As a new owner of a stock 91 Corvette base model, I am loving all the info on this website. Thanks to any members who have knowledgeable info to contribute (unlike myself ahem).

I am absolutely loving the car (minus some water getting into the interior, but thats for another day). The problem I've encountered a couple times is that the manual ZF6 transmission won't let me into 1st or 2nd from a stop.

The most recent scenario this happened went like this: start up the car, drive down the road getting up to 4th gear, approaching the stop light I put my foot on the brake and when it starts to bring the RPM too low for the engine I then push the clutch to the floor and put it in neutral, letting the clutch back to its resting point and completing my stop. All is good up to this point.

Once the light is green I push the clutch to the floor, slide the shifter to the left and go to engage 1st gear. It went to the left position (for 1st and 2nd) just fine. But when I go to enter 1st gear it is like I hit a wall. Obviously this is not normal so I tried 2nd gear. Same thing, totally locked out of 2nd. I let out the clutch and then re-entered the clutch (pedal to the floor) and same thing. Locked out of 1st and 2nd completely. At this point I go for 3rd and 4th, they went fine, so I go back to 1st and it went in no problem.

This is quite scary to me since the ZF6 can be quite pricey to repair and replace. I did as much research as I can with my current knowledge on the subject.

I've heard getting locked out of 1st in this scenario on these cars can be normal. However the solution people offer is to let out and back in the clutch and then 1st should go no problem. It didn't in my scenario as you read above.

I've tested the clutch hydraulics the best way I know how: clutch, wait 5 seconds with clutch pedal to floor, try shifting, and it always works fine and gives no resistance. The clutch master cylinder is full, but the fluid looked gross.

Just to clarify, 99% of the time everything works perfect as far as I can tell. There have only been a few times in the 1k miles I've driven the car that it has done this to me.

Any suggestions, experiences, questions, information is all welcome with open arms! I want to know whats going on here.

 

Hmm

Do you know if your skip shift 1-to-4 CAGS system is functioning? Maybe that system is malfunctioning or something?

A quick thing to try is to perhaps unplug the skip shift solenoid. That might help if the system is malfunctioning for some reason.

 

I havent looked into that system. I assumed one of the PO did the 1-4 skip shift bypass because I will still get the light on my dash that says "ONE-TO-FOUR" but it will still let me go into 2nd when it lights up with that.

I'll start my list of things to investigate with that. Thanks for the suggestion.

 

It does sound like the clutch master is going. Could also be the fluid has lost its hydraulic pressure if it is so nasty. Try putting it in first gear, start it with the clutch pedal down and see if it stays in that neutral like stage. When my master cyl went bad it would engage the clutch with the pedal completely down.

 

I'm on the side of Top Fuel I would start with the Clutch Master, not a replace right off but a look at. The Clutch Master have a Reservoir equal to a good Shot Glass so it doesn't take much loss or used during wear, so check the level and top off and see if it has improved.

If your unsure were the Clutch Master is, its to the side of the Brake Booster and it uses Brake Fluid.

 

Definitely don’t drive it like this. If your clutch fluid is low top it up and pump the daylights out of the pedal. This will self bleed. If this works if you keep it topped off you can safely drive it for a bit.

If you confirm low fluid/bad hydraulics, replace both master and slave together.

 

The fluid level is good in the master. The fluid looks nasty but the level is right. I've tried the hydraulics test listed in #1 and the previous suggestion of clutch all the way down while in gear and the vehicle still doesn't move. Doesn't seem to lose hydraulic pressure. I'm lost here.

 

Gotcha. Could be shift linkage…maybe give zfdoc (Bill) a call…

When this happens can you get into reverse? Is it easily reproducible?

Make sure the skip shift solenoid is unplugged. I’ve heard of it being force plugged in backwards and locking out gears but it should be every time. Just make sure it’s not plugged in just in case.

Have you changed the trans fluid?

I’d flush, fill and bleed the hydraulics too.

 

I didn't test reverse as this happened in traffic. I can't reproduce it either. Now I am pretty mechanically ignorant but it felt to me like something wasn't quite aligned right, hence it locked me out of 1 and 2 on multiple attempts back to back. Is it possible that once I went to 3 and 4 it "lined something back up" and then allowed me to get in 1 and 2?

The PO had the car 2 years and gave me some Amsoil synchromesh fluid. So I'm guessing it has not been changed for at least 3 years. Clutch hydraulics fluid is so nasty it's probably been longer for that. Both of these fluids are first on my list of to do's along with checking the skip shift solenoid is unplugged.

 

There is a FAQ section on zfdoc.com. It has 2 scenarios where it addresses not going into gear. One is due to the powder coating on an aftermarket shifter and the other is due to the hydraulics. The FAQ is very informative about these transmissions.