'74 ELDO THM 425

[Viator Trudeau]

I want to change the trans. filter.  Got one, its really called a strainer, not a typ. felt filter that many GM filter are, just in blowing thru it, it must have large holes to only collect large pieces of metal as I feel no resistance, has any forum posters ever cut one open to see how its made?   Thanks for any info.

V.Trudeau

[The Tassie Devil(le)]

These filters are very nicely made, and have a by-pass valve.

As you can see, this one was made in Taiwan.

Bruce. 

[bcroe]

Nice pictures!  Bruce Roe

[Viator Trudeau]

Bruce; thanks for sending the exploded view pictures, they really show the guts of the filter W/ the mesh strainer and the pleated oval filter.

V. Trudeau

[The Tassie Devil(le)]

The Mesh Strainer is only there to stop large pieces of junk from going past the pressure relief valve in the event of the filter mesh being totally blocked up.

Bruce.

[Viator Trudeau]

Hello,  I needed a new filter for my '74 eldo W/ the THM 425.  Got one from Rock Auto.  The one in the trans. was a Delco unit, a PF 169 where the out let of the filter goes up into the trans body and held up into the hole by a snug fit of the "O" ring and pipe and resting on the bottom of the oil pan. The filter I got from R. A. has a smaller OD on the pipe and  the "O ring and are a too loose fit, probley wont fall out because the filter is held in by the pan.   I am afraid the loose fit into the trans. will suck air and starve the trans for oil.      Has any of the poster encountered this situation??? W/ this filter from Rock Auto.
Most hydromatic filters were the flat filters.  The 425 uses the A C PF 169 on Eldorado's,  that slightly smaller pipe has me worried.
V. Trudeau

[The Tassie Devil(le)]

I have used the Filters from Rock Auto, and they fit perfectly in position.   I have another one as a spare that I can measure if you like.

If they were to fall out, you would have trouble holding them in place before you were able to install the pan.

Bruce.

[bcroe]

Most hydromatic filters were the flat filters.  V. Trudeau

The original TH400 through most of 67 had a filter very much like that one.  By the end of
67 they had converted to the flat filter, but the TH425 never changed.  Bruce Roe

[The Tassie Devil(le)]

I don't think it is possible to fit a TH400 Filter into a TH425, as the 425 has a number of alterations that stop it taking the flat one, or the Factory would have definitely used it.   Cost more to make up new stuff.

Bruce.

[Viator Trudeau]

Hello all::  the AC  PF  169 is the original equipment filter for the THM -425.  The R A filter I have is the correct one.
My concern is the O D of the outlet pipe and O ring being a sloppy fit into the tapered lead hole and could suck air instead of oil.  The pipe is .010 to .015 smaller in O D.
I looked in my statch of o rings and found one W/ a bigger cross section that push fits up into the hole tightly and feels like the P F 169.  I am going to go with that.
The supplier of the filter in the picture you sent is different then the one R A uses, stamping on filter I have is C W in a circle  followed by #109


V,Trudeau

[bcroe]

I don't think it is possible to fit a TH400 Filter into a TH425, as the 425 has a number of alterations that stop it taking the flat one, or the Factory would have definitely used it.   Cost more to make up new stuff.  Bruce.  

Right, 400 & 425 filters were never close to interchangeable, though the earlier ones used
similar construction.  When i tried to buy a TH425 filter from NAPA many years ago, their
computer had no idea what was the right part number (had the same issue on other older
things).  Bring one in and they might find it, usually works.  Or they can cross reference a
number from the box.  Nobody seemed interested in updating the computer data, or pulling
out the old paper books.  Bruce Roe

[Viator Trudeau]

When installinfg a new filter, the THM 425 takes about 6 QT. or more of fluid for the refill of the pan.  How does one remove the rest of the fluid, in the torque converter??  for a more complete change of fluid?

V. Trudeau

[The Tassie Devil(le)]

In a normal oil change, this is not necessary.

The only real way to completely change the oil is to strip the trans, split the converter in a lathe, clean out everything, including the Transmission cooler and lines, and rebuild.

This way means the trans has to be removed from the car, and the converter cut and re-welded back up.

There may be a pressure point that will allow compressed air to purge the Converter, but you will still be left with the oil in the bottom half of it, as there is no drain hole, as in the older Converters, and Fluid Drives.

When a trans is being rebuilt because it has been totally destroyed through completely burnt fluid, or internal pieces floating around, the Converter is split, and rebuilt/re-welded or an exchange one fitted.

I have opnly seen one converter requiring complete replacement, and that was in a Dragster with a full manual shift valve body, and the Reverse position was selected whilst doing 80 Mph forward.   The internal vanes were totally stripped.

Bruce.

[Viator Trudeau]

With the new and the old oil mix the end result is a cleaner total fluid.
Another question think all modern braking systems have a proporation valve, is its function to apply the rear brakes slightly before the front for added control as to not cause the car to swap ends in a panic stop or on slippery surfaces??


V. Trudeau