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.