1970 M-20 Issue

[Stephen B.]

I rebuilt my 70 Corvette M-20 transmission, including new gears, etc. The transmission shifted through all of the gears fine on the bench.


Now that it's installed in the car, I am not able to shift fully into reverse. I can shift fine in the 4 forward gears; however, the engine dies when I let out the clutch. It appears that something is not working right in the reverse mechanism.


Please give some suggestions as to what I need to look for and/or adjust.


Thanks in advance,

[Edward J.]

Re: 1970 M-20 Issue

Steve. Is the shift linkage set up properly? What it sounds like is the reverse gear is not hooked the internal lever, this will permit the arm to move but not permit the gear to engage and disengage. if the driveshaft is out, put the transmission into neutral and see if you can spin the output shaft.

[Stephen B.]

Re: 1970 M-20 Issue

The linkage is set up properly.

The driveshaft is out, and the transmission will not spin freely.

It's strange that the transmission shifted perfected on the bench. No problems. I checked multiple times to ensure that the reverse lever was in the proper slot. Maybe I did not put it on right. If so, then how did the transmission work so well on the bench.

Thanks for your reply

[Edward J.]

Re: 1970 M-20 Issue

Steve, You can shift the trans on the bench but if you had spun the tail shaft you would have noticed that it would not spin, the reverse gear is engaged and like not hooked.what is happening is your engaging into two gears at one time. (one forward and reverse)

[Stephen B.]

Re: 1970 M-20 Issue

Thanks Edward

[Domenic T.]

Re: 1970 M-20 Issue

Ed is right. You can just reach and move the reverse lever by hand and your done.

Dom

[Gary R.]

Re: 1970 M-20 Issue

I rebuilt a 69 M22 for my buddy a few years ago, in this case he bought new gear set for closer ratios that the original CR M22 had. Who knows why?

Anyway I got it assembled on my setup stand and the forward gears were good, shifted into reverse and locked up the trans. Pulled it apart and rechecked everything- same problem. Replaced the new import rev idler with the original used GM idler and solved the problem. I didn't see anything odd with the new one but it was the cause of the problem.

[Stephen B.]

Re: 1970 M-20 Issue

I used the original idler on the reverse gear. I've heard of having that problem with import gears.

[Stephen B.]

Re: 1970 M-20 Issue

I've done that by hand. In addition, the shifter does not appear to go far enough when i put it in reverse. However, on the bench everything shifted fine and the gears turned properly without any lock up.

[Mark E.]

Re: 1970 M-20 Issue

Clarifying what Dom suggested:

What happens when you disconnect the reverse gear rod, and engage/disengage the reverse lever by hand?

[Stephen B.]

Re: 1970 M-20 Issue

Mark, The reverse lever moves about the same, and the rear yoke will not turn.

[Mark E.]

Re: 1970 M-20 Issue

Mark, The reverse lever moves about the same, and the rear yoke will not turn.

What if you disconnect all shift rods, position 1-2, 3-4, reverse levers in neutral by hand, and disengage the clutch? This should be the same state as when you tested it on the bench.

Are the shift levers tight and indexed properly with their shafts?

[Gary C.]

Re: 1970 M-20 Issue

If you did not engage reverse fork onto reverse gear when you installed the tailhousing, it would shift into reverse but not shift back out when in the car. On the bench, if the xmsn was laying down with the two mounting ears and tailhousing on the table, the downward angle of the tailshaft may let the reverse gear slide back and disengage when shifting on the bench but in the car, the tailshaft is horizontal so reverse gear does not move back from gravity.

[Stephen B.]

Re: 1970 M-20 Issue

Here's an update on my additional analysis of the transmission:

1. When the clutch is disengaged, the transmission will move when turning the yoke by hand in any gear, including reverse. This tells me that the transmission is not locked in two gears. Does anyone agree with this conclusion?

2. When clutch is engaged, the transmission will not turn by hand in neutral or any gear. Shouldn't the transmission turn by hand in neutral with the clutch engaged?

3. I started the engine with 1st/2nd and 3rd/4th levers in neutral position (and the shift linkage removed). Then I shifted between reverse and neutral. The engine did not bog and quit when I released the clutch in either position; however, the rear wheels did not turn when in reverse. The shifter does seem to be moving far enough to engage the reverse gear. As you recall earlier in this post, when i shifted into any forward gear, the engine would bog. Any ideas?

Thanks in advance...

[Domenic T.]

Re: 1970 M-20 Issue

I think Gary has the answer. Reverse fork. That's always a feel thing.

Dom