I posted this also in the towing section but I figured I would try here also.
Hello all I was loaded and ready to wheel tonight and after moving trailer and motorhome around several time I was ready to leave and the motorhome will not move at all. It is a 1984 gmc p30 chassis motorhome with a 454/ t425 trans. It will try to move in reverse at times when its in manual 2. I checked fluid and it was in safe range but I added a qt. just in case. Any Ideas before I tear it out I had no warnings and the fluid is not burnt. I use this thing every couple weeks so its not like it sits much. I am just looking for some ideas of if it might be valve body related of just a smoked trans.

I'm just interested to find out what a t425 tranny is

its a turbo 400 with bolt on yoke for the output shaft

isn't that just a 2wd th400? I've had a slip yolk 400 and bolt on yolk 400 never heard it called anything other than a 400.

sorry for the side track.

I could be wrong but thats what I have always been told they where. Internally it is a 400 from what I understand.

so it just wont move?
If so sounds like TC drainback, especially if it has been sitting for a while
Let it run a while and see if that wont help

its a turbo 400 with bolt on yoke for the output shaft


Where the hell did you get that?:shaking:

The 425 was/is a front wheel drive auto for the caddys.



He is probably talking about a 475, which is a 400 with straight cut planitaries. Basically a 400. I would venture to guess that the trans need to be rebuilt

The 425 was/is a front wheel drive auto for the caddys.



He is probably talking about a 475, which is a 400 with straight cut planitaries. Basically a 400. I would venture to guess that the trans need to be rebuilt

That is my understanding also.... plus the TH475 should have a deeper 1st gear than a regular TH400.

425 was the front wheel drive oldsmobile and caddy yes

the 400 with the bolted on yoke is just another 400

the bolted on yoke style is extremely dirt common see them all the time in the 80s trucks mainly

same 32 spline shaft as a slip one but that the hole in the end of the shaft is drilled and tapped for the yoke bolt

it is a 400

the 475 in the light duty(1 ton and under) trucks wasnt until 87 and was known as the 3L80HD
common tag code on these was LJA, i own one out of a 87 c-10 and we have another one in the garage out of an 88

they are 34.5" long overall in which is longer than a regular short tail 2wd 400 and they are 32 spline but have the normal electric kickdown wire and everything else a 400 has

and they have a large raised HD on the bellhousing

if no HD on it then its not a 3L80HD and is just a regular 400(3L80)


the th400 name was done away with by 87 and changed to 3L80

just like the 700r4 was done away with and renamed 4L60 and 4L60E





good luck

Thanks for the Info I have been told for many years that these where 425 and that the 475 was the frt drive version. I am not a trany man so I do not know if this it true or not.
The condition suddenly occured it was working fine 2 weeks ago and the on friday It was fine to run to town get propane and gas. I hooked up to trailer and backed it up to shop loaded tools and the pulled forward and loaded bronco and then again backed up out of way and left it idling while everything else was loaded. After about 1/2 hr of idling we where ready to pull out and at this time put it into drive and nothing. try all gear positions and still nothing. then noticed then moving shifter through gears from 1 to 2it sometimes will engage reverse and actually back up. I am thinking possible valve body problem but I am not a auto trans man so figured i would see what advice I could find on this.
Thanks again

the 475 in the light duty(1 ton and under) trucks wasnt until 87 and was known as the 3L80HD
common tag code on these was LJA, i own one out of a 87 c-10 and we have another one in the garage out of an 88

they are 34.5" long overall in which is longer than a regular short tail 2wd 400 and they are 32 spline but have the normal electric kickdown wire and everything else a 400 has

and they have a large raised HD on the bellhousing

if no HD on it then its not a 3L80HD and is just a regular 400(3L80)


Your description and application is right for a TH375 (check the spline again ;) )

The turbo 375 is a 400 with a 27 spline output.

The turbo 375 is a 400 with a 27 spline output.

Agreed. :)

Ok I got caught up in the 375/425/475 discussion at first, then I re read and come across this...

I checked fluid and it was in safe range but I added a qt. just in case.

A QT just in case???

I'm betting you now have too much ATF... not that, that would cause your symptoms, but ...

Im thinking either #1 needs a rebuild or something obvious linkage loose or similar

I don't figure fluid level has anything to do with it. It was in safe range but on low side. I added fluid as a last ditch waste to get it to move. I am hoping someone has ran into this and found a stuck valve in the valve body or something like that. The trans gave me no signs of slippage or anything. My last Motorhome lost all but 1st and reverse and I was ready to pull and rebuild but was advised to inspect governor gear for apple coring and that was the fix. It saved me alot of money and that trans is still going strong about 8 years later. I keep hoping for some quick fix on this one to.

Well I figured I would post what I found on this one I talked to local trans shop and they said check the linkage inside trans at valve body. It could either be the nut came loose or the nail that holds the shaft inplace has came out. Well I found the nail to be gone I replaced it and all is well once again.