Is it safe to coast backward in forward gear m813?

[monstertruck287]

I just bought my first MV its 1977 m813. I've read mention of not to coast backward while in forward gear. Will it break the tranfer case? I tried to search but couldn't find a difinitive answer. I also read TM-10 and couldn't find anything about it in there either, but could have missed it. Is this true or am I confused?

 

[Shoprat 19th Engr]

Because They a sprag that engages the front axle not like a Duece that is air operated to engage the front axle

 

[wreckerman893]

Rolling backward in any gear except reverse is asking for trouble. If you are rolling backward in a forward gear and your foot slips off the clutch the next sound you may hear is metal grinding and have a rapid deceleration.

 

[monstertruck287]

So if the sprang gets turned backward will it hurt it? Or the transfer case? I understand the foot slipping off the clutch can definately snap parts, and should be avoided.

 

[monstertruck287]

For example if you were trying to crawl up out of a creek bed onto a bank, but didn't make it and only got the front end over the top. Would it physically hurt any thing to push the clutch in and coast back down to give it another shot? Just want to make sure I'm not gona blow anything out if the situation ocurrs.

 

[swbradley1]

For example if you were trying to crawl up out of a creek bed onto a bank, but didn't make it and only got the front end over the top. Would it physically hurt any thing to push the clutch in and coast back down to give it another shot? Just want to make sure I'm not gona blow anything out if the situation ocurrs.

You can do it....once.

 

[doghead]

The sprag is an internal component of the transfer case.

You will damage it if you roll in a direction that your not in gear for.

 

[Floridianson]

Or it will lock up and the front end will hopp like a bunny.

 

[dmetalmiki]

The short (and only) answer is you WILL do damage to the sprag clucth mechanism. In our club We have a few deuces with no front wheel drive deu to (having had ) this happen , And also probably when they were used setting off in second or third from reverse. Another BAD practice.

 

[jcappeljr]

Avoid it..

 

[monstertruck287]

Ok thanks guys. I remember reading something but wasn't sure if it was true. Now I know, I'm just trying to learn what not to do the easy way.

 

[73m819]

You will forget ONLY ONCE, after the truck jumps when the sprag finally unwinds, the first thing you will do is pullover and LOOK under it, thinking that there will be a trail of parts, after the BIG bang, you WILL think about the rolling in the wrong direction EACH time your are in the truck.

 

[ThatXJGuy]

If I understand how it works right, rolling backwards in a forward gear (or rolling forwards in reverse) will lock up the sprag because of the gearing difference between the front and rear outputs of the transfercase, but putting it in the right gear and continuing to roll would unlock the sprag and it would continue to over-run like it is supposed to (8% as I heard). I actually learned exactly what it does when that happens in an interesting way. I pulled off the air hoses to the transfercase because one of them was leaking, when I put the new ones back on I swore I had it right but I was wrong. I pulled outside the shop to test the front wheel drive on the snow (which would only work in forward before) and I didn't have any front wheel drive either way... Then I drove it around the building and every about 15 feet when the force finally became enough, it dragged the rear axles a few inches. Luckily I had almost no traction, no weight because of the lack of a bed, and crappy tires or the dragging would have been more dramatic but it just dragged the rear tires. Same thing in reverse... Rolling backwards in a forward gear or forwards in reverse just binds up the transfercase like a normal truck in 4wd going around a tight corner, but it happens on the straight because of the gearing difference, and because the bind is bigger, the transfercase is bigger, the truck is bigger and heavier, the force is bigger and so is the resulting reaction which is hopefully a slide and not a bang but that would depend on your traction, luckily I was on snow with crap tires and no weight. What I did was cause the transfercase sprag to be in the reverse position when I was in a forward gear and the forward position anytime I was in reverse.

 

[MyothersanM1]

The short (and only) answer is you WILL do damage to the sprag clucth mechanism. In our club We have a few deuces with no front wheel drive deu to (having had ) this happen , And also probably when they were used setting off in second or third from reverse. Another BAD practice.

Once shift lever is removed from the reverse position the poppet valve will activate the declutching piston to the forward movement position. The "PFFFT" sound should alert you to this. One does not have to shift to first gear before forward movement to avoid sprag issues. Neutral or any forward gear is safe.

Also, if you remove air completing from the declutching (sprag) system, the system will be rendered neutrel and forward or reverse movement will not be hindered.

 

[monstertruck287]

Once shift lever is removed from the reverse position the poppet valve will activate the declutching piston to the forward movement position. The "PFFFT" sound should alert you to this. One does not have to shift to first gear before forward movement to avoid sprag issues. Neutral or any forward gear is safe.

Also, if you remove air completing from the declutching (sprag) system, the system will be rendered neutrel and forward or reverse movement will not be hindered.

My truck only makes this sound shifting into reverse and again back to neutral, no sound at all into or out of first. Sometimes it just continues to hiss the whole time while in reverse and i have to shift in and out of gear to make it stop. Front axle only engages in reverse, not at all in first. I looked through the TM and am gona take the airvalves apart and clean and inspect when I have time.

 

[Tow4]

My truck only makes this sound shifting into reverse and again back to neutral, no sound at all into or out of first. Sometimes it just continues to hiss the whole time while in reverse and i have to shift in and out of gear to make it stop. Front axle only engages in reverse, not at all in first. I looked through the TM and am gona take the airvalves apart and clean and inspect when I have time.

Sometimes the transfer case air cylinder leaks. I had this problem. The seals are easy to change.

http://www.steelsoldiers.com/showth...Transfer-Case-Air-Cylinder-Rebuild&highlight=

 

[dezert ratt]

With air to the sprag, neutral and reverse are the only positions that change the sprag engagement therefore you will only hear the air when going between neutral and reverse and not any forward gears. So neutral is still going to put the sprag in forward position and you don't want to roll backwards In neutral either.

 

[Sephirothq]

I never knew about the don't roll backwards in neutral? So you can only roll backwards in reverse?

 

[MyothersanM1]

Sometimes the transfer case air cylinder leaks. I had this problem. The seals are easy to change.

http://www.steelsoldiers.com/showth...Transfer-Case-Air-Cylinder-Rebuild&highlight=

I rebuilt mine with a kit from Memphis Equipment. Also, do yourself a favor and replace the plumbing from the poppet valve to the cylinder. Leaks will occur there also. I replaced my PITA copper lines with DOT reinforced nylon and push-to-connect fittings. No more leaks.

So you can only roll backwards in reverse?

Yes. OR, you can roll any direction if you disable the air supply to the poppet valve...renders the sprag to neutral. The declutching cylinder has two springs which center the piston when no air is applied.