Welcome to the Corvette Forums at the Corvette Action Center!

Question: 1990 ZF 6-speed intermittent balk upshifting to third gear

90SixSpd

Member
Joined
Sep 28, 2014
Messages
9
Location
Gilbert, AZ
Corvette
90 Conv HT
My '90 6-speed balks (no grinding) occasionally when upshifting from second gear to third gear. It works best if just relying on the spring loaded shifter mechanism, and pushing straight forward out of second gear (slowly enough to avoid going into first gear). Downshifting from fourth gear into third gear is perfect. This transmission never grinds. The clutch is good, with no crunching/bad noises when shifting from first into reverse. The car has approximately 20k miles, the CAGS bung has been removed, and the spec gear lube level is full. Has anyone else experienced and remedied this problem, and if so, how? Thank you very much!
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
First thing to do is verify that the clutch hydraulics are in good order. Many cases of missed shifts or high shift effort are due to faulty clutch hydraulics or faulty clutch. Rule that out first.

When you say "balks" do you mean when you push forward then over and forward to third, the shifter hangs in the neutral gate and will not go into third?
 

90SixSpd

Member
Joined
Sep 28, 2014
Messages
9
Location
Gilbert, AZ
Corvette
90 Conv HT
I appreciate your reply; thank you! The clutch and related hydraulics have been reconfirmed as functioning as designed/within spec. By "balking," what I mean is that in performing the 2-3 upshift, during the movement from the neutral detent toward third gear, the shifter feels as though it runs up against a stop at the point which it should be engaging/selecting third gear. There are two choices at this point: 1) force the gear selection (unacceptable), or reselect neutral and complete the third gear selection. This fault is utterly random and occurs probably 5% of the time. Of interest, downshifts into any gear are always utterly smooth. Hopefully someone can confirm that there is some adjustability or identify components that wear over time, based on the way the shifter mechanism attaches to the rest of the transmission. As an aside, I have 50+ years experience driving dog-ring/teeth type synchromesh and tend to be very gentle with gearboxes (two-part shifts/imperceptible neutral pauses, etc.), so this car is not being speed shifted by one who's trying to beat the synchros. Would much appreciate any available guidance. Thanks again!
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
My guess is you are driving the car such that skip shift is enabling.

The conditions under which it will enable are listed in the service manual. I'm on a road trip right now away from my manual collection otherwise I'd key them into this post.

Suffice to say skip shift enables at low engine speed, low vehicle speed and light throttle.

Try to not short shift out of first and you'll probably solve the problem. Light throttle low rpm shifts out of first will usually enable skip shift which forces a shift to fourth.

Also, skip shift doesn't hurt anything so, if you drive such that you get skip-shift just go into fourth.
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
I appreciate your reply; thank you! The clutch and related hydraulics have been reconfirmed as functioning as designed/within spec. By "balking," what I mean is that in performing the 2-3 upshift, during the movement from the neutral detent toward third gear, the shifter feels as though it runs up against a stop at the point which it should be engaging/selecting third gear. There are two choices at this point: 1) force the gear selection (unacceptable), or reselect neutral and complete the third gear selection. This fault is utterly random and occurs probably 5% of the time. Of interest, downshifts into any gear are always utterly smooth. Hopefully someone can confirm that there is some adjustability or identify components that wear over time, based on the way the shifter mechanism attaches to the rest of the transmission. As an aside, I have 50+ years experience driving dog-ring/teeth type synchromesh and tend to be very gentle with gearboxes (two-part shifts/imperceptible neutral pauses, etc.), so this car is not being speed shifted by one who's trying to beat the synchros. Would much appreciate any available guidance. Thanks again!


You need to let the centering springs do their job, if you try to shift too fast or grip the shift knob you may run into this issue.

Remember to upshift with the palm of your hand (I usually open my fingers at the same time to keep them away from the knob) and down shift with your fingers, gripping the shift knob with your fingers can over ride the centering springs.
 

90SixSpd

Member
Joined
Sep 28, 2014
Messages
9
Location
Gilbert, AZ
Corvette
90 Conv HT
Thank you for your sharing your insights!
Following up on my car's skip shift feature, the Computer Aided Gear Selection bung (electronically-activated skip shift mechanical component within the transmission) was removed, disabling the skip shift feature. An aftermarket electronic disabling part (redun) was also inserted in the harness.
I agree with the input to minimize side-to-side input, and that is what I was referring to as just pushing the shift lever straight forward out of second gear, and I should have included "with no side-to-side guidance." This is how the shifter works best, but regretfully still does not fully prevent the third gear "balking."
Thanks again for your insights!
 

90SixSpd

Member
Joined
Sep 28, 2014
Messages
9
Location
Gilbert, AZ
Corvette
90 Conv HT
Problem solved. Draining the previously recommended Castrol 10-60 and refilling the transmission with Amsoil Synthetic Manual Synchromesh Transmission Fluid immediately and totally cured the problem. I have since tried to induce the previous fault while shifting and been unable to do so. I have no relationship with the product and bought it from a dealer who was a complete stranger, and frankly, I was more than a little skeptical. The fluid meets GM specs 9985648, 9986105, and 9985535 according to the manufacturer. The label also references GM part numbers 12345349 or 12345577, 12377916, and 1052931. The manufacturer's product code is MTF-QT.
 

Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Win Both Corvettes!

Win Both Corvettes and Get Extra Bonus Tickets Now!

Supporting Vendors

Dealers:

MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!

Parts/Accessories:

Dead Center Foundation

Vetteskins

Advertise with the Corvette Action Center!

Partners

Top Bottom