904 trans: Here is the SYMPTOM, what does it mean?

[Kern Dog]

My Brother In Law's 72 Duster has a 360 and a 904. Both are junkyard fresh units, cleaned up and regasketed. The trans has a new filter and ATF as of 8 years ago but has maybe ONE mile on it since the car has sat here WAITING for him to get OFF OF HIS *** to work on it.
He claims to want to bring it to his place in Phoenix and maybe use the car as a daily driver for a few weeks a year when the sun doesn't blister a man in 15 minutes.
The trans acts strange to me though. It sometimes acts as if it is NOT in gear even when it clearly is. It will slip and freewheel, then kick in and move the car. While engaged, it feels great and moves the car out quickly. The fluid level is always fine when this happens, I have tried adjusting the KD linkage, the shifter linkage, moved the shifter back to 1st and up to reverse when it free-wheels. I am not great with diagnosing electrical or transmission problems so I can only suspect a front pump or valve body issue. The trans was from my stash and I don't know where I got it. I don't know the year model except to say it is a traditional slip yoke style. Again, it sometimes slips but once engaged, it responds normally.
I want to make the car as reliable as possible for the unlikely event that he actually takes the car back home. This project started Feb 15 2007 and has been taking up space here since then. A slipping trans would surely result in him not wanting to drive it OR the possibility of a failure when he drives it and..... A big bill to PAY SOMEONE ELSE to replace the transmission since he has only moderate skills and a rookie tool selection.
So....What usually causes this type of slip?

 

[miller]

Look through the trouble-shooting guide in the service manual, under the 904 trans.
That will give you all the possible causes, and what needs to be done.
Unfortunately, like anything else, if it sits for awhile, usually needs some attention.

 

[493 Mike]

This sounds to me just like the issue in another post a couple days ago. I believe it is the rear clutch slipping/not engaging causing the trouble. Those rubber seals get hard with age and allow oil to bypass the apply piston.
Mike

 

[Lefty71]

This may sound stupid, but if I were you, I'd just drive it around myself, and see if its just a matter of sticky sealing rings from sitting, or if the problem manifests itself by getting considerably worse. A few heat-cool cycles bringing it all the way up to temp for a while may do it some good, especially going through all the gears and reverse, or they may bring the problem to a head. I doubt you are damaging any hard parts, and if it has to come back apart, you're gonna change all the wear items anyway. My two cents.

 

[Darter6]

Yeah, what Lefty said, My first thought was servo sealing rings/O rings.

 

[towtruckAl]

try letting it idle in neutral for a couple of minutes after start up to build up pressure . works for me with pu truck . I dont drive it often .

 

[Greg Ducato]

It is probably obvious but bears repeating that these transmissions are checked in neutral with the engine running, if it is checked in park it will be low on fluid, by as much as three qts. Slipping out of gear denotes pressure loss, this can be low fluid, loose filter or incorrect filter, sticking converter relief valve or pressure regulator valve.

 

[YY1]

does it only do it when cold?

the converter can leak down

not sure how to fix that other than new converter

 

[Cranky]

Had a car once with only 40k miles and the converter would drain back in two days and then the fluid would start leaking out of the shifter seal. I fixed the seal and just let it idle in neutral for a minute or two before taking off. Wasn't about to replace the converter or do a tear down....

 

[Dipstick]

This sounds to me just like the issue in another post a couple days ago. I believe it is the rear clutch slipping/not engaging causing the trouble. Those rubber seals get hard with age and allow oil to bypass the apply piston.
Mike

BINGO !!!