Transmission behavior into Reverse?

[PineClone]

Me again, with another question to the brain trust. Just getting familiar with my 85 720 4x4...

 

Shifting into reverse, the gears want to grind.

 

Example 1: Driving in 2nd gear, stop, depress clutch, shift out of 2 and move directly to R, GRINDING!

Example 2: Driving in 2nd gear, stop, depress clutch, shift out of 2, release clutch, depress clutch, shift to R, no grinding.

 

Thanks!!

 

 

[iceman510]

I don't know this for certain in the case of this vehicle, but some transmissions have a reverse brake (sort of like a synchro with friction) that stops the shaft spinning.  Sounds like your second scenario allows time for the shaft to stop and thus you are not getting the grinding in that case.

 

Maybe the reverse brake is worn, or you just need to shift slower.

[ElliotV]

It's a older truck you just need to take your time with it. Me personally I have developed a habit of going into first gear then shifting into reverse in all my manuals. This forces the gears to stop moving and let's it get into reverse easier. 

[Crashtd420]

I also believe that reverse has no synchro ..

[bottomwatcher]

If it's new to you it may be taking several pumps of the clutch to get it to fully release. Master and clutch slave cylinder are wear items. Do they look old? Having a tough time getting into reverse is usually a sign they are wearing out. I usually wear out the slave before the master due to the exposure to the elements the slave gets more of. 2 slaves per master. I have also rebuilt them but they don't seem to last very long compared to new.

[ggzilla]

14 minutes ago, bottomwatcher said:

Master and clutch slave cylinder are wear items

This is a good point. They usually last 100,000 miles or 10 years but do wear out. This is why the Nissan maintenance schedule says to change the hydraulic clutch fluid every 12 months. Not because the fluid wears out, but to remove contaminates from the system

[datzenmike]

Try second, come to stop, depress clutch, count to five, shift into reverse. This gives the transmission oil time for the spinning countershaft time to slow down. You might want to check the transmission oil level. It should be filled up to the fill bung. If low this will not provide as much drag to slow the gears

[ggzilla]

1 hour ago, Crashtd420 said:

I also believe that reverse has no synchro ..

yes like most transmissions from most automakers, Nissan reverse has no synchro. You have to fully stop before shifting into reverse. All the forward gears you can shift into if the vehicle is (slightly) rolling. Lamborghini made headlines when they came out with synchro reverse

[ggzilla]

I'm trying to remember ... I think my Datsuns had this problem when the clutch was not adjusted to spec. As the clutch wears it gets less and less pedal free play and if i remember correctly, the problem first shows in reverse

 

You can test this with your pinky. Push on the clutch pedal with your pinky finger, how far does it move before it stops? There should be 1" free play or is it 3mm free play? Check your service manual. If there is none or almost none, it is adjusted down at slave cylinder on the transmission

[NC85ST]

3 hours ago, ggzilla said:

it is adjusted down at slave cylinder on the transmission

There’s no slave cylinder adjustment on a 720, like there is on a 521. The only clutch adjustment is at the pedal.

[PineClone]

3 hours ago, ggzilla said:

You can test this with your pinky. Push on the clutch pedal with your pinky finger, how far does it move before it stops? There should be 1" free play or is it 3mm free play? Check your service manual. If there is none or almost none, it is adjusted down at slave cylinder on the transmission

I have about 1" of play. But I tested it with my index finger....pinky finger seemed a little highfalutin.

[ggzilla]

Maybe too much? My 620 FSM says maximum of 3mm (that's less then 1/8"), and @NC85ST is right, there is no adjustment at the slave cylinder, i totally forgot about that. I don't have a 720 manual

 

if i recall correctly:

* no play: clutch may slip under load, when pedal is up

* too much play: gears can grind during shifting, when pedal is down

[datzenmike]

Clutch pedal play is adjusted on the clutch push rod into the master cylinder. Pedal play is a few mm. Only the 521 and 510 had adjustable play at the slave push rod and maybe earlier vehicles but all after were not.

