M923 A2 Trans slippage?

[Kanakaman]

My neighbors and I purchased from auction a M923 A2 several years ago and we transformed it into a community fire truck with a poly 1600 gallon watertank on board. The truck sits a lot. I maybe put 10 miles a year on it. It snows where I live, so in the winter I don't drive it at all, and in the spring when I start it up and put it in gear, I have to mash the accellerator almost to the floor to get it to move. Until recently, once it was warmed up it seemed to run fine, but lately it seems like the trans may be slipping. I drive it down my road at 20 mph and when I step on the accellerator pretty much nothing happens- the rpms don't increase (much) and it moves forward but not like you'd expect when you step on the throttle- rpms stay the same, and the truck slowly speeds up... it just feels like (perhaps) the Allison trans is slipping. The truck supposedly only has 6000 miles on it and is in great condition. Engine bay looks very clean, all the fluids are clean..... do any of you guys have any ideas?

 

[fuzzytoaster]

My neighbors and I purchased from auction a M923 A2 several years ago and we transformed it into a community fire truck with a poly 1600 gallon watertank on board. The truck sits a lot. I maybe put 10 miles a year on it. It snows where I live, so in the winter I don't drive it at all and in the spring when I start it up and put it in gear..

If the transmission was slipping you'd be able to rev the throttle/rpms and not get any acceleration out of it if not loose some. When moving and the rpms staying the same we are looking at a different problem so lets start with the simple stuff first. I'd recommend start with your fuel filter under the driver front fender (big thing) and open it up to take a look. Since the truck sits a lot, it's up north, and stuff grows in diesel with moisture present it can easily gunk up with algae. It may even sit on the bottom of the tank and not get stirred up until you take it for a drive. There is a drain plug on the bottom of the fuel tank too and it wouldn't hurt to drain a few gallons to get an idea if the fuel is contaminated. Also, there's a small filter on the driver side rear of the engine that would need to be changed too if the system is contaminated.

I have to mash the accelerator almost to the floor to get it to move. Until recently, once it was warmed up it seemed to run fine, but lately it seems like the trans may be slipping. I drive it down my road at 20 mph and when I step on the accellerator pretty much nothing happens- the rpms don't increase (much) and it moves forward but not like you'd expect when you step on the throttle- rpms stay the same, and the truck slowly speeds up... it just feels like (perhaps) the Allison trans is slipping. The truck supposedly only has 6000 miles on it and is in great condition. Engine bay looks very clean, all the fluids are clean..... do any of you guys have any ideas?

When doing this do you feel resistance on the foot pedal? What are the rpm's when doing this? Is the truck loaded? What color is the exhaust when foot is to the floor? What I'm getting at is two things:

1) The 8.3 in your truck doesn't build boost aka "power" until around 1000-1200 rpm. If it's not getting there something is restricting the fuel or air.
2) There was a MWO for the A2 trucks due to throttle linkage issue and thus seemingly poor acceleration. Inspect!

Judging by what you've said so far it seems more like a fuel delivery issue than transmission issue. It my idle fine but when the fuel is demanded by the IP it falls on its face.

 

[Kanakaman]

Interesting... thanks for the reply....I do feel resistance on pushing the throttle down. The engine doesn't rev even as I push the throttle down- the rpms stay pretty much the same. I'll take the truck out for a drive to check the rpms tomorrow. The behavior is both loaded and not. I'll also check the exhaust when I do this.... this thing is pretty smoky even at idle. Now.... come to think of it.... we bought the truck at sea level and we are operating it at 6000 feet... I guess it is possible it is running too rich? Is there any 6x6 mechanic in the Reno, NV area that would be willing to come up and take a look at this thing up near Lake Tahoe? ... I gotta tell you this story: I called the local truck repair service, and they sent a guy out.... we couldn't find the trans dip stick (and this guy works on big diesel trucks) and when we spent over 10 minutes looking for the dipstick, this guy goes and tells me that he heard that the military sometimes didn't put dipsticks on their trucks (and I assume that would also mean they never had to change or top off the trans oil?) and they would run them until the trans went bad..... really? That doesn't sound like a smart way to run a fleet of millions of trucks.... so the guy spends about 10 minutes looking at the truck and tells me I most likely will have to pay him to remove and rebuild the trans.... then he said he had to go do more research, and he takes off and leaves.... and I am thinking "that was pretty nice of him to just give me his opinion"... but then 10 minutes later his dispatcher calls me to tell me that the valuable information I got from the mechanic would only cost me $250.... because he liked me.. he wasn't going to charge me the standard $350 they charge to show up. .....duh... I'm not so sure I want these guys working on this truck... it could cost me $10,000. Now THAT's A good deal, eh?

