresh and clear like the brakes. It will be a day or two before we can get it up on our hoist to see what the bottom side looks like and I will see if anything looks loose.

Will keep all posted
Thanks wro87

PS forgot to mention that the O/D light does not come on but we know that it is engaging.

s carter

First start with I don't feel it has anything much to do with the clutch, but as above have said check the fluid. get the car up off the ground enough to get under it, check the linkages make sure the linkages fully engage into there respective gears, and move freely. and the fluid level is good. I hope I am wrong but I kind of feel your 1-2 syncros may be having some kind of a problem.

you may want take the side cover off and take look at the shifter forks and shifter shafts, and try to see as much as you can of the syncros,

wro87

I have the car on the lift to see if the 1/2 shift fork might have come loose. Before I take the side cover off for inspection, what type of sealant do they use for the side cover. Also what type of filter does it have I have tried to locate one and Chevy says no longer available. Also should I refill it with synthetic ATF or standard?

And just in case, any recommendations on who to contact for a rebuild or parts?

Thanks wro87

WVZR-1

Side cover on the M4 is a gasket and you will have no problem locating, if you're removing the side cover I'd replace the shift-shaft seals also. The M4 uses conventional GL-5 fluid for the fill.

The filter for the OD is the same as a Ford FMX, information in this older post:

https://www.corvetteforum.com/forums...post1571645223

No synthetics for the fill

wro87

OK Gang we have it on the lift and popped the side cover off the 1/2 syncro looks to be the problem! It wiggles and can get cocked around on the back one (toward the o/d unit). The front one is nice and tight and does not move. I DO NOT think that is should do this. When we drained the oil it was real clean did not smelt burnt and not a lot of debri on the drain plug magnet. So we are thinking of dropping the tranny and get some new parts for the1/2 unit. Where is the best place to GET PARTS and in a HURRY?

Thanks wro87

wro87

PS: the shift forks look like there is no wear at all and we are thinking of using synthic gear oil in the tranny and just plain old ATF in the o/d unit. This tranny only has 43k miles on it.
wro87

C4+3=/=C7

The BW Super T-10 has brass synchro rings and bearings which are better lubricated by traditional MT fluid. You hear anywhere from 1.05 to 1.5 qt. capacity, so buy two bottles of MT-90 or similar (75W-90 conventional gear oil) and fill with a fluid pump until the fluid starts flowing out of the fill hole. IIRC, the overdrive unit uses something like 1.7 quarts of Dexron III; not VI, which is synthetic. FMX filter along with RTV for the gasket and you're good to go. It's probably wise to change both fluids and the filter every 10k miles (at latest) or couple of years and probably clutch in to engage overdrive, although that's a source of contention. Here's a useful guide, if you really want to be thorough: https://www.google.com/url?sa=t&sour...38jl9RhN6FT1mQ

I try to be cautious with my 4+3 as it seems to show signs of wear (whine mostly), although the GM shop manual indicates that the source of the symptoms is most likely gear meshing and non-structural.