M-817 4WD Operation

[PeteNora]

My first post here so I will start out with a hello. I just purchased a m-817 dump. I do not have it home and know very little about them. Once the snow goes down I will go and put batteries in it and drive it home about 60 miles. Until then without any seat time I know very little. So here are a couple basic questions.

1-Batteries. I was thinking that 2 group 31 batteries should work. Once November hits it will be parked for the winter. So will the Two 31 batteries work?

2-How does the four wheel drive work?

3-Is there a basic operators manual available for the M-817?

Thanks in advance for any suggestions.

 

[simp5782]

5 ton and up tm section is where you want to start listed as m809 series trucks.

two group 31s will do fine.

The rear two axles pull all the time. There is no interlock like civilian heavy trucks. The 6x6 works thru a sprag operation in the transfer case. You should hear an air clunk when you put the truck in 1st or reverse.

 

[BKubu]

1-The military calls for four 6TL batteries, but you don't need them. In fact, they are expensive and you can find cheaper civilian batteries. Just be sure you are getting batteries with sufficient CCAs. Group 31s should be fine. Two should work, and definitely will in warmer weather. But, I'd suggest four for cold weather.
2-The front wheel drive is engaged via a sprag unit on the transfer case (assuming it is working). It engages automatically when the rear wheels slip a certain percentage. You can test to see if your sprag is working by getting on soft ground (e.g., sand) and popping the clutch. Or, if you have stout jack stands, you could jack up all wheels and see if the front wheels spin when you put it in gear. That is a lot of truck to have up in the air, though, so I'd suggest just letting the truck do what it was designed for...that is, engaging the front wheels when the rear wheels slip. Then, look to see if there are marks/holes where the front tires were. You should also try it in reverse as I've seen sprags that worked in one but not the other.
3-The operators manual, often called the "10 manual," is available on this site with the manuals. Definitely read it. For example, the manual will tell you not to let the truck roll back while in a forward gear or forward while in reverse gear. It will cause the sprag to bind up and is often the cause of failure. You can also find hardcopy (i.e., paper) manuals. I like to keep hardcopy manuals in the cab of each truck. But, the electronic version is nice because it is searchable.

Good luck!

 

[BKubu]

I was typing as Wes was replying. Sorry for the duplicate response.

 

[PeteNora]

Thanks for the quick replies.

So I will start with the 2 group 31 batteries and see how it works, So there is no cab controls for the 4wd? So the rear has no power divider? With that being said do both rears work all the time? Are they open differentials or some type of locker? Is there any way to manually control the sprag for the front engagement?

 

[simp5782]

Thanks for the quick replies.

So I will start with the 2 group 31 batteries and see how it works, So there is no cab controls for the 4wd? So the rear has no power divider? With that being said do both rears work all the time? Are they open differentials or some type of locker? Is there any way to manually control the sprag for the front engagement?

You can modify the sprag air with a TR1 valve on the dash. There are threads on this.

The rear probably has both open diffs. They work together. Lockers in a dump truck is rare unless added by someone..

 

[Floridianson]

Or you get lucky and it's a USMC and has lockers.

 

[PeteNora]

[QUOTEYou can modify the sprag air with a TR1 valve on the dash. There are threads on this.][/QUOTE]

Hate to be a beggar. Is there a good link you could lead me to?

 

[fasttruck]

There is a picture on the dashboard that tells you where the gears are. 5th is in the front 4th is in the back. Understand how the light switch works. In "off" no brake lights or turn signals. There are 2 fuel tanks. There is a switch on the dashboard for fuel left or right. This changes the gauge. It does not change the fuel feed. This is accomplished with a valve on the floor between the drivers seat and door near the handbrake. Maker sure it matches the tank that has enough fuel to get where you are going. Run one tank dry and the engine will not start even with 50 gallons of fuel in the other tank. Then you will learn how to prime a NH250 Cummins on the side of the road which is not easy with the OVM that comes with the truck. Download or get the -10 operators manual and read it. The relevant TM is 9-2320-260-10. Also get an organizational maintenance and a lubrication order. Depending on whether the truck has a winch, there are at least 45 grease points on a M817. Neglect them at your own peril.

 

[Floridianson]

[QUOTEYou can modify the sprag air with a TR1 valve on the dash. There are threads on this.]

Hate to be a beggar. Is there a good link you could lead me to?[/QUOTE]

Here is just one I found with a search in the 5 ton mod forum. forum.https://www.steelsoldiers.com/showt...rag-Switch&highlight=sprag+air+cut+off+switch

 

[PeteNora]

Thanks for the help. I am trying to grasp the 4wd operation. I do not like the sprag concept. I either want 4wd or I don't. So a manual way of activating the 4wd makes better sense to me. I would like to find a simple theory of operation write up to read. Not having the truck home makes it harder to visualize whats going on. Can any one tell me what senses the rear slippage and how it knows to lock in the front. To me that sounds like a lot of in and out operation and would be hard on parts. Are the rear and front the same gear ratios?

 

[BKubu]

Then, you want an M939 series truck.

 

[simp5782]

Thanks for the help. I am trying to grasp the 4wd operation. I do not like the sprag concept. I either want 4wd or I don't. So a manual way of activating the 4wd makes better sense to me. I would like to find a simple theory of operation write up to read. Not having the truck home makes it harder to visualize whats going on. Can any one tell me what senses the rear slippage and how it knows to lock in the front. To me that sounds like a lot of in and out operation and would be hard on parts. Are the rear and front the same gear ratios?

