[67drake]

My ‘55 has no noticeable downshift if I accelerate while cruising, or even come up to a steep hill and have to put my foot into it. Is there a procedure listed online? I just got the car so don’t have a shop manual yet.

And if anyone has a manual suggestion, I’d appreciate it. Seems like there’s multiple manuals listed. What’s the “go to” one?
Thanks

[Bill Hanlon]

While you are trying to decide on which books to get, fellow Pontiac enthusiast Safari Larry (Larry Gorden) has these available on his web site:

https://www.pontiacsafari.com/55/ShopManual/index.html

https://www.pontiacsafari.com/Hydram...ange/index.htm (this is a '56 manual, but very close to '55)

https://www.pontiacsafari.com/L1Gara...ic_1946-55.pdf (a pretty good manual for external adjustments)

I have a dual-range HydraMatic manual from NAPA that has a good troubleshooting section. I'll look through it for your symptom.

[67drake]

Thanks.
Not used to this 4 speed automatic yet, but I would imagine it would downshift similarly to a 350 or 400 transmission on accreditation. Hopefully just an adjustment.

[Bill Hanlon]

I looked through the NAPA manual as well as the "On-The-Car" and HydraMatic books on Larry's web site. First choice for no forced downshift in the books is the throttle linkage connecting to the transmission is too long. Other choices involve internal workings of the transmission.

Question: Is the upshift speed OK at full throttle? Too long of a linkage adjustment should also lower upshift speed.

There is a gauge to make this adjustment, but they are hard to find. Try shortening the adjustment one turn at a time, then testing until you notice a change. Don't forget to count turns so you can put the adjustment back where it started.

Maybe SteveP will check in with additional info.

[stevep]

The first thing to do is to make sure that you are getting the carb to open fully when you floor the throttle.
When you accelerate more aggressively, do the upshifts come in later than on a slow acceleration?

[Bill Hanlon]

Just curious: Did the '55 Pontiac w/HydraMatic have a strange looking device on the carb end of the throttle linkage where the shaft of the linkage went through a spring? My '57 GMC V8 (Pontiac) HydraMatic had such a device. It allowed the throttle plates of the carb to stop at fully open and there was still additional pedal movement that was transferred to the throttle input of the tranny. I can't find any pictures or drawings of the Pontiac carb linkage.