Auto Trans Overdrive Issue?

[chemist]

So, my 2003 AWD Astro has been having an issue on the highway and downhill runs for some time. Simply, you put it in Overdrive, it runs through the first three gears fine. But on the last gear, it never goes in... At 60+ it'll make a rattling or vibrating noise when you take your foot off the gas pedal, until it slows to about 50 or 55 at which point the noise becomes less apparant. No other mechanical issues noted (other than something where heater won't switch from top vents, that I don't really care about right now...) Oh, and the fluid doesn't seem burnt; it's not gritty either--both good signs I hope... I did the drain and fill about 10K miles ago, new filter and cleaned magnet when the issue started, and that did not improve.

It's been freaking me out a little... What could this mean?

Am I looking at an auto trans rebuild, and if so, am I in better shape to try to fix now, or should I wait until the sucker doesn't run--hopefully after my wife gets her RN...

If I flush it, is there a danger that it will sieze up entirely like my Buick did?

Could this be something completely stupid and not tranny?

[zook94]

The vibration noise is probably the viscous coupling in the transfer case. I would check for a bad shift solenoid, or broken wires inside the transmission. Replacement solenoids are not that expensive, but I can't tell you the procedure for testing them on an '03.

[LiftedAWDAstro]

The vibration noise is probably the viscous coupling in the transfer case.

His being a 2003, there is no viscous coupling in the transfer case.

[zook94]

See? I told you I didn't know, and I wasn't kidding.

[ihatemybike]

Using the SC2, I verified that Grumpy is not shifting into OD this morning. I had thought so for a while, but being able to see the RPMs really confirmed it. New trans filter and gasket is scheduled for delivery today. Hopefully new fluid and Seafoam will will clear it up. I'll also check for loose wires. 17.4 mpg sucks.

[COMP]

updates ???

[chemist]

Sorry to bump such an old post. But I do have something of an update on this, and would love some input.

The transmission issue is no longer as pronounced--not as much vibration at higher speeds, although still present. It's actually worse if you drive around in 3rd... I'm thinking it is going into OD based on downhill speeds with my foot off the gas.

On another note I've noticed, on rare occasion, that it doesn't just drop into first and take off from a dead stop--there can be something of a pause between when I push the gas and when the van moves. It's not related to the engine or throttle as far as I can tell. But it's yet to simply take a dump on me and not take off though...weird.