Bought new to me 1979 Mack Dump Truck

[Gunny65]

Soooo, I was looking to get a repair and/or service manual for this truck. $304.00???  Totally ridiculous. I found some others that I Think are for the correct years but not sure. Still kind of expensive but better.  Anyone know where to get manuals for my truck in a more reasonable price range?

[Joey Mack]

I buy some from, mypowermanuals.com....  they are downloads, but I put them on C.D.'s and load them into my shop laptop..  

[JoeH]

Not sure what book you're looking at.  There's a whole slew of individual component books. Book for pretty much every engine, trans, chassis, suspension, fuel, etc. Usually each book is $40 or $50 at the dealer, but you gotta know which ones you want. They're numbered.

[JoeH]

If you go to your local Mack dealer they should be able to look up book # 5-672 and order it for you. It's an engine overhaul book for 1981. Pretty much identical engine to what you have, just 2 years newer should cover multiple HP ratings.  Not sure what transmission you have, but that book will likely be available as well. Same on rears. Other than that you're largely on your own to figure things out by asking around here.

[doubleclutchinweasel]

The type of clutch you have (Eaton, Mack, etc.) will determine the exact adjustment procedure.  Most are fairly simple.

That 5-speed has a HUGE step between gears.  Upshifting at 1800 will probably drop down to 1200 or so before it meshes up.  Downshifting will be about the opposite...pull down to 1200 and then get up to 1800 before slipping into the next gear.  You'll get the hang of it.  You'll also get the hang of "floating" the gears (shifting without the clutch).  Pretty easy.  Just feather the throttle to release the load on the gears, slip it into neutral (no effort required), let off or increase throttle to get the RPM right, and slip it into the next gear (again, no effort on the stick).

And, like everybody said, only depress the clutch fully to apply the clutch brake while sitting still (otherwise, they can be a PITA to get into gear).  No need to push it all the way to the floor if using it to shift.

[Gunny65]

Finally, here are some pictures.

[Gunny65]

[Gunny65]

[Gunny65]

[terry]

237 hp engine.  Terry

[doubleclutchinweasel]

Like Terry said, "685" means ENDT-675 237HP Maxidyne engine.

"S" means "6-wheel" (tandem drive axles).

"X" means "heavy-spec".  Translation: "beast".

[doubleclutchinweasel]

On 10/13/2023 at 7:19 AM, JoeH said:

this is a Constant Mesh transmission

So are the car and truck transmissions, just for future reference.

[doubleclutchinweasel]

On 10/13/2023 at 3:32 PM, JoeH said:

 

In a synchronized transmission (not what you have) unused gear sets are not spinning until you try pushing it into a gear.  Synchronizers match the gear speeds together for you.

 

Again, car transmissions ARE constant mesh.  All the gears are meshed, and are turning any time the input shaft is turning.  They are NOT tied to the output shaft until you engage a sliding clutch (which is splined to the output shaft) with a gear (which is powered).  Then you transmit power.

Only primary difference (from a functional standpoint) between a car transmission and a truck transmission is the car transmission uses blocking rings ("synchronizer rings") to match the speed of the gears to the sliding clutch.  That's why they start grinding when the blocking rings wear out and no longer have the friction against the cones to synchronize the gear speeds.

Lots of the school buses I drove in my youth had worn out synchronizers in them.  There were only a couple of us guys who could drive a non-synchro transmission, so we always got those buses!  One of them was our activity bus.  So, I got to make lots of trips in that one.  None of the coaches or teachers could shift it!  Benefit of growing up in Macks, I suppose.

[doubleclutchinweasel]

Kinda like this...

This example shows the transmission in 1st gear.

Input shaft on left turns counter gear ("cluster gear") on bottom.  Cluster gear turns each driven gear on mainshaft.  Gears spin free on mainshaft.  Inner part of sliding clutches (the "hubs") are splined to mainshaft.  When outer part of sliding clutch is moved, it locks into smaller teeth on gear extensions and that gear's rotational speed is transmitted to the mainshaft.  The brass "blocking" rings can be seen between the sliding clutch and the gear.  The sliding clutch ring, the blocking ring, and the tapered teeth on the gear extension make up a "synchronizer".

Notice that the reverse gear is operated by a separate fork, and is not constant mesh.  That's why they could grind when going into reverse.

Incidentally, this one looks to me like a Muncie.  And, from the look of the shallow helix angles, I'd say we're looking at an M22 "Rock Crusher". Strong, but kinda noisy!  Yeah, I've had a few (dozen) of these apart!

[JoeH]

13 hours ago, doubleclutchinweasel said:

Again, car transmissions ARE constant mesh.  All the gears are meshed, and are turning any time the input shaft is turning.  They are NOT tied to the output shaft until you engage a sliding clutch (which is splined to the output shaft) with a gear (which is powered).  Then you transmit power.

Only primary difference (from a functional standpoint) between a car transmission and a truck transmission is the car transmission uses blocking rings ("synchronizer rings") to match the speed of the gears to the sliding clutch.  That's why they start grinding when the blocking rings wear out and no longer have the friction against the cones to synchronize the gear speeds.

