My 69 VC, has developed an issue, when under power everything works as it should, but when letting off gas, and the tr@nsmission drops to a lower gear, then giving gas again there’s a noticeable hesitation between the gears, but only when not under power, when under power the transmission shifts as it should. The vacuum line is as old as the car, it’s my first thought on a cause, could that be the problem, or is there something else.

 

That doesn't sound right. It should shift to the next higher gear under light load.

Which transmission is this - TH350 or TH400?
Have you verified the fluid level is full?

 

Low transmission level might be the issue (it's a common issue). As mentioned, verify transmission level. Ensure the transmission is at normal operating temperature when taking measurement.
Examine the transmission fluid. It should be a brilliant red just as the color would be in a new container of ATF. If it's a dark red (especially), change the transmission filter & install new ATF.

 

This is a very well written, easy to understand dialogue which should be read to set a tone of things to consider as you move forward w/ diagnostics/troubleshooting. It sets the pace for understanding what might be at play. Nothing lost & much gained by taking the car to a reputable transmission shop & let them take it for a spin. Good Luck!

Why Is My Transmission Slipping? Common Causes and Signs

 

New filter and fluid, it only does it when when letting of gas and reapplying acceleration, it’s just a quick hesitation, that just started

 

When shifting to next gear after dropping RPMs then powering back up. Not sure if it’s transmission or if the carb is causing the hesitation

 

Your vacuum reading as measured with a vacuum gauge should be a “steady” reading between 15”Hg - 21”Hg. There could be the chance your automatic transmission modulator is buggered up, clogged, leaking? Pull it & evaluate.

 

Thanks that’s a good starting point

 

Yes, it's a good start & beneficial to know your engine's vacuum characteristics. It may not resolve your current issue; yet, vacuum is a critical engine component for (1) carburetor A/F mixture, (2) ATM valve operation, (3) HVAC, (4) Brake Booster, etc. Perform the test in the red box (diagram below). A key characteristic for a stock engine w/ a stock cam is ability to maintain a 'steady' vacuum reading. Cracks in vacuum hoses are quite common in 50+ yr old cars. If in doubt replace hoses with vacuum hoses - yes, they are different than fuel line hoses. Most auto parts stores have reels of vacuum hose - have them cut a length which will accommodate your needs. Locations where vacuum hoses attach to metal are always suspect. Again, vacuum may not resolve your current issue, but yes it's a good start & quite beneficial to overall engine health. Good Luck.

 

I’ve replaced all the vacuum lines this yr except the transmission line. Car has set in my garage, 21 yrs and never started, this yr it was time to come out of the mothballs so to speak, I’ve driven it 600+ miles and the transmission issue just started. Changed filter and fluids, with my initials all fluids change last yr. The car has 241,000 miles on it, it was my fathers and as far as I can remember the transmission has never been worked on.

 

As stated, ‘measure’ the vacuum so you ‘know’.

 

The problem existed before or after changing the fluid and filter? Questioning the filter/oil pickup.

Asked by Fun71, what transmission TH___?