Trouble Downshifting to First Gear?

[red67]

Newbie here. I recently purchased a 1971 240Z with a later model 5-speed trans..  Shifting into first is somewhat of a crapshoot, sometimes no problem, and more often it just does not want to go in, period. I've  had to take off in 2nd, if I'm at a light, which I do not like to do! I've done some searches, and come up with bleeding slave units and such. Just thought I would bounce it off you guys, before I started. Thank You!

[Zed Head]

Does it grind or does the lever just not move?  Also, does it go in to reverse easily?  Reverse is the true test of your clutch since it does not have a synchro.

Might just be worn bushings on the shift lever, letting things get out of alignment.

[red67]

Goes into reverse easily!  First is only issue. No grinding, just won't go in.

[AK260]

Goes into reverse easily!  First is only issue. No grinding, just won't go in.

 

 

With engine off and clutch down:

 

Try going into second (and push hard home) then try getting into 1st.

 

If it works: try again with engine running.

 

If not working at all: S14a time!! [emoji12]

[red67]

Does not seem to be a sloppy shifter, but when I get home I'll Try.

[Zed Head]

1 hour ago, red67 said:

Goes into reverse easily!  First is only issue. No grinding, just won't go in.

Could be something to do with the check and interlock balls.  Balls that are pushed in to notches to keep the gears in the correct place.  I think that you can check those without removing the transmission.  EuroDat knows more @EuroDat

[Zed Head]

I had a similar problem with a Ford transmission and while I was figuring out a solution I would just match speed and RPM when approaching a light to get in to first and just sit there with the clutch pedal down.  You've probably considered it already but just in case.

The solution wouldn't apply to your problem, it's a design problem with the Ford (actually Mazda) transmission I think.  In that case changing fluids to Pennzoil Synchromesh worked.  I've never heard of anyone having your problem though.  It seems mechanical, maybe a broken synchro or a jamming ball.

One thing you might do right off the bat is to check the fluid level, then drain the fluid.  Check the magnet for fragments.

Edited October 1, 2020 by Zed Head

[red67]

Thank you, I'll give that a try, and report back.

[EuroDat]

Hi Red67,

Not seeing it, hearing it or feeling it happen makes it hard to diagnose. From your description I think you have a problem with the teeth on the side of first gear.

 

A little background on how a borg warner type synchro works. 1st gear: The FS5W71B Synchro mechanism consists of:

Hub (26),  Sleeve (28),  Baulk ring (29),  3x inserts (30/31),   Constant mesh gear (19). 

The hub is connected to the main shaft. In the hub are three grooves for the inserts and springs the push the inserts outwards towards the sleeve. The inserts have a tab on the outside so they will "centre" in the sleeve.

The baulk ring has three slots where the inserts fit into. This makes the synchro turn at the same speed as the hub.

The constant mesh gear has teeth on the side and a cone that fits neatly into the baulk ring.

 

Now how does that all work when you change from neutral to 1st gear. Basic description: 

The sleeve moves over the hub and pushes the inserts and baulk ring onto the cone section of the constant mesh gear. The friction then causes the hub and constant mesh gear to synchronise by speeding up or slowing down the first gear to the same speed as the hub.

When you push further the sleeve slides over the insert tab and meshes with the teeth on the constant mesh gear. The teeth on the constant mesh gear are pointed to guide the sleeve into position.

Now first gear is connected through the constant mesh gear à Sleeve à Hub. The baulk ring and inserts have no function. The detent balls in the selector mechanism hold the sleeve in position.

 

What can cause your problem:

1. The baulk ring is worm and under minimum tolerances. It can not synchronise the hub and the constant mesh gear. Result is grinding gears.

2. Poor quality oil makes the baulk ring inefficient. Oil does make a difference.

3. The teeth in the constant mesh gear and sleeve are blunt and rounded. That way the sleeve hits the blunt teeth and you require more force to engage the two. You have to force the gear a little left or right to get it home. Occasionally they will line up and it go in easily.

 

From your description, I would think the 3^rd problem is the main issue and maybe some of it comes from the other two problems.

I would try changing the oil for starters. The rest will require dismantling the transmission.

A member on this forum "Chickenman" recommends an oil from AC Delco or GM. The part numbers are AC DELCO USA part number is: 88900399. GM USA part number is: 12377916. I can not order the GM or AC delco oil here in Europe so I have no experience with it. I'm using Redline MT 75W-90 and find it very good. Here is a thread on the oil subject

https://www.classiczcars.com/forums/topic/51698-how-should-a-71-240-4-speed-feel/

I don't think your selectors are an issue atm.

 

 

 

[jfa.series1]

This is a highly recommended transmission oil, perhaps the same one referred to by @EuroDat

[red67]

So today, with engine off went thru gears, with no problem. Weather would not permit a drive, so I didn’t start it. 

Thanks all for the help. How many quarts do I need. I know...search.

Edited October 2, 2020 by red67

[Pilgrim]

I've used only Mobil1 or Amsoil synthetic gear oil for years in my 280ZX Turbo.  Makes a significant difference in ease of shifting. Make SURE that you use type GL-4, NOT GL-5. 

Transmissions need the GL-4 type gear oil. 

Incidentally, when I looked up the gear oil for my '83 ZXT on the Amsoil site, it shows an ATF synthetic for the '83 5-speed. I'm pretty sure that is wrong!

Redline is an excellent lube, one of the very best. I believe this gear oil is correct: https://www.redlineoil.com/mt-90-75w90-gl-4-gear-oil

Edited October 2, 2020 by Pilgrim