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67 Cadillac trans issue

Started by richardbergquist, November 15, 2018, 06:06:43 PM

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richardbergquist

Maybe someone more knowledgeable than me on transmissions might tell me what could be behind my little issue here. When getting the gear into Drive when engine is cold it’s all very smooth, don’t even feel it. However when engine is warm getting into Drive makes for a violent jolt. I’ve experienced something similar in the past with old cars, the classic clunk when getting lever into Drive but I’ve never noticed any difference that depended on the engine being warm. Higher pressure in trans fluid when warm could cause this yeah?

bcroe

First thing to try is a new filter, your switch pitch probably uses the
earliest heel print pan TH400 filter.  Unless it is an Eldo.  I suggest if
the trans is original, you may have ancient harding or shredding
rubber seals, about to let go.  When cold the thicker fluid doesn't
leak so badly at the bad seal, but warmer it leaks worse and
malfunctions. 

Save your trans with a $30 (internal) seal kit, before it is damaged. 
Bruce Roe

richardbergquist

A seal kit, OK. This involves taking down the transmission?

Dave Shepherd

Yes out and disassembled to reseal the unit.

savemy67

Hello Richard,

Engine cold = higher idle speed (choke engaged), engine warm = lower idle speed (choke disengaged).  An engine running at higher RPM in Park would often invoke a "clunk" when shifting to Drive.  However, your description seems to be the opposite - you note a jolt when the engine is warm.

Transmission cold = thicker fluid, transmission warm = thinner fluid.  As Bruce mentions, thinner fluid is apt to flow more easily past any rubber seals that are in bad condition.  When the THM400 is shifted into Drive, the front clutch unit is engaged (among other things - see the shop manual for fluid flow description).  If the rubber seals (there are two) of the clutch are failing, fluid pressure has to increase in order to overcome a failing seal, so the clutch may engage with a noticeable jolt.

Pressure increase is not a function of fluid temperature per se.  There are several devices that control different pressures in the THM400.  And, of course, rubber seals in good condition work in conjunction with the pressure control devices.

As Dave said, the transmission needs to be removed from the car in order to replace all the rubber seals.  Whether or not you plan to do this yourself, you may want to read my adventure rebuilding my '67 THM400 here:

http://forums.cadillaclasalleclub.org/index.php?topic=133691.80

Scroll down to reply #88.

The attached photo shows some seals I removed from my transmission.  While they were flexible enough to twist into a pretzel shape, they were also starting to become hard after being in my transmission for half a century.

Respectfully submitted,

Christopher Winter
Christopher Winter
1967 Sedan DeVille hardtop

bcroe

#5
That is a great write up of what will be encountered in a TH400.  I think
my first time took 6 weeks, but each time gets faster, cleaner, and more
efficient.  A drawer with a bunch of simple home made tools really does
speed the process. 

Having seen so may TH400s scrapped, I saw many changes made over a
decade and a half.  Some changes were good, some poor cost cutting. 
Those in my cars are a combination of the best over all the years, not
exactly stock.  One thing I did was convert any switch pitch to use the
68 up filter and pan style, just one hole tapped into the valve body. 
Much more practical with lots of options.  I painted my first TH425,
but probably never a TH400. 

I find it a lot easier to find specific parts now using the internet, than
when I started decades ago. 

Some seals I removed from that vintage actually broke when bent,
others were starting to separate lengthwise.  My diagnostics don't come
down to which seal is the cause, I just determine if the trans must come
out.    Bruce Roe