-Hydramatic drops gear, what's wrong?

[SwedenStefan]

Hi. I have just bought an 1964 bonneville. And if I put my Hydramatic in to Drive, and accelerate,  it works just fine, it change gears from 1 to 4 just fine. But, if I then move my foot from the throttle,  so that the engine goes down in to idle, the transmission drops gear!? So then, I have to put it in to 2:nd to make it drive, and when I get som speed, I can put it on to Drive.

 

What to do? Can it be a slide in the hydraulic that are worn out, wrong kind of oil? or is is an complete overhaul that has to be done? In that case, can I do it by myself?  Is there an good place to buy overhaul kit from?

 

I'm very grateful for tips and tricks.

Edited March 11, 2018 by SwedenStefan (see edit history)

[JACK M]

first thing I would do is replace the filter and fluid.

[Grimy]

If it's a Turbo Hydramatic 400 as on my long-since-sold '64 Cadillac, the upshift is controlled by a vacuum modulator screwed into the right side of the transmission case, and connected to steel vacuum tubing by a 3-inch piece of vacuum hose which deteriorates.  IF you have a vacuum modulator, replace the vacuum hose, and check for oil inside the vacuum tube part of the modulator with a cotton swab.  If you find oil there, replace the modulator.

[TexRiv_63]

Not sure but I think the 64 Bonneville was the last use of the old design 4 speed Hydramatic, the lower models used the 3 speed Roto.

[Grimy]

Don, you're probably right.  I should have described my symptoms of 40 years ago:  Wouldn't upshift out of first, but take it to 30 mph, move shifter to Neutral, then back to drive--then got top gear (3rd on TH400).  A 4-inch piece of vacuum hose doing a 90^ bend from the modulator to the metal tube from the intake manifold had failed.  I wonder how many transmissions got "rebuilt" (meaning removed and painted) for want of 10 cents of rubber tubing?  So if anyone with a vacuum modulator has upshift problems, I recommend checking the integrity of this vacuum hose.  Does the old 4-speed have a vacuum modulator?

[Roger Zimmermann]

The "old" Hydramatic has no vacuum modulator. If that transmission was never overhauled, the seals are as hard as stones and are leaking. The shop manual has instructions how to check the oil pressure in various circumstances. I would do that first. If you have some experience in mechanical work, you can do it yourself. There are some special tools; the first I overhauled were done without those tools and are still functioning. The shop manual is however a must; usually, Pontiac manuals are well done.

[SwedenStefan]

Thanks guys I doent know exactly which transmission I have, but it is 4-speed, and I can read: PBS-64- 98195 on it.

I start with an oil and filter change, and a check of the vacum- hose and modulator. Then I think I have to order the shop manual from Rockauto.

[poci1957]

On ‎3‎/‎12‎/‎2018 at 7:24 PM, TexRiv_63 said:

Not sure but I think the 64 Bonneville was the last use of the old design 4 speed Hydramatic

 

Don is correct, this should be what the manual calls a "Super" hydramatic, also known as a Jetaway and offered in Pontiacs from 1956-64.  Parts are available and reasonable at www.autotran.us , good luck, Todd C