Well now, I call this an excellent Flash of Insight without any Wild
Speculation.
Nice work Rick, and thanks for this keeper.
Page 7-11 of the shop manual gives several reasons for a final drive -
transmission leak which tends to tell me that a vent hole between these two
seals is not there.
Page 3C-2 gives a good picture of the seals in the bearing carrier.
Looking at the pictures, It appears a vent from between these seals to the
outside world would be difficult.
Looks like that bearing carrier has gear oil on one side and transmission
fluid on the other.
A transmission vent fix has already been posted and seems like a good idea
to reduce the fire hazard.
If the final drive were vented into the same area as the transmission vent
then at least internal air pressure in the final drive and transmission
would be equal which might help.
If nothing else comes of this, it points out a weak design area and
strengthens the case for additional transmission and final drive cooling
along with use of superior lubricants.
Don Miller
75 Glennbrook
Shenandoah Valley of Virginia
> WARNING, FOLLOWING CONTAINS WILD SPECULATION, OR MAYBE A FLASH OF
INSIGHT
>
> When I read the above message, I went running for my shop manuals. You
>see, for over 20 years I was in the foreign car repair business, and this
>brought back some things I should have remembered before. A few foreign
cars
>(Subaru comes to mind) used a fore-aft engine driving an automatic FWD
>transaxle. Some (eg: Saab) even use a chain drive like the GMC. What I
was
>concerned with was the connection from the transmission to the final drive,
>especially when the two use different kinds of fluid. Some cars (again, I
>think Subaru) just put two oil seals back-to-back between tranny and final
>drive to keep their respective fluids separated.
>
> The trouble with this design is that if one of the seals fails, the
>resulting leak will just pass through the other seal, as the oil comes to
it
>backwards. It was very common on 1980s vintage Subarus to find the final
>drive overfull and leaking ATF, which caused accelerated wear of the final
>drive gears and bearings. Worse yet, occasionally final drive gear oil
would
>find its way into the transmission. Automatic transmissions don't tolerate
>Extreme-Pressure gear oil very well. The stuff soaks into clutch discs,
>causing them to slip and eventually burn out. Even one-way roller and
sprag
>cluthes may be made to slip by enough EP oil. The presence of gear oil was
>easily detected in the transmission by its strong sulfurous odor on the
>dipstick. Of course once a transmission was burned-out from slippage, the
>strong burnt-fluid/clutch odor might mask the gear oil smell, so no one
would
>know the true cause...... And of course if someone used synthetic gear
oill,
>it may not smell as strongly as dino-base gear oil. Hmmmm....????
>
> Now some later automatic transaxles, and some other makes, solved this
>problem by separating the two seals (ATF and gear oil) by just a millimeter
or
>two, and drilling a drain hole to the outside. Now any leakage would go to
>the ground, where it might be noticed, instead into the wrong gearcase
where
>it would wreak havoc. Looking at the pictures in my X-7525A manual
(Section
>3-C, Fig. 1, A), the GMC's seals are back-to-back in a housing, and no weep
>hole is visible..... Can any transmission people confirm or deny this???
The
>pinion bearing/seal housing is part of the final drive assembly.
>
> My speculation is that this may explain some of the sudden transmission
>failures we see. Given the well-known problems with clogged final drive
>vents, gear oil could easily be forced into the transmission, ruining one
or
>the other unit. (The transmission by contamination/slippage, the final
drive
>by oil starvation.) Don't want to start another myth, but this idea really
>has me wondering....
>
> Given the placement of the trans/final drive seals in the pinion
bearing
>holder, I'm not sure if a permanent fix is possible. I'd only suggest that
>everyone be alert for any gear oil smell in their ATF, check frequently for
>any sign of gain or loss of final drive oil, change ATF often, and replace
>those seals carefully if you ever separate the transmission and final
drive.
>
> My .02. Comments anyone? Perhaps Jim Bounds can share some experience
on
>this??
