A Slow-Shifting Skylark: A Lesson on Older Automatic Transmissions

Dear Car Talk

Dear Car Talk | Apr 28, 2016

Dear Car Talk:

I have a 1963 Buick Skylark V-8 that is very slow to shift into high gear. I need to go approximately 50 mph before it'll shift. I have tried additives. Is there some other simple fix? Thanks.

-- Dave

The crusher's not simple enough for you, Dave?

Actually, there might be a very simple fix.

Most cars of this era had transmissions that used vacuum modulators to help them figure out when to shift. In order to shift at the right time, the transmission needs to know how hard the engine is working. And one measure of that is the amount of vacuum it's producing.

As your foot pushes down on the gas pedal, engine vacuum decreases. That's because as the throttle opens, lots of air is let in.

So when you stomp on the gas to enter a highway or climb a hill, the vacuum modulator detects the drop in vacuum, and calls for a downshift.

Conversely, when you ease off the gas and level out your speed, engine vacuum increases, and the transmission upshifts.

In your case, the line that connects the intake manifold to the vacuum modulator may have fallen off. Or it may be cracked or broken. Or the diaphragm in the vacuum modulator itself may have decided that five decades of that kind of work is enough, and retired to Boca.

And sometimes, leaky vacuum modulator diaphragms can allow transmission fluid to get sucked back into the intake manifold, resulting in voluminous clouds of blue smoke coming out of the tailpipe. If you're nodding your head now, Dave, saying, "So that's what all that blue smoke is!" I think we're on the right track.

You probably still can find a new vacuum modulator for short money if you're lucky. That's pretty simple, right?

We'll keep our fingers crossed for you, Dave.

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