5 Speed Twin Screw

[twin_screwbulldog]

,

i have a 73 R-685 , its a twin screw tractor with a short wheel base.

It shifts up ok , 5 speed single stick. But as i slow down , it wont shift or even find a gear , whats wrong? Should i have it looked at ?

R 2 4

1 3 5 , the pattern is like this

[Lmackattack]

are you useing the clutch? or are you downshifting "by feel"?

[b61fred]

are you useing the clutch? or are you downshifting "by feel"?

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The 5 speeds have a big rpm drop between gears, You would have to snap it out of gear as soon as you let off of the gas and then get the rpm up quite a bit so it will go in to the next lower gear. I sometimes use the clutch to get the trans out of gear, slowly increse the rpm and float it in to gear without the clutch. It will go in when the rpm is right, It also has a lot to do with how much slop is in the drive line.

Fred

[twin_screwbulldog]

So I am doing 50 mph and i have to slow down becuse of a car in front of me , I want to go from 5 to 4 , So you say let off the gas right away and mash the clutch in and put it in 4? And dont touch the brakes

[twin_screwbulldog]

By the way my RPM guage isnt working , so i try to go by noise and feal

[b61fred]

By the way my RPM guage isnt working , so i try to go by noise and feal

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this is is the way I do it if I use the clutch at all,

I will try to put it a step by step

1, you want to down shift, as you let up on the throtle push the clutch in about half way, and kinda snap the gear shift into nutral, In time you can do this with out the clutch, DO NOT PUSH CLUTCH TO FLOOR!, why? some trucks have thing called a clutch brake, Its there to stop the transmission gears from turning I will get to that later. on to step 2

2, You are in neutral, let the clutch up, this will spin the gears in the trans to the same rpm as the engine.

3, raise the rpm up to lets say 1600 rpm and try to slip the gear shift into 4th, You will feel it grind a little you can hold it there right when it starts to engauge the gear, and the gear shift will kinda bounce a little in your hand, or it might go right in,

4, kind play with the throttle some, raise the rpm up some and it should scratch a little and then go in.

some guys will put the clutch all the way to the floor between step 3 and 4. this is a double clutch technique, this uses the clutch brake to slow the trans gears down so it will go in. I never had it work for me, maybe I dont do it right.

let me know how you come out

Fred

[Lmackattack]

I shift like Fred does. I only use the clutch if its hard to get out of gear or when im at a full stop and trying to start. If you are driving at 50 mph you will not be able to down shift to 4th untill you slow down some more (it might go in if you force or bang it in but you will over speed the engine...not good) depending on your rear end ratios (likely 4.17s) and the governed engine RPM (likly 2100) 40-42 MPH is when you can down shift to 4th. the RPM gauge will help with learning but after you get the hang of it its all feel from there.

I have a 2300rpm engine 4.17 rears with my 5 speed and I down shift as following

5th - 4th is around 42 moh

4th - 3rd is around -30mph

3rd - 2nd is around 18mph

2nd - 1st is near 8 mph

and like Fred said the engine rpms need to match the trans speed at the next lower gear you select so when you pull it out of 5th gear mash the pedal to the floor quickly and "feel" for the grind with light press on the stick in the 4th gear position.

Just an FYI bob tailing is sometimes more difficult to lern shifting as there is no weight to keep the truck rolling at a steady speed.

let us know how it goes

Trent

[twin_screwbulldog]

First chance I get , I will get the RPM guage fixed. That sould do it with the help you guys gave. I will let you know how it worked out in a few days . Thanks

[jim]

I shift my 9 speed road ranger the same as Fred and Trent. Down shifting is always more tricky than up shifting. I think my syncros are a bit on the tired side also. If you have plenty of normal slow down distance it's easier to go down one gear at a time,the tricky part is in the event of a sudden slow down that requires you downshift to the second or third gear lower imediatly from the one your in.In that case the speedometer will come in handy. I will at times put my left foot on the brake and rev the engine with my right foot to get the speed and rpm's in line with one another and the feel my way into the downshift. On my truck I keep practicing different situations. I always try and give myself plenty of gear reduction

room in a comfortable manner so I can get into gear as smoothly as I can one gear down at a time. I very rarely use the clutch. I also use the brake as little as possible. I use gearing to slow the truck down as often as I can.

JIm !

[b61pickup]

Sounds to me like it used to sound when the 5 speeds first came out. Everyone hit a hill let it fall to about 1700 yanked it out of 5th gear and stoped and started over. My advice would be fix the tach and let it fall to 1300 before you downshift, wether you used the clutch or not. If you take it out of gear too soon it will not go down till the truck slows down a lot like Lmackattack saids. Dont feel bad tho because 80% of the driver that first got them when they came out had the same problems,including me.

[Lmackattack]

Sounds to me like it used to sound when the 5 speeds first came out. Everyone hit a hill let it fall to about 1700 yanked it out of 5th gear and stoped and started over. My advice would be fix the tach and let it fall to 1300 before you downshift, wether you used the clutch or not. If you take it out of gear too soon it will not go down till the truck slows down a lot like Lmackattack  saids. Dont feel bad tho because 80% of the driver that first got them when they came out had the same problems,including me.

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LOL.. I had it the other way around

6 years ago when I turned 21 I got my CDL i was driving a 2 stick 6 speed mack trans. when I changed jobs and went to work for my current job I was given a 96 internationa eagle with a 18 speed. I missed gears all the time because I let the revs fall to far...

trent

[twin_screwbulldog]

Yeah ,

I tell you , when im slowing down , as when i always get the yellow then quick red light I pull it out of 5 and then it wont go into another gear until i almost come to a stop, then it hits 1 again.

Its going to be a little while before I get a chance to change the speedometer and RPM guage in it. But I wont give up on the old dog. I just need more time behind the wheel just to have a open road and can do what I need. Thanks

[Superdog]

What's the VIN # on your '73 R685?I know of one just like it,they might be twins.

[twin_screwbulldog]

, 1973 Mack R-685ST463423 , I hope you have found the twin

[Superdog]

I was 98% sure that's what VIN # you'd give me.I have one more question for you.Why didn't you use your old name on this board,Sarge?

[Lmackattack]

Ha Ha.

[Freightrain]