Allison Leak

[Thumper Pilot]

Hey all - I had my truck parked nose-down in my driveway (which is probably ~35 degree angle) since this past Sunday. Haven't driven it all week. I get home this evening and see a spot on the driveway about 2" diameter. Crawl under and it is tranny fluid and it appears to be coming from the center bottom, in front of the pan, where there looks like some kind of tin plug?

I assume that's the bell housing 'weep hole'?

Anyone have any idea why it would just start leaking after not having driven it for 5 days?

Could it possibly be because I had the truck parked nose-down that all the fluid is forward and gravity is pulling it through this weep hole? I've parked it this way before but probably not for 5 straight days without driving it.

I've read other threads about bolts coming loose and pump seals, but why would it take 5 days of parking for the leak to show if it were those problems?

[More Power]

It's common for the Allison torque converter to bleed down while sitting for a few days. This causes the pan to overfill, which can allow some ATF to weep out the dipstick pipe grommet. There were some minor mods GM offered for the bleed-down problem (and a TSB), but it's not usually worth the trouble as long as you know that's the cause. There are quite a few threads here in this forum on this subject.

Some of the fixes: 1- Vented dipstick. 2- Mod in the valve body. 3- Run the trans up to 1 quart low.

If it leaks at the bellhousing area while you're driving, then that's another story...

Jim

[Thumper Pilot]

Thanks for the reply Jim. I hadn't heard or read of a bleed-down problem before. Maybe that's what it is.

I did call a local Allison-approved dealer and they said there's no fluid behind that plug in the center-bottom, it's just an access port but it could have fluid behind it if some came out of an overflow port due to being overfull or from overheating the fluid. I haven't pulled anything so I don't see it being due to overheating the fluid.

It has only done this after being parked for several days without being driven, so I don't see it as an overfull situation either.

I just got back from having it parked, nose-up this time, in my driveway for 6 days sitting idle. There were no signs of any leaking at all after the 6 days. I drove it about 30 miles or so, parked it again nose-high, and the next morning there's a new fluid stain on my driveway - a swath about 2" wide tapering to 1" wide and about 8" long.

I wonder if it would do this if I parked it on flat ground for several days? I have no way to test this though, not a flat spot for me to park it at my place.

[SoTxPollock]

MP is right about the converter bleed back. If you could roll your truck to level ground without starting it up and pull the dip stick, you'll find it showing way too much fluid in the pan, the problem is that as soon as you start the engine it pumps the fluid out of the pan back to the converter so you can't tell its too full. I've experienced that with my old antiques that have Automatic transmissions. Let them sit for a month and you got fluid coming out, start them up every couple of weeks, no fluid leaks. Frustrating, because I hate leaks on my garage floor, But the work to reseal everything makes it not seem so bad, a little oil dry and its ok again, but still frustrating. Remember once the process starts, air allowed into the system, the cooler being mounted much higher than the transmission also just bleeds back to the lowest point too, that being the pan and your driveway.