Automatic transmission changes roughly

[Reverend]

My friend has -73 240z with auto transmission, and im asking this on behalf of him as he dont speak english. It changes gears really roughly, i mean the whole car goes like BUMP. I already suggested that he changes oils in it but is there any other adjustments you can make to it externally to perhaps smoothen it up?

[Chickenman]

An oil and filter change is good advice. Automatic Transmission fluid is often over looked as a Maintenance item. But it has to be changed just like engine oil. Once every 24,000 miles should be sufficient. Change the filter as well of course.

 

Rock Auto carries the Filter and Gasket and will ship Internationally. You may be able to get the parts from your local Auto Parts stockist as the Transmission is more common than people normally think. Use Dexron III fluid.

 

One other thing to check is the piping or vacuum hose to the Vacuum modulator. This is a Vacuum valve on the side of the transmission that sense load on the engine according to Intake Vacuum. High load = Low or Zero Intake vacuum, like at WOT. The vacuum modulator then increases line pressure to the clutches and discs to prevent slippage.

 

There is a Vacuum fitting on the Intake manifold that connects to the Vacuum modulator. I think it comes off the brake booster Tee fitting.  If the hose or piping is  damaged ( or not connected ) the transmission will think you're at Wide Open Throttle all the time and shifts will be very harsh.

 

Check the connection at the Transmission end as well. Usually the problem is a perished hose. The Vacuum Modulators themselves seldom fail. 

Edited June 23, 2015 by Chickenman

[Chickenman]

Of course check the rear diff mount to see if it is broken. It won't cause actual rough shifting... but it will make a big BUMP noise if it's broken. Report back with what you find please. Good Luck.