Lots of the school buses I drove in my youth had worn out synchronizers in them.  There were only a couple of us guys who could drive a non-synchro transmission, so we always got those buses!  One of them was our activity bus.  So, I got to make lots of trips in that one.  None of the coaches or teachers could shift it!  Benefit of growing up in Macks, I suppose.

All due respect and to keep things straight for reference, but the industry refers to those as Synchronized Transmissions, not Constant Mesh. The transmission in this DM is a Constant Mesh, and using that terminology to search for shifting advice will yield different shifting explanations than searching for Synchronized Transmission.

[doubleclutchinweasel]

"Constant Mesh" simply means that all the gears are constantly meshed.  The alternative is a transmission in which gears actually slide in and out of mesh while shifting.  Those are called "sliding mesh" boxes.  Obviously, neither the Mack transmission in question nor the Muncie of which I posted a picture has gears which slide in and out of mesh while shifting.  So, both are "constant mesh" gearboxes.

Synchronization is irrelevant in whether or not a transmission has "constant mesh".  The car transmission is a "synchronized constant mesh" and the Mack transmission is a "non-synchronized constant mesh".  If you want to get super-technical, the "synchronized constant mesh" box can be called a "synchromesh" box, which is basically a contraction.

But, make no mistake, these are ALL "constant mesh", by the very definition of the words.  The gears ARE meshed...CONSTANTLY.

If you choose to call it something else, that's fine with me!  Hey, it's a free country!  Go for it, my brother!

[doubleclutchinweasel]

https://www.mistertransmission.com/manual-transmission-constant-vs-sliding-mesh/#:~:text=The gears used in a,less wear on the gears

More info...

It adds the "synchro-mesh" box to the discussion.

Edited October 17, 2023 by doubleclutchinweasel

[doubleclutchinweasel]

By the way, Gunny, sorry we kind of went off-track there!  Didn't mean to hijack your thread!  But, it happens sometimes on here!  Squirrel!

No offense meant to you either, Joe!  I actually enjoy these discussions.  Nobody around here (work) who can even join in on these kind of things!

[Geoff Weeks]

To make thing less clear, some transmissions use sliding mesh on the lowest gear and constant mesh on the rest. Makes it easy to use 1st gear on the mainshaft for reverse as well, most often straight cut gear.

The main advantage other than wear (you are going to get wear somewhere, constant mesh just move the wear to the dog clutch) is it makes the trans easier to shift, you are moving less mass.

[doubleclutchinweasel]

1 hour ago, Geoff Weeks said:

To make thing less clear, some transmissions use sliding mesh on the lowest gear and constant mesh on the rest. Makes it easy to use 1st gear on the mainshaft for reverse as well, most often straight cut gear.

The main advantage other than wear (you are going to get wear somewhere, constant mesh just move the wear to the dog clutch) is it makes the trans easier to shift, you are moving less mass.

Good point, Geoff.  The old "granny gear" boxes in pickups, buses, and medium-duty trucks often had an unsynchronized (sliding mesh) "LO" gear.  Many were, as you mentioned, on the "reverse" fork.  I drove quite a few of those.  Our school buses all had "3-speed plus granny" boxes, but the reverse gear was on a different fork in those.  Still probably driven off the 1st gear, though.

Edited October 17, 2023 by doubleclutchinweasel

[doubleclutchinweasel]

Incidentally (and WAY off topic!), isn't it interesting that the newer high-performance racing transmissions are dual-countershaft...ala RoadRanger?  This one is a Liberty.

[Geoff Weeks]

15 minutes ago, doubleclutchinweasel said:

Good point, Geoff.  The old "granny gear" boxes in pickups, buses, and medium-duty trucks often had an unsynchronized (sliding mesh) "LO" gear.  Many were, as you mentioned, on the "reverse" fork.  I drove quite a few of those.  Our school buses all had "3-speed plus granny" boxes, but the reverse gear was on a different fork in those.  Still probably driven off the 1st gear, though.

More or less comes down to: do you slide the reverse idler gear (separate fork) or do you slide the main shaft gear one way for lo, the other to engage with the reverse idler (same fork).  Two way to get the same result.

The F51 in my IHC is sliding gear for low/rev. They did make some two speed reverse gear trans, now it gets complicated. Mine is sliding gear for low/rev and constant mesh (but no syncro's) for 2nd-5th. Shifter throw is much greater on the rev/lo gate than the rest.

[doubleclutchinweasel]

1 hour ago, Geoff Weeks said:

More or less comes down to: do you slide the reverse idler gear (separate fork) or do you slide the main shaft gear one way for lo, the other to engage with the reverse idler (same fork).  Two way to get the same result.

 

You nailed it right there!  As usual!

[Gunny65]

I came here to learn so no big deal to me if it goes further than my intended questions.  This is all helpful.  I read up on how to adjust the clutch engagement and clutch brake.  Hopefully I can get to it in the next few weeks.  You know, before the snow falls.  I have to work on it outside so weather is a factor.  Hopefully the clutch isn’t to far gone to adjust it properly.