 

[fuzzytoaster]

Interesting... thanks for the reply....I do feel resistance on pushing the throttle down. The engine doesn't rev even as I push the throttle down- the rpms stay pretty much the same. I'll take the truck out for a drive to check the rpms tomorrow. The behavior is both loaded and not. I'll also check the exhaust when I do this.... this thing is pretty smoky even at idle. Now.... come to think of it.... we bought the truck at sea level and we are operating it at 6000 feet... I guess it is possible it is running too rich? Is there any 6x6 mechanic in the Reno, NV area that would be willing to come up and take a look at this thing up near Lake Tahoe? ... I gotta tell you this story: I called the local truck repair service, and they sent a guy out.... we couldn't find the trans dip stick (and this guy works on big diesel trucks) and when we spent over 10 minutes looking for the dipstick, this guy goes and tells me that he heard that the military sometimes didn't put dipsticks on their trucks (and I assume that would also mean they never had to change or top off the trans oil?) and they would run them until the trans went bad..... really? That doesn't sound like a smart way to run a fleet of millions of trucks.... so the guy spends about 10 minutes looking at the truck and tells me I most likely will have to pay him to remove and rebuild the trans.... then he said he had to go do more research, and he takes off and leaves.... and I am thinking "that was pretty nice of him to just give me his opinion"... but then 10 minutes later his dispatcher calls me to tell me that the valuable information I got from the mechanic would only cost me $250.... because he liked me.. he wasn't going to charge me the standard $350 they charge to show up. .....duh... I'm not so sure I want these guys working on this truck... it could cost me $10,000. Now THAT's A good deal, eh?

I don't say this often but he's an idiot. Don't let him near anything with oil in it: engine, transmission, or even a bottle of olive oil. The Army has dipsticks on nearly everything and it's on the passenger side near the starter (both engine and transmission). Altitude won't be hindering it that much and the TM -10 should cover any power loss per gain in altitude. Luckily the 8.3 has a turbo, unlike it's predecessor the cummins 250, so it's not likely to be starving for air unless the filter/intake is restricted. The fact that you're not getting any throttle response confirms it's more likely fuel or linkage related. It may be wise to pull the intake boot off the front of the turbo to visually inspect the turbo and make sure it spins freely. In the off chance the turbo is seized (not saying it is) then it's not breathing and will be incredibly anemic to the point it wont move out of its own way. Let us know what you find.

 

[Russ Knight]

IOW's, the "mechanic" was the dipstick...

 

[wheelspinner]

First-Fuzzy is 110% correct! Do not let that whole shop near your truck.

Second-a lot of sitting and moisture can get the brakes dragging. It will feel exactly like you described. Even if the wheels are turning they could be dragging. Or, a spring brake can is failed and a brake is dragging.

Both will take some work to diagnose but take a look.

Good luck

 

[BKubu]

The dip stick is under the hood, back by the firewall on an M939A2. On the earlier trucks, it was accessed in the cab through a door in the middle of the cab floor.

I concur on checking/replacing the fuel filters. One additional curve, however (that is in addition to what FUZZY said): not all of the A2s have the large fuel filter under the fender. The trucks originally only had a spin on filter on the side of the engine. I believe they later added a small inline filter before the spin on filter. I have had a bunch of A2s and only one of mine had the "upgraded" large fuel filter under the fender. Interestingly, the large under fender fuel filter is a throw back to the M939A0 and M939A1 series, as well as the earlier M809 series.