Will the overdrive unit of a 4+3 slip when it needs serviced or is my clutch slipping only under hard acceleration in top gear with overdrive engaged?? Thanks in advance for your help.

 

The more experienced will chime in but if it doesn't slip in any of the four manual gears beforehand, especially in the lower gears where there's more torque, it's probably the unit that needs servicing. Maybe give the fluid level a check.

 

Usually if reverse works OK the direct clutch plates in the over drive unit are OK? Going forward there is a sprag clutch in the OD that will allow forward motion. It sounds like your clutch is going out, it will not hurt to service the OD unit, a $9. filter and some AT fluid and your all set.

 

 

It's supposed to kick out of overdrive under hard acceleration.

 

I agree. I think I saw somewhere the OD kicks down at 60% throttle.

 

Grinds bad going into reverse. The only way to do it is shift quickly form a forward gear, I replaced master and slave cylinders but it didn`t seem to help much. Does the grinding going into reverse mean anything to you?? Thanks, Dan

 

Thanks to everyone else as well for you input.

 

Did you bleed the slave correctly? you have to have the slave OFF the bell housing, tilted up about 45 deg. so the bleeder screw is facing up at it's highest position. You can even gravity bleed it in this position, don't let the master cyl. run dry.

 

It's normal for it to grind in reverse. You have to shift into a gear before putting it into reverse. But if you keep your foot on the clutch, shift into gear then back to neutral and wait a little before putting it reverse and if it still grinds then your clutch is not properly disengaging. It should grind going from neutral to reverse because reverse gear is not synchronized. It should not grind however if you first start the car and hold the clutch in while you put it in reverse. Any time you release the clutch while in neutral you will have to go into a gear before going into reverse.

 

My 87 with the 4+3 has a 6 year old well-abused clutch and the trans will not grind while going into reverse from neutral. I don't shift into a forward gear first, and when I want go backwards, I depress the clutch pedal, raise the lockout ring, and shift into reverse.

Any grinding when shifting into reverse is typically associated with a worn-out clutch disc. The 4+3 is no different that way just like any B-W Super T-10 (early or late versions), or even a Muncie. The OP's problem is due to a clutch disc that is worn out.

Slipping under load/acceleration that he sees is a definite indication of a worn disc. Hard acceleration (i.e. WOT) in 4th OD should cause the OD unit to kick out of OD. When that happens and the clutch is weak, slipping will occur. Try driving up a steep hill in 4th with OD disengaged using part throttle. Is there any indication of slippage? If so, start looking for clutch/flywheel assemblies

 

Transmission should not grind going in and out of reverse. Grinding is an indication that there's air in the clutch yet (at least in my case) and the clutch is not fully disengaging when the pedal is being pressed.

Here's a quick test that may or may not prove it: pump the clutch 5 or 6 times and see if it goes in clean or not. If it does, then it's likely air still in the system.

If it is air and you've bled the system and it's still grinding, here's a trick a former GM mechanic taught me and it worked on mine. Engine off, top off the clutch master cylinder and on level ground. Pump the clutch a half dozen or so number of times and hold it to the floor for about 5 minutes. At the end of the 5 minutes, slide your foot off the clutch and let it pop up on it's own. The theory is the air is the line will come back to the 180 degree turn right over the master (while holding the pedal down) and when you let the pedal go, the fluid in the slave will push the air back out of the line into the master.

I did this on my 86 then waited a little bit and did it again for good measure. Once done, I had a nice solid clutch pedal and it went into reverse without so much as an indication of grinding. It's held up for almost a year now without any indication of problems. Be sure to keep your master filled to the right level, the one I have has an awfully small reservour so it's worth keeping an eye on over time.

 

This is going to be pretty hard to explain over the internet but I'll try anyway. If you notice in the picture of the parts laid out on the table, the input shaft is separate from the main shaft. These two shafts go together, but there are bearing that allow them to spin at their own rate. When 4th gear is selected, it combines the input shaft with the main shaft. The force is not transferred through the counter shaft in 4th gear.

When you release the clutch, the input shaft is always spinning. No matter what, any time the input shaft is spinning, the counter shaft is spinning.

Lets put it in neutral now. The input shaft and counter shaft are the only shafts spinning at this time. Notice the Reverse gear. Under the reverse gear is a gear on the counter shaft for 1st gear. The reverse gear is always spinning when the counter shaft is spinning.

There is a shaft that runs through the reverse gear, which is attached to the over drive unit. Between the overdrive unit and the reverse gear, is another gear for reverse. These two reverse gears spin together at the same time. On the outside of the manual transmission between the transmission and the over drive unit is a lever, with a 5" ish gear that is splined to the main shaft.

When you select reverse, the 5" gear meshes with the reverse gear on that is between the transmission and over drive.

Since the input shaft is spinning, the counter shaft has to spin, causing the reverse gear inside the manual trans to spin, causing the reverse gear on the outside of the transmission to spin. When you mesh the 5" reverse gear which is not spinning, with the spinning reverse gear, you get a grinding.

This is why you have to put it in first before you select reverse, so that you stop the counter shaft from spinning, which in turn stops everything else from spinning.

 

If your trans grinds going into reverse, you have other problems; usually with the master/slave. Bleed it, and then bleed it some more.

 

Did you not read what I just posted?

 

My 85 does not grind going into reverse. I have owned it for 20 years. Wear on the clutch pedal pivots and/or air in the lines, or clutch wear will not allow the clutch to fully disengage is the most likely reason for this.

 

On my other car, owned it for 26 years, when that happens with reverse its been an early indication the clutch needed to be replaced. Dont know why exactly but the lower gears show up first. Bleeding it properly first to be sure sounds like the right thing.

My old car doesnt have any hydraulics or self adjuster for the clutch. Once took it somewhere and the guy had the master cyl for the brkes open thought he was checking my clutch fluid, even after I told him it didnt have a hyd clutch, and what he had opened was attached to the brake booster. I told him just close it up,Im leaving

Do these Corvettes have an automatic adjustment on their cable?

 

The clutch is hydraulic. No cable or adjustment.

 

I bet most people have you on "ignore" now, Daniel.


While you're THEORY is mostly right, the one thing you didn't account for is friction. In "real life" friction in the gears, oil, bearings, etc. generally bring the input shaft/clutch disk to a stop w/in a few seconds, so unless you simultaneously push the clutch pedal and move the shifter into reverse, you shouldn't get gear grind going into reverse.

If you HAVE to put it in another gear first, in order to prevent gear clash going into reverse, something is not right in your clutch system.

 

If I push the clutch in and wait about 5 - 10 seconds, then no, it doesn't grind.