 

Most common cause of grinding is air in the hydraulics causing loss of clutch arm travel. Also the easiest to fix by bleeding. Slave push rod travel should be just over 1 ".

 

The JDM S13 or S14 had a synchro reverse on the 71C. Corvettes have a synchro reverse but I don't know when they were introduced.

[DwayneOxford]

Always adjusted mine to  need to mash to floor or almost to disengage and engage with heel on floor. The sooner one disengages the more likely you are into throw out brg. and can slip easy in high under hard load.

[banzai510(hainz)]

bleed first

 

 

[Thomas Perkins]

What I use.

Edited April 16 by Thomas Perkins

[PsychoNaut04]

I shift to 3rd then reverse and it helped. 

[PineClone]

Reporting back on how I have addressed the grinding into reverse problem. Figured i'd share my experience just in case there is anyone out there as stupid as me who is fumbling through similar problems....

 

To recap, I was having problems shifting into reverse without significant gear grinding. I tried changing out the clutch fluid, but that only made it worse. As in MUCH worse. My guess is that I didn't do a good enough job of bleeding. After that...

 

1) Replaced master and slave cylinders and hose (exact parts as shown above by Tommy Perkins).

2) Drained gear oil. The old gear oil was dark brown with some visible brass glitter (I'm a little concerned about this). The plug magnet had typical gunk on it, but no big chunks of metal. I suspected that the gear oil level might be low (causing the reverse problem), but it was definitely full when i removed the fill plug. Filled with Redline MT-90.

3) Bled the lines with a one-man bleeder kit.

 

After much bleeding and many trips back and forth from the reservoir to the slave, it seemed that all of the air was out of the line. Took it for a test drive.

 

Right out of the gate (putting into reverse to back out of the garage), major grinding. Not a good sign. Decided to take it for a drive anyway. Shut off engine, put in reverse, start engine, back out of garage. Drive a mile. Try reverse. Works much better! Not perfect, but better. Drove it to work the next day. Tolerable, but still grinding. 

 

Watched a few videos on clutch bleeding and discovered that you also may need to depress the slave push rod to expel any air that might be trapped in there. Got the wife to watch the reservoir while I depressed the push rod. BUBBLES!!

 

This seems to have solved the issue. Shifts like a dream. Still intermittent grinding, but shifting into 1st and then into R seems to work every time. Sometimes don't need to do it at all. Definitely coming to a full stop and a brief pause seems to work best.

 

The end.

[Thomas Perkins]

Great job.

[ggzilla]

Thanks for the update. Another Nissan taken care of

A little glitter is just 10 years of normal wear. No worries.

1 hour ago, PineClone said:

Drained gear oil. The old gear oil was dark brown with some visible brass glitter (I'm a little concerned about this). The plug magnet had typical gunk on it, but no big chunks of metal.

 

[datzenmike]

Place in neutral while idling and let clutch pedal up. Depress and wait 3 seconds, shift slowly into reverse. If there is even the slightest gnashing or grinding of gears the clutch is still rubbing the PP and/or the flywheel. This could be air still in the hydraulics or the clutch disc or the PP or both are damaged. The job of the synchro is to connect two different speed parts and acts itself like a miniature clutch. If just the inertia of internal transmission gears this is easily within it capabilities, but if being driven by the engine this will put a lot of load on them and shorten their lives.

[PineClone]

Rubbing the PP? That seems like a good thing right?

 

😀

[DwayneOxford]

21 minutes ago, PineClone said:

Rubbing the PP? That seems like a good thing right?

 

😀

Depends on your company.

[DwayneOxford]

Unless I'm badly mistaken Datsuns were designed for right side drivers necessitating the hydraulic clutch actuation.

[datzenmike]

Japan is RHD but for the world market it's easy enough to 'mirror image' parts so they work LHD. 

 

Hydraulic clutches work universally for LHD and RHD. Designing a clutch linkage for LHD with the clutch arm on the right side would be a nightmare.