M915 - what a ride!

[NMC_EXP]

The M915A1 has an Allison transmission, but the M915, M916, M919 and M920 all had the Cat transmission you are talking about. There are several threads on that transmission here on Steel Soldiers and they all mention that the controller (picture) is the root of a lot of problems with the pneumatic shift system.

At the time, I had design control responsibility for o-rings. The pneumatic controller used a "floating piston" design sealed with o-rings to open and close the various air pathways.

With the "floating piston" there is an aluminum piston with an o-ring (acting as a piston ring) inside a cylinder. The "floating" designation comes from the fact that by design the O.D. of the o-ring is smaller than the I.D. of the cylinder, i.e. there is a gap between the o-ring and cylinder. Therefore the design has a certain amount of leakage past the o-ring.

However, too much leakage and the piston will not move and it will not shift. If the piston o-ring is too tight, then it will stick in the cylinder.

I ended up measuring a big pile of o-rings and sorted out pieces at nominal dimensions. These were used in some test builds and there was little or no improvement in failure rate.

I concluded that the problems were due to: design (wrong o-ring to cylinder clearance), or out of tolerance pistons and cylinders. I recommended that they measure all of the metal parts in the valves and verify they met the print.

Never heard back from them after that. Either I was right, or they gave up.

Regards

Jim

 

[wreckerman893]

One of the worst faults of the CAT tranny was that if the air pressure dropped below 80 psi it would shift into NEUTRAL.....bad news going down hill with a heavy load on......no engine braking capability.
There was a Modification Work Order that added an extra air tank to these vehicles but I don't know if every vehicle in the inventory had it done.
I liked the CAT transmission as long as the Army owned it......I am glad my M915 has the Allison since very few civilian mechanics have ever seen the CAT.

 

[NMC_EXP]

One of the worst faults of the CAT tranny was that if the air pressure dropped below 80 psi it would shift into NEUTRAL.....bad news going down hill with a heavy load on......no engine braking capability.
There was a Modification Work Order that added an extra air tank to these vehicles but I don't know if every vehicle in the inventory had it done.
I liked the CAT transmission as long as the Army owned it......I am glad my M915 has the Allison since very few civilian mechanics have ever seen the CAT.

Might be a good thing to retrofit with an aftermarket electric air pump. Pressure switch it to kick in at 90 or 100 PSI.

Regards

Jim

 

[ZRT556]

How are you going to handle registration? Will TX allow you to register a tractor trailer as a non commercial vehicle?

 

[NDT]

How are you going to handle registration? Will TX allow you to register a tractor trailer as a non commercial vehicle?

Heck no. USDOT registration, TXDOT registration, medical card, random drug testing consortium, $750,000 liability. All for a hobby vehicle. Not puckering up when you see a state trooper? Priceless.

 

[ZRT556]

Heck no. USDOT registration, TXDOT registration, medical card, random drug testing consortium, $750,000 liability. All for a hobby vehicle. Not puckering up when you see a state trooper? Priceless.

Check up on the Federal Motor Carrier Safety Regulations. If you label the truck as "Not For Hire" you should be able to legally avoid some of the above hassle.

I'm not up on that portion of the law, as all my farm tractor trailers and straight trucks get registered as "Farm" or "Commercial."

I do feel your pain.

 

[cundupa]

Very few CAT mechanics have ever seen or know about that trans, at least around here.