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‘70 Deville Transmission Jerk

Started by Houston747, February 24, 2022, 03:45:37 PM

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Houston747

I have a question to see if anyone has experienced something similar in their classic Cadillac. In my '70 Coupe Deville, I noticed when changing gear whether from park to drive or the other way around, the car would kind of shake/and jerk forward. The car itself doesn't move when that happens, it just jerks and doesn't seem right. The transmission itself shifts through the gears great, and I have no problems when driving it. But I am not sure if this normal for a Cadillac, because this jerk that comes when changing from park to drive and the other way around doesn't seem normal for this car.
A.AlRub

chrisntam

Make sure your idle speed isn't too high.

My '70 seems to go into drive hard.  I talked to the shop that rebuilt the transmission, he said Turbo 400s all had that trait.  Mine goes into park as I think it should.
1970 Deville Convertible 
Dallas, Texas

Dave Shepherd

Basics, check the trans and motor mounts for any looseness or deterioration.

Cape Cod Fleetwood

2 excellent suggestions above. One assumes your fluid was checked 'hot' and is at the proper level, as well as the fluid being fresh and the filter clean.

The TH400 can be a bear when its REALLY cold, let your car warm up. Additionally if your car like mine has a WORKING choke stove it can be pretty violent going into gear before that spring has unwound and you've properly kicked it down with the throttle. (The body shop The Ark is currently at has commented on this. "You should adjust the carb". No honey, its working properly, don't touch it.)

Additionally fluid can impact the 'feel'. The Ark's transmission was completely rebuilt (it just needed a new seal, the owner read the article on the car in the March 2020 "Self-Starter" issue and got carried away, its all good) last summer. I specified using FORD transmission fluid as I have been using. Its greasier, its more 'slippery', and not as sensitive to temperature hot or cold. I felt a difference in smoothness of shift with the FORD fluid vs Dexron. And it wasn't because I paid more for it, its the same price. If you consider changing fluids, be advised it will take a couple of full fluid and filter changes to get all the Dexron out and the FORD fluid all the way through the system.

Let us know what worked in your car.
There are 2 kinds of cars in the world, Cadillac and everything else....

The Present -1970 Fleetwood Brougham

The Past -
1996 Deville Concours
1987 Sedan De Ville "Commonwealth Edition"
1981 Coupe De Ville (8-6-4)
1976 Sedan De Ville
1975 Sedan De Ville

The Daily Driver and work slave -
2008 GMC Acadia SLT *options/all

TJ Hopland

Does it change hot to cold?   Does it change depending on the speed you go from P to D?   What does it do between R and D?  I'm wondering if its partially starting to engage R as you pass by it. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

bcroe

Shops may rebuild a trans to shift stiffer than before, just so
you know its rebuilt they say.  One of the things they do is
replace all the steel clutches with flat pieces, but the original
clutches each used one wavey plate (a bit of a spring) to
soften the engagement.  I DO NOT want to modify my TH400s
in this way, so I always put back the wavey plates. 

They may also change out a perfectly good vacuum modulator,
not bothering to adjust it to match the old one.  Could even
install a shift kit. 

In truth many transmissions show no wear on many of the hard
parts, just need a seal refresh.  But shops have one plan, replace
everything (including parts you do not need) with possibly inferior
parts, charging for the most expensive kit.  This is part of why I
rebuild my own transmissions.  My favorite is a 70 Cad used in
my brand O.  Bruce Roe

TNdeville

Quote from: Houston747 on February 24, 2022, 03:45:37 PMI have a question to see if anyone has experienced something similar in their classic Cadillac. In my '70 Coupe Deville, I noticed when changing gear whether from park to drive or the other way around, the car would kind of shake/and jerk forward. The car itself doesn't move when that happens, it just jerks and doesn't seem right. The transmission itself shifts through the gears great, and I have no problems when driving it. But I am not sure if this normal for a Cadillac, because this jerk that comes when changing from park to drive and the other way around doesn't seem normal for this car.

Hi, apologies for bringing back an older post, I wanted to share my experience as well.  My 1970 CDV was doing this, and I initially thought the idle was too high, turned out that the gas pedal was sticking and I figured it out by lifting the gas pedal with my foot before shifting.  Hopefully by now you've ironed this out.
T Clark