Testiment to the 904 Trans

[Flashback]

I have been slowly refurbishing my '66 wagon that spent 25+ yrs in the Texas Desert. One of the items I noticed just moving the car around my larger circular driveway was the transmission was slipping when cold and I could not tell about upshifts as I did not have enough driveway to find out.

Finally, having got the brakes squared away, engine running well I trailered the car to a local empty school parking lot. After just a few 1-2-3 shift (the first couple were pretty choppy) it started shifting perfectly!! I could not believe it?? I wanted to get it warm and through the gears before changing the fluid/filter so I knew what I had.

Now, I'm wondering if I should mess with it at all? It will not get serious drive time...just putt around for ice cream etc. But it sat so long the fluid has to be severly degraded. I'm thinking of just draining pan & changing filter...leave torque converter with it's fluid...so there is a mixture as I believe straight new fluid will take out what remains of the clutches. Anyway that's my plan so far.

But man, between the trusty slant six and the 904 no wonder you couldn't kill these things back in the day...

 

[slantsixdan]

New fluid will not eat up or finish off the clutches, or otherwise kill a transmission that is fundamentally sound. If the transmission is walking-dead, it will die soon with new fluid or old. If it is still basically sound but is being beaten up by varnish and lack of lubricity (old fluid), it will not be basically sound for long. On the other hand, old fluid will contaminate new fluid. For the same reason, we do not change just the oil without changing the filter; four quarts of clean oil plus one quart of dirty oil equals five quarts of dirty oil. Do a complete fluid and filter change, including draining the torque converter, and a proper band and linkage adjustment.

Nice new pans with unwarped rails for the A904 can be had from the dealer under p/n 52118 779AD, and they even include a spiffy magnet to catch metallic shavings. Whether you go with a new pan or not, save yourself a bunch of current and future hassle: Discard the floppy cork or rubber pan gasket that comes with the filter kit. Instead, from the Chrysler dealer get the really nice double-seal, reusable rigid pan gasket P/N 4295 875AC.

Fluid selection: a reputable brand of ATF+4 or Dexron-VI; see here.

 

[JohnRR]

Do a fluid change and cross your fingers . Anyone I know that did a fluid change on a trans that had had the fluid changed in over 100k miles or decades was doing a full trans rebuild soon after .

 

[Flashback]

Do a fluid change and cross your fingers . Anyone I know that did a fluid change on a trans that had had the fluid changed in over 100k miles or decades was doing a full trans rebuild soon after .

Yep RR, that's what I'm concerned about. Agree, overall with SlantDan but was told years ago by a Transmission guy that high mileage trans should not use all new fluid unless they are completely rebuilt. Have always remembered that so in a quandary now. Does not feel right to keep running it with old fluid, but will be really ticked if new fluid takes me backwards! That's why I'm leaning towards an in between route and new filter...

 

[TrailBeast]

/6 Dan was really close on this, but just a detail or two more if I may.

It slips cold because the rubber seals are getting hard and unflexable until they get warm.
New fluid will kill a trans that has burned friction discs because the new detergents will disolve the adhesives that hold the clutch material to the discs.
Some converters have a drain plug so you don't have to pull it to drain it.

If it was my trans, I'd spend for all new fluid and a filter and see what happens.
Worst thing that could happen is that it would need rebuilt, and if it was so close that the new fluid killed it then it needed it REALLY soon anyway. Right?

Just the slipping when cold tells us that it's not long for this life anyway.

 

[Flashback]

Well, completed the fluid change, filter and installed new Chrysler pan gasket as suggested. I drain most of torque converter as well ( yes left some old in)...just hedging. Like stated here, given cold slipping, it likely is not long for this world but perhaps I'll get a season or two out of it before re-doing. It still seem to perform better, 1-2 shift is barely noticeable it's so smooth, 2-3 is a bit more noticeable. Reverse is perfect and when warm, no slipping at all in any gears.

It does have whirring sound ( had before fluid change too) that I can't quite isolate. Sounds like from trans??? Anyway, thanks for the help and we'll see how long she lasts?