78 Eldorado Final Drive (Differential) Problem

[Jakes]

The THM 435 tranny was rebuilt a couple of years ago and it was discovered that the flange at the bottom left corner of the Final Drive (differential) - FD -where it joins the gear box was cracked and leaking (see pics below).  The shop believed that the tranny had been removed years earlier but proper procedure wasn't done to separate the tranny housing from the FD flange.  The tranny shop pulled the FD and welded the crack but was unable to stop the leakage despite many returns.  Based on what is collected in a floor pan, it leaks about 2 oz. of DEXRON VI fluid every time I take the car out on its 30-mile local/hwy exercise run, but who knows how much more while on the road?  I'm always checking fluid level and topping up.

The solution seems to be to find a replacement FD housing or an entire unit.  I've searched online, including Ebay, and so far it seems that only entire units taken from old, high mileage '70s Cadillacs and Toronados are available, and they're not cheap.... at least about $200 incl. shipping to where I live.

To complicate matters... according to the '77-'78 shop manual these FDs are not serviced but replaced as an assembly.  In fact, the Cadillac '76-'80 parts catalog shows "Group 5.505 (FINAL DRIVE - HOUSING, Differential), '76-'78 Eldorado, no. 231301" as an entire unit (which back then cost $536!).  There are no internal parts listed.  If that means the internals can't be swapped for the purpose of having a usable housing, I'd be very reluctant to swap my low mileage (22K miles) unit for one with high mileage.

So.... two questions:  1) Can anything be done with the FD now on the car to arrest the leak that persists from the welded area(...as we speak, the car's being looked at another highly recommended tranny shop), and 2) do you know of any parts sources around the country that might have a low mileage unit that would replace the one on the car?  As always, many thanks for you help and advice.


Paul Jacobs
Huntington NY

[bcroe]

Looks like someone owes you a final drive.  Had a related problem myself
decades ago, stripped bolt holes were not retaining the Final Drive and a
lot of trans fluid was being lost.  I did manage to salvage stuff by retapping
all the holes and using customized long bolts. 

That looks like a braze which probably isn't as strong as the original build. 
But if it is holding, I suggest using a generous amount of something like
that blue gasket former to see if can be sealed up, no gasket.  Be sure no
edges are sticking up and pushing the mating surfaces apart.  Here is
VALCO product we use to seal the difficult chain cover on the back of a
TH425  trans. 

Failing that, I would just pick up a 70-78 Toro/Eldo final drive that hasn't
been run out of oil, and install it.  These things are super tough, same one
is used to pull a 6 ton motorhome.  Doubt you will ever wear it out, never
have seen that happen.  There is a rather slim chance you could get a bad
one, but by far the quickest cheapest route.  You won't avoid the shipping
charges regardless.  Must be 6 of those under my bench. 

Don't look at 66-67 which have different ratio and mounting.  Most are
2.73:1 ratio, a few 3.07:1 were used, mostly in motorhomes.   Bruce Roe

["Cadillac Kid" Greg Surfas 15364]

Too bad you are all the way up there in NY.  Ive got a pristine final drive I bought for my Eldo, but after 500,000 miles (when I sold it) I still had no need.  Bruce is right, they last forever if kept in oil.
Greg Surfas

[cadillacmike68]

$200 is cheap by my book. Try $600 for a turn signal switch.

["Cadillac Kid" Greg Surfas 15364]

I think $600.00 plus shipping is about the right price for a good unit.
Greg Surfas

[bcroe]

Long ago I upgraded some cars from 350 to 403, which I usually got
from a yard 77-78 Toronado (cheap).  The yards were so lazy, they
would leave the trans and final drive still attached for no charge, that
was just fine with me.  Got a lot of TH425/final drives, most are still
here and one went into my 79 Eldo after some surgery.  I doubt I
ever paid more than $400 for the set. 

With the right packaging you can ship a final drive UPS, under 150lb
at about 106lb.  Bruce Roe

[Greg C]

I'm stripping a 72 Eldo  if the ratio is the same as yours an will fit It's yours   cheap
I'm a couple hours away  06457
Greg

[Jakes]

Gentlemen..  I'm pleased to report that the serious leak from the Final Drive (FD) and tranny pan has been stopped!!  What a relief!  The shop I used came through big time.  Some of you had recommended certain RTV sealants, which they weren't familiar with, so the "amateur" decided not to tell the "experts" what to do.  They pulled the tranny, installed a seal kit, used an epoxy compound on the FD flange in the area where the cracked flange had been welded to smooth and level the surface and plug any porosity, assembled everything with new FD and pan gaskets and carefully torqued the bolts.  Now I'm noticing a very slight leak of gear oil from the FD cover -- it probably was masked by the tranny fluid leak.  See my latest post on this subject.

Many thanks, everyone, for your advice and parts "offers".

Regards..
Paul Jacobs