>
>Rick Staples
>'75 Eleganza
>Louisville, CO
Speculation.
Nice work Rick, and thanks for this keeper.
Page 7-11 of the shop manual gives several reasons for a final drive -
transmission leak which tends to tell me that a vent hole between these two
seals is not there.
Page 3C-2 gives a good picture of the seals in the bearing carrier.
Looking at the pictures, It appears a vent from between these seals to the
outside world would be difficult.
Looks like that bearing carrier has gear oil on one side and transmission
fluid on the other.
A transmission vent fix has already been posted and seems like a good idea
to reduce the fire hazard.
If the final drive were vented into the same area as the transmission vent
then at least internal air pressure in the final drive and transmission
would be equal which might help.
If nothing else comes of this, it points out a weak design area and
strengthens the case for additional transmission and final drive cooling
along with use of superior lubricants.
Don Miller
75 Glennbrook
Shenandoah Valley of Virginia
> WARNING, FOLLOWING CONTAINS WILD SPECULATION, OR MAYBE A FLASH OF
INSIGHT
>
> When I read the above message, I went running for my shop manuals. You
>see, for over 20 years I was in the foreign car repair business, and this
>brought back some things I should have remembered before. A few foreign
cars
>(Subaru comes to mind) used a fore-aft engine driving an automatic FWD
>transaxle. Some (eg: Saab) even use a chain drive like the GMC. What I
was
>concerned with was the connection from the transmission to the final drive,
>especially when the two use different kinds of fluid. Some cars (again, I
>think Subaru) just put two oil seals back-to-back between tranny and final
>drive to keep their respective fluids separated.
>
> The trouble with this design is that if one of the seals fails, the
>resulting leak will just pass through the other seal, as the oil comes to
it
>backwards. It was very common on 1980s vintage Subarus to find the final
>drive overfull and leaking ATF, which caused accelerated wear of the final
>drive gears and bearings. Worse yet, occasionally final drive gear oil
would
>find its way into the transmission. Automatic transmissions don't tolerate
>Extreme-Pressure gear oil very well. The stuff soaks into clutch discs,
>causing them to slip and eventually burn out. Even one-way roller and
sprag
>cluthes may be made to slip by enough EP oil. The presence of gear oil was
>easily detected in the transmission by its strong sulfurous odor on the
>dipstick. Of course once a transmission was burned-out from slippage, the
>strong burnt-fluid/clutch odor might mask the gear oil smell, so no one
would
>know the true cause...... And of course if someone used synthetic gear
oill,
>it may not smell as strongly as dino-base gear oil. Hmmmm....????
>
> Now some later automatic transaxles, and some other makes, solved this
>problem by separating the two seals (ATF and gear oil) by just a millimeter
or
>two, and drilling a drain hole to the outside. Now any leakage would go to
>the ground, where it might be noticed, instead into the wrong gearcase
where
>it would wreak havoc. Looking at the pictures in my X-7525A manual
(Section
>3-C, Fig. 1, A), the GMC's seals are back-to-back in a housing, and no weep
>hole is visible..... Can any transmission people confirm or deny this???
The
>pinion bearing/seal housing is part of the final drive assembly.
>
> My speculation is that this may explain some of the sudden transmission
>failures we see. Given the well-known problems with clogged final drive
>vents, gear oil could easily be forced into the transmission, ruining one
or
>the other unit. (The transmission by contamination/slippage, the final
drive
>by oil starvation.) Don't want to start another myth, but this idea really
>has me wondering....
>
> Given the placement of the trans/final drive seals in the pinion
bearing
>holder, I'm not sure if a permanent fix is possible. I'd only suggest that
>everyone be alert for any gear oil smell in their ATF, check frequently for
>any sign of gain or loss of final drive oil, change ATF often, and replace
>those seals carefully if you ever separate the transmission and final
drive.
>
> My .02. Comments anyone? Perhaps Jim Bounds can share some experience
on
>this??
>
>Rick Staples
>'75 Eleganza
>Louisville, CO