They are all 6.44s. A 939 series dump truck will cost you 13 to 25,000$ respectively

 

[PeteNora]

They are all 6.44s. A 939 series dump truck will cost you 13 to 25,000$ respectively

I realize the newer 939 is a lot more and for my intended usage that's more than I care to spend. I bought the 817 and intend to use it. Its a hobby toy on my hunting property. I just figured they had a simple transfer case. Hi-lo, 2wd-4wd. I read about this sprag failing, don't roll back and all these failures can happen. I am in swamps on hills in holes rough this rough that and I just don't need to break it. So I figured I could make in a manual either front working or not. I also read that it spins the front faster on turns by planetary in the TC. I rear the best I could and saw the tear down but did not see any planetary drive. I have had all kinds of trucks and equipment. This is a new game with these trucks and I am just trying to be able so grasp the theories of operation.

 

[Scrounger]

As Wes has posted all the differentials have the same gear ratio.

The intermediate and rear axles do almost all the work, remember there is no interlock.

Under normal nonslip conditions, when the truck is being driven forward the rear drive shaft will turn at X speed. The front wheels will be turning the same speed of the rear tires. The result is the front drive shaft will be turning the same speed of the rear drive shaft, X speed.

Now if the intermediate and rear axles start to slip the rear drive shaft will start to turn faster than the front shaft. Remember the front shaft is normally being turned from the front.

The sprague unit detects the differences in the speed between the front and rear driveshafts and engages. When that happens, the front axle is now driven from the transfer case. The truck is now in six-wheel drive. The sprague unit will stay engaged till there is no load on the drive train and the axles are spinning at the same speed, then it will disengage. This happens automatically and often you won’t even know when it happens.

The same thing happens when in reverse.

Easiest examples of what not to do. Don’t drive up a hill and coast backward until you shift the transmission into reverse. Don’t back up a hill and coast forward till you shift the transmission into first.

 

[PeteNora]

As Wes has posted all the differentials have the same gear ratio.

The intermediate and rear axles do almost all the work, remember there is no interlock.

Under normal nonslip conditions, when the truck is being driven forward the rear drive shaft will turn at X speed. The front wheels will be turning the same speed of the rear tires. The result is the front drive shaft will be turning the same speed of the rear drive shaft, X speed.

Now if the intermediate and rear axles start to slip the rear drive shaft will start to turn faster than the front shaft. Remember the front shaft is normally being turned from the front.

The sprague unit detects the differences in the speed between the front and rear driveshafts and engages. When that happens, the front axle is now driven from the transfer case. The truck is now in six-wheel drive. The sprague unit will stay engaged till there is no load on the drive train and the axles are spinning at the same speed, then it will disengage. This happens automatically and often you won’t even know when it happens.

The same thing happens when in reverse.

Easiest examples of what not to do. Don’t drive up a hill and coast backward until you shift the transmission into reverse. Don’t back up a hill and coast forward till you shift the transmission into first.

Thanks that helps a lot. One of my intentions was to put a 10 foot Boss hydraulic v-xt snow plow on it. With pushing on ice and snow I could see this shifting in and out a lot and I did not want to beat it to death.

 

[simp5782]

You can search some of the older junk yard for some mack toploaders that will have a power divider option on them that will swap into the 5 ton axles you have. You would just need to simply match the ring and pinion ratios to the others on the truck. Certain units share the same ring and pinion and can be interchanged.

I am about to modify my 5 ton rear bogie to the rear drive unit of a mk15a1 with 5.57s. Simply for the parabolic springs and increased load capacity. I have acquired an older mack unit for the front that is a 5.55 ratio.

 

[PeteNora]

As Wes has posted all the differentials have the same gear ratio.

The intermediate and rear axles do almost all the work, remember there is no interlock.

Under normal nonslip conditions, when the truck is being driven forward the rear drive shaft will turn at X speed. The front wheels will be turning the same speed of the rear tires. The result is the front drive shaft will be turning the same speed of the rear drive shaft, X speed.

Now if the intermediate and rear axles start to slip the rear drive shaft will start to turn faster than the front shaft. Remember the front shaft is normally being turned from the front.

The sprague unit detects the differences in the speed between the front and rear driveshafts and engages. When that happens, the front axle is now driven from the transfer case. The truck is now in six-wheel drive. The sprague unit will stay engaged till there is no load on the drive train and the axles are spinning at the same speed, then it will disengage. This happens automatically and often you won’t even know when it happens.

The same thing happens when in reverse.

Easiest examples of what not to do. Don’t drive up a hill and coast backward until you shift the transmission into reverse. Don’t back up a hill and coast forward till you shift the transmission into first.

So the sprague does this mechanically? I thought there was am air actuator that that applied? Sorry I am new to this and sometimes have a thick head.

 

[autoshopteacher]

Go to YouTube "Sprag Clutch" and there are some videos of how this unit works. Older IHC Scouts had a Warn locking front hub system that had "Open" "Lock" and "Auto" settings that used a sprag. In "Auto" you got 4 wheel drive only when the rear wheels spun faster than the front, thereby activating the oblong rollers in the front hub. There was no engine braking in this design however.
In most sprag clutch drive systems as long as the inner race and the outer race turn at the same speed there is no power applied. If the rear turns faster, the driveshaft spins the inner race faster and the oblong rollers rock up into the corresponding groove locking the 2 races together. In the 809 series trucks it is complicated and expensive to make the front axle manually selective.

 

[Scrounger]

So the sprague does this mechanically? I thought there was am air actuator that that applied? Sorry I am new to this and sometimes have a thick head.

The sprague unit works mechanically. The reason for air is to set the direction. When one shifts the transmission into first gear it sets the sprague into forward. When the transmission is placed into reverse the sprague is then in reverse. That is why one can’t coast backward when the unit is in forward. Same thing applies for reverse. Unlike the M35s, the five-ton sprague units are usually quite durable.