Tranny diagnosis question

[Eric von Kleist]

I'm wondering if I may need to replace my transmission, but I am not sure how to diagnose the situation -- or even if there's a problem. I have an '85 Thomas-built bus on an IH S-1800 chassis. The engine is a 9.0 liter V-8 that was rebuilt not long ago, and that has a dyno certification sticker on it for 180HP. The tranny is an AT-540 (according to the line ticket), but I'm not sure that is the original tranny, so it possibly could be an AT-545. It is a 4 speed tranny, for sure. The rear end is 6.50:1, so I know it it cannot possibly go above 50 MPH or so in that configuration, but speed is not the issue.

The bus won't maintain speed going uphill. I can get up to 45 or 50 going down a hill or on a stretch of straightaway, but when I get to a hill, it bogs down and bogs down and bogs down, and by the time I'm at the top, it has slowed to 20 or 25 MPH. I would think that with the 6.50:1 rear end, that it would be able to slog uphill without even noticing the grade. Eventually I would like to move to a 4.11 or 4.86 rear end, but I'm afraid to even consider that at this point: wouldn't a lower rear end ratio make the bus tend to lose speed even worse going uphill?

The hills I am talking about are moderate, maybe a hundred feet in elevation over a half a mile...4 or 5 percent grade, probably.

Does the tranny sound like it's going/gone, or am I expecting too much from 180 horses?

Oh, yeah, it appears that the tranny is leaking, as well, but I haven't determined the source yet.

[fmtaylor]

this probably won't help you alot, but my 7.0 liter (429) went over the rockies and never slowed below 30 MPH. I don't know what is in it as far as tranny or rear end but i know it will do 70+ going down hill and will cruise at 65 without a problem. for most hills it slowed to about 40-45, but that was to be expected as I was using it as a moving van. The places where it went really slow had those "slow truck" lanes. Considering the size of your engine (around a 550 ?) you should not have the problems you describe unless you are seriously overloaded or something else is wrong. But you already guessed that.

[Eric von Kleist]

Hmmmm....

I don't know diddly about automatic transmissions, except that you have to keep them full of pink stuff.

My rear end gearing limits my top end to a theoretical 50 MPH (if the final ratio on the tranny is 1:1), and 50 MPH is the maximum it has done on a flattish piece of ground.

My thinking was that the high ratio would let it pull uphill all day long without slowing much, given enough power. Maybe it just doesn't have enough power.

If it's max speed is 50MPH in high gear, and the engine isn't strong enough to push it uphill in high gear, and then it drops down to 3rd gear, I guess the rear end gearing is going to mean it has to go slow in lower gears, too????????

I doubt it's overloaded. I took all but two of the seats out. The only substantial weight I added to it is about 7 sheets of 3/4" plywood in the floor, plus some 5/8" interior wall panels (about another 2 sheets of 5/8" ply, total).

Plus the accelerator pedal is incredibly hard to push. I have to brace my back against the seat and push down hard with my leg to get it floored.

[Eric von Kleist]

Thanks, but I don't think it's a case of overheating. My bus has a transmission cooler mounted on it already. I'm thinking it's more of a gearing issue than anything else.

I may have found the source of my leak. One of the bolts on the pan was loose. I really should tigten it.

I hope that's what the source was...

[trentet]

I hope that tightening works for you, but I have found that once a leak starts around a gasket just tightening rarely stops it. I end up having to remove the ofenting gasket and replacing it, using gasket sealer for good measure before I am able to get a good seal again. It can be quite an undertaking and often proves frustrating. Hope i didn't jinx things.