What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

Blazer jerks when shift into gear

J

JW-Tex

Registered Member
Joined
Mar 2, 2022
Posts
16
Reaction score
8
Location
Kingwood Texas
I have been working on an old 1973
blazer and it always jerks when shift from park to any gear (turbo 350). When I first got it it would not move so added transmission fluid and got things moving. I think I I need to add some more fluid.

Since I have been driving it a few hundred miles now was thinking of dropping the trans pan and putting in a new gasket and filter (I have never replaced trans filter before). Plan to put all new fluid in.

Anything else I should do while I am doing this?
 
New vacuum modulator they are cheap and check vacuum line going to it. Also you can disconnect tranny lines and flush out using compressed air or just allow to drain. They also make an aerosol to clean them. I like to disconnect top line at both ends and bottom line at Trans. Low pressure blow air through top line and into radiator Trans cooler so you get all new fluid.
 
Thanks for everyone input.

This afternoon I did the following:

Topped up the fluid- no change.
Put hands on driveshafts and shook them back and forth no movement.
With engine running shifted into gear...saw no movement of the engine on the engine mounts.
Not. Sure how to check transmission mount but did no see any obvious movement.
while engine running did see the driveshaft turn slightly when placed in gear both front and rear and noise and jerk occurs.
 
Does it jerk the same if you put it in neutral and go into gear from there. I assume since you said front and rear shaft both turn do you have full time front end no hubs and a 203 tease? I f so you could have a little slack in tease chain.
 
Neutral to reverse it does it and neutral to drive it does it.

Front hubs are unlocked manualluy...but did have high lock on earlier.

Went out and put in high. Same noise and jerk .
 
I could be wrong, but weren't 73 th350 np203s? So either the 203 has a part time modification kit, or it has np205, or possibly another model t case. Installed.
How much back lash in the rear diff? You say the motor moves, question is how far. The clam shell motor mounts won't let the engine move to far, but enough to feel it flop around.
Check the trans mount by prying up on the t case adapter.
If it moves up the mount are compressed. If it's covered in oil and squishy they're no good.
 
Last edited:
Wouldn't a tight torque converter or a performance trans upgrade that "chink" sound when you drop it in gear?
 
Thanks for all the ideas and input.

Not sure which Transfer Case I have other than the engine being changed out the truck is pretty stock but will try and determine which one I have.
Motor does not move when shifting and jerk happens.
I am going to focus on the Tranmission mount and check that throughly.
Once in Drive the shifitng is smooth as it should be.

Thanks JW
 
They're asking you about the vacuum modulator and vacuum lines because if that is broken or disconnected, the transmission acts as though you are at full throttle, so it shifts very hard. A classic symptom of a bad one is oil leaking past the diaphragm, also consistent with your symptom of low oil in the transmission (i.e. the engine has been sucking it out and burning it). However, nice shifting on the road doesn't really support that. It's possible somebody mis-routed the vacuum to the transmission to a ported vacuum source on the carb. Quick check is to pull the line off the modulator with engine running and feel for vacuum or hear change in idle speed.
 
Last edited:
Blue 85 the Vacuum line could be an issue. When I first got the truck it was not hooked up and if I remember correclty I do not think it was upshifting automatically at all. I believe I plugged in to a port on the carb and the transmission started upshifitng. This is from memory from 6 months ago so will check it out when I get home.

Thinking about this I also belive the jerk/lurch was more prononuced when I first got the truck prior to solving the upshift issue (pluging the vaccum line into the carb).


To confirm what I think you are telling me is that the modulator vacuum hose should be routed to the intake manifold and not a carb port?
 
It may go to the carb or the manifold. I would find the 1973 vacuum diagram, which will look something like this (which appears to be from 1971)

1971 vacuum.JPG
 
Update. Added energy suspension transmission mounts and about 80% of the jerking is gone.
Rerouted modulator vacuum hose to correct port per Holley carb website.

Putting new modulator on later today and new transmission filter and gasket.

I think new motor mounts will be my next upgrade!

Thank you everyone for all the ideas and tips to help me fix this issue!


JW
 
Manifold vacuum is correct for the th350 modulator. Intake or base of carb, as long as you get full vacuum at idle.

If you are going to tackle the engine mounts I highly recommend updating to either ORD or DIY4x competition engine mounts. No more sagg

https://diy4x.com/product.php?productid=17692&cat=252&page=1

www.offroaddesign.com

Competition Style Motor Mounts for '73 to '87 GM - Small Block or Big Block

Competition style motor mounts for '73 to '87 ('91) GM Fits gen I/II small block, big block or 6.2L/6.5L diesel Includes all bolts and hardware for mounting to the frame and engine block.
www.offroaddesign.com www.offroaddesign.com
 
You must log in or register to reply here.

Latest Posts

Top Bottom