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Turbo 400 without park
#1327658 Wed Sep 25 2019 12:22 PM
Joined: Jul 2016
Posts: 55
Wrench Fetcher
I have a 1968 Chevy C30 cab and chassie with a twelve foot flat bed dump. It has a 400 turbo auto transmission. The transmission isn't equipped with the park

It's my understanding GM omitted it to prevent transmission damage from heavy loads being parked without the use of the emergency brake or chocking the wheels.

I would like to add the park feature to the trans for convenience. Is their a simple modification that can be done by dropping the pan ?

Thanks ,Rich


68 C30 CST dually flatbed dump
58 Viking 6500
Re: Turbo 400 without park
viking58 #1327708 Wed Sep 25 2019 07:56 PM
Joined: Sep 2011
Posts: 770
Shop Shark
The only simple modification I know of is to install a later TH400.

1960 Chevy C10 driver 235 T5 3.73 dana 44
1949 GMC 250 project in waiting
1960 C60 pasture art
G.M dealer tech since 1980 & counting
Re: Turbo 400 without park
viking58 #1327757 Thu Sep 26 2019 01:01 AM
Joined: Mar 2010
Posts: 9,449
Master Gabster
I would be shocked to find out that your TH400 could not simply have a park pawl and return spring installed to give you that feature.
It may well require removal and some disassembling to do it.

Re: Turbo 400 without park
viking58 #1327771 Thu Sep 26 2019 02:13 AM
Joined: Jul 2019
Posts: 24
New Guy
I agree with 52Carl. Been a bit since I've been in the bottom of a 400 but iirc there will be a linkage running from the selector back to a tapered pin that forces the parking pawl up to engage one of the drums. this is held in place by a bracket and the pawl itself has a spring to retract when the pin is moved. I think the pivot pin for the pawl gets driven in from the external rear of the case and the hole is plugged with a small cup plug.

Not seeing many good pics in a google search. The only problem I could see running into trying to install the setup would be some of the holes not being drilled out.


Too many to work on
4 bolts, 2 square bodies, 4 GMT400s...and those are the keepers
Re: Turbo 400 without park
viking58 #1327772 Thu Sep 26 2019 02:27 AM
Joined: Mar 2019
Posts: 1,070
Shop Shark
Get a set of wheel chocks. Will last 4ever unless you forget to pic em up like I have.

Re: Turbo 400 without park
viking58 #1327775 Thu Sep 26 2019 02:37 AM
Joined: Jul 2000
Posts: 1,062
Shop Clutterist

1954 1/2 ton 235 4 speed
Re: Turbo 400 without park
viking58 #1327806 Thu Sep 26 2019 11:45 AM
Joined: Mar 2005
Posts: 1,716
Shop Shark
I'm not sure why a later 400 would be needed. 65 and up had park except for the heavier trucks. But, you might drop the transmission pan and with a flash light look towards the back of the transmission to see if the slotted disc for the park pawl to engage is there. You should be able to see the slotted edge just barely. They may have simply left the actuator rod off so it could not be placed in park. You might get lucky and simply install the linkage inside to complete the parking pawl engaging the slots in the disc.

Or find another 1968 TH400 and install it. Check overall lengths and whether or not the driveshaft yoke slides in or is held on with a nut.

You might have to remove the tail cone to install a spring and part of the actuator, but that's not hard to do,.

One other option is to take the transmission down and take it to a rebuilder and they can get the parking mechanism set up.

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