915 end of shift in 5th, then bzzzzzzt

[targa44]

After looking at 10 pages of 915 questions, I'll try this...

A 1974 915 trans that was rebuilt 3 years ago (new ring and pinion and synchos) and was just fine until recently.

Now, when completing my upshift to 5th gear (just after engagement) I get a bzzzzt vibration and noise in the shifter. It also occurs when downshifting back to 4th gear (after engagement) - bzzzzzt. It's okay going up to 4th, but up to 5th and then down to 4th gets the high freq. vibration and noise at the end of the throw. When I take my hand off the shifter at that point, the bzzzzt goes away.... those gears seem to be barely kissing something.

Anyone have this experience and fixed it?

I'm going in for some other work one of these days and will drain the trans oil and check for metal.

thanks,

[DRACO A5OG]

Oh my goodness.

I would open up the inspection port and take look at shift fork, it may be rubbing against something. I would also check the coupler, was that or the bushing looked at on the rebuild?

[Flat6pac]

Something you can do to check is, the 5 th gear is located under the end cap of the trans. Possibly, the fork has become loose and not engaging 5 th completely. This is inspection and adjustment, just a pain because you have to pull everything to get to inspect.
Bruce

[Matt Monson]

Sounds like you might be nicking reverse. What could cause that is a variety if things. Is start with the inspection plate and the attached guide fork.

[Bill Verburg]

I'd also look at the clamp, w/ age they lose tension and thus adjustment.

Wevo has a nice replacement

[targa44]

Wow. What a distinguished panel of experts. Great input. I appreciate your responses.

thank you.

[Jon B]

Originally Posted by targa44

Now, when completing my upshift to 5th gear (just after engagement) I get a bzzzzt vibration and noise in the shifter. It also occurs when downshifting back to 4th gear (after engagement) - bzzzzzt. It's okay going up to 4th, but up to 5th and then down to 4th gets the high freq. vibration and noise at the end of the throw. When I take my hand off the shifter at that point, the bzzzzt goes away.... those gears seem to be barely kissing something.

Several things to consider...

You don't indicate that you have shifting issues, just noise & vibration issues that occur during shifting.
Inside the transmission, 4th and 5th gear are on opposite ends of the cluster, and do not share a common fork.
Inside the transmission, the direction of both shifts are the same- toward the rear. This is because the shift linkage motion is reversed in 1st through 4th, but not 5th.
The shift into 5th does not involve the guide fork on the "inspection plate".
Upshifts and downshifts usually involve different drive dynamics; gears and shafts thrust in different directions on power versus deceleration.

"those gears seem to be barely kissing something"
If this is occurring, it could be due to a shaft developing excessive movement in the fore-aft (axial) direction, and the "push" during shifting is now causing some unwanted contact between gears on the two shafts.
Just a thought.

[targa44]

thank you Jon

[Jon B]

Originally Posted by targa44

It's okay going up to 4th, but up to 5th and then down to 4th gets the high freq. vibration and noise at the end of the throw.

One additional thing to consider...

The synchro gears of 4th and 5th are on different shafts; the synchronized 4th gear is on the input shaft, the synchronized 5th gear is on the output (pinion) shaft.

[targa44]

actually, my mechanic found it.

The bzzzzt sounds coming up through the shifter didn't start at the transmission, but at the shifter.

I have a Seine Gate Shifter and the lock out mechanism/tab for 5th gear was doing the buzzing. Up into 5th and the tab springs back..... down from 5th to 4th and the tab springs back.

thanks all,

[911pcars]

The factory lockout pawl for reverse has a torsion spring to keep it in the ready position. If the pawl is vibrating, the spring might be either broken or jambed.

MHO,
Sherwood

[targa44]

Thanks Sherwood - you've helped me a couple of times in the past. I hope you know I wasn't being critical of the Seine shifter mechanism... both my mechanic and I like it. The car came with the shifter when I bought it years ago and I've no idea how old it is.

cheers,

[911pcars]

KJ, not at all. In fact I wish I owned the design patent for the lockout pawl. :-)