How many auto 720s out there?

[TimmyG]

Anyone know what percentage of 720s were built with an auto trans? I know it's not extremely common. I didn't even know it existed for years. 

[datzenmike]

Well all 4x4, all diesel and all Cab Chassis options are NOT automatic so only 2wds had them. After '81? they had a torque converter lock up above 45? .

[Charlie69]

45 MPH or 4500 RPM?

[datzenmike]

 

 

 

No, I think it will lock at or around 45 MPH if light gas pedal. The lock-up is so slight that the only true way to check for it is the tachometer drop of 200-400 RPMs

 

They had converter lock up half way through the '82 model year so 11 '81 build dates and on

[TimmyG]

I couldn't even tell you when my converter locks up. Hard to tell. 

[datzenmike]

Perhaps is doesn't. Subtle is the word. Do you have a tach option? That would show the small change.

 

Lock-up isn't just a very slight improvement in mileage I think mostly it reduces losses from slippage and generates less heat.

[TimmyG]

I added a tach. I do think it locks up at highway speed. Otherwise, most of the time I'm not taking it easy on the throttle...at least not lately. 

[datzenmike]

There is a connection between engine vacuum (load) and the 2 to 3 shift point. At about 4 in of Hg or more, it up shifts at about 2,100 drive shaft RPMs. (aprox 43MPH depending on differential)  Less vacuum means heavy foot on gas and the shift point can extend to 55 mph. Lock up only happens in 3rd and may be so close to not be noticed.

[TimmyG]

If I were WOT, would the converter immediately lock up after up-shifting into 3rd? I only ask this specifically because either that is what's happening or there is some very slight slippage for that brief moment. Its subtle, tho.

[Charlie69]

You have a 35 year old truck with unknown maintenace history and you want it to run like it did when it came off the show room floor.  You sir have a lot of work and parts o go through !  LOL

[datzenmike]

Ahhh I was reading the shift chart wrongly.

 

The 2 to 3 shift at light to moderate throttle input is 1,000 drive shaft RPMs or just under 20 MPH. If the intake vacuum is below 8 inches of mercury (in. Hg.) The up shift is delayed anywhere up to 2,500 drive shaft RPMs or about 55MPH.

 

Torque converter lock-up happens at 2,100 drive shaft RPMs or about 43 MPH (and no lower) when already in 3rd. If the up shift to 3rd is delayed due to low intake vacuum and heavy load the converter lock-up happen as soon as 3rd gear is reached.

 

So if driving normally the 2 to 3 shift is at about 20 MPH and as you speed up the torque converter locks at about 43 MPH. If you are accelerating hard and still in 2nd gear at 43 MPH lock-up does not occur until the transmission shifts into 3rd. So yes you should be able to barely notice a slight change in engine RPMs at about 43 MPH driving normally.

 

I had an old Chrysler and at about 45 the torque converter would lock up. You couldn't really feel it but the engine sound would change.

[TimmyG]

Thanks, Mike. I always like to know how things work. Seems mine is working correctly "like it did when it came off the showroom floor" 35 years ago. 

[datzenmike]

The Nissan JATCO 3 speed is a pretty good transmission. It's tough and reliable. Change the fluid every 30K.

[TimmyG]

2 minutes ago, datzenmike said:

....Change the fluid every 30K.

 

It's on the list, boss. 😆 Along with the rear diff fluid.

 

Currently have the bed off doing some rust removal and changing out the gas tank. 

[datzenmike]

I believe the oil pump supplies the torque converter, which is next to impossible to empty without removal. From the torque converter it goes to the rad cooler so if you change the transmission oil you should be able to flush much of the converter oil by pulling off the return line off and idling the engine while it pumps it out. If one line is larger diameter, that's the return. Maybe 10 seconds, stop check level add more fresh and continue till it runs clear red. 

[canon3383]

On 10/11/2019 at 6:00 PM, datzenmike said:

Well all 4x4, all diesel and all Cab Chassis options are NOT automatic so only 2wds had them. After '81? they had a torque converter lock up above 45? .

I need you to define "lockup", to me that means it doesn't turn anymore. I have an automatic transmission, I have had leaking issues in the past as the cooler lines were incorrectly replaced, since fixed, and totally replaced all the fluid. Shifts great with plenty of torque to turn the tires and peal out of i need to. I've had it up to 80 on the interstate and i am unsure if i should do that again with a possibility of locking up. 

[datzenmike]

Torque converter 'lock-up' is simply a way to direct drive past the torque converter and eliminate any slippage than normally occurs. The engine is (more or less) directly coupled to the drive shaft and this allows for better efficiency, maybe slightly better mileage and the transmission runs cooler.

 

It's not that it 'doesn't turn anymore'.... think of it as not slipping anymore.

[85"720]

On 10/11/2019 at 8:32 PM, datzenmike said:

 

 

 

No, I think it will lock at or around 45 MPH if light gas pedal. The lock-up is so slight that the only true way to check for it is the tachometer drop of 200-400 RPMs

 

They had converter lock up half way through the '82 model year so 11 '81 build dates and on

Mines an 85 and it will lock up  at 45mph. At light throttle  its hardly  noticable. you can definitely feel it with moderate  throttle input.

Edited November 3, 2019 by 85"720