[Bill Hanlon]

From a stop, with the gas pedal pressed down 3/4 of the way or so in left drive approximately what speed are you going when the upshift takes place? (RPM reading from a tach would be even better).

What speed will the car run at in the highest working gear at half throttle?

Don't go to stupid speeds to find out!

Just trying to figure out if your 2 gears are:
... a. 1 and 2
... b. 1 and 3 (not likely)
... c. 1 and 4 (really not likely)
... d. 2 and 3 (really, really not likely)
... e. 2 and 4 (really not likely)
... f . 3 and 4

[Bill Hanlon]

Can you choose from one (or more) of these problems?

[Jack Gifford]

Originally Posted by Bill Hanlon

... ... b 1 and 3 (not likely) ...

Wouldn't this be likely with front clutch failure?

[Onestone]

I have a tach, so that's no problem. The gear it shifts up to is 2500rpm at 35mph - which gear is that?

[johnta1]

Don't know the rear gear ratio or what the tranny gear ratios are, but think it is 1.86, possibly 2nd gear?

[Bill Hanlon]

I can't find '55 Pontiac HydraMatic ratios right now, so I'm going to use the ratios from my '57 GMC truck. They may not be exactly right, but they should be close. I'm going to use a rear end ratio of 3.23:1 and a 27" tall tire.

Maybe Larry can find the ratios for a '55 Pontiac tranny and the most likely rear end ratio.

Using the chart shown below I'm trying to answer Onestone's question of "The gear it shifts up to is 2500rpm at 35mph - which gear is that?"

Assuming his 2500 rpm is AFTER the shift completes and his speedometer is near-correct at 35 MPH I'm guessing that the shift that just occurred was INTO 3rd gear. I know the numbers don't line up, so if Onestone could give me the tire diameter and Larry can find a most likely rear end ratio I should be able to come closer.

Onestone: Please measure tire diameter and answer .....

1. What is engine rpm just before the shift at 35 mph?
2. What is engine rpm just after the shift at 35 mph?
3. What is actual speed (use the GPS function on a cell phone) at 35 MPH indicated?

[Bill Hanlon]

Jack is much more the Dual-Range HydraMatic expert than I am, but ....

A. If the front clutch failed "engaged" and the front band was operating normally the front unit would be trying to be in direct drive and reduction mode at the same time. Assuming normal operation of the rear unit I would think the transmission would either lock up or the front unit would disintegrate once you left idle and the torus started to work.

B. If the front clutch failed "disengaged" and the front band was operating normally the front unit would work normally in 1st gear. When 2nd gear should engage the front unit band would release, but because the front unit clutch is off no power would be transmitted through the front unit, resulting in no power transmitted to the rear wheels.

[Bill Hanlon]

Onestone:

If you are so inclined you could measure the rear end ratio by ...

1. Block the right front tire to prevent movement of the car.
2. Put the left rear wheel off the ground on a jack-stand. Be safe doing this.
3. Put a chalk mark at the 6 o'clock position on the left rear tire.
4. Put a chalk mark running linear on the driveshaft where you can see it.
5. Put the transmission in neutral.
6. Have an assistant rotate the tire forward exactly 1 turn while you count driveshaft rotations.
7. Estimate the last partial rotation. 1/4, 1/2 or 3/4 is probably close enough. Add that to the number of driveshaft rotations.
8. Divide the number of rotations by 2 giving rear end ratio.

By the way, this won't work if you have a positraction rear end (doubtful).

[Bill Hanlon]

Attachment from 1955 Pontiac Shop Manual. Shows size of 7.10x15 and 7.60x15 tires as well as four "likely" rear end ratios.

[Bill Hanlon]

I have updated the chart's HydraMatic ratios and tire size based on info found here:
https://over-drive-magazine.com/2023...urtesy%20lamps.

[Bill Hanlon]

MY POST # 48 ABOVE IS WRONG !!!!!! It should say ....

Onestone:

If you are so inclined you could measure the rear end ratio by ...

1. Block the right front tire to prevent movement of the car.
2. Put the left rear wheel off the ground with a jack-stand under the left rear axle. Be safe doing this.
3. Put a chalk (or tape) mark at the 6 o'clock position on the left rear tire.
4. Put a chalk mark running linear on the driveshaft where you can see it.
5. Put the transmission in neutral.
6. Have an assistant rotate the tire forward exactly 2 turns while you count driveshaft rotations.
7. Estimate the last partial rotation. 1/4, 1/2 or 3/4 is probably close enough. Add that to the number of driveshaft rotations giving the rear end ratio.

By the way, this won't work if you have a positraction rear end (doubtful).

[Onestone]

Thank you so much for your help!

I already thought the same way - how to determine which gear 2500rpm at 35mph is

So what I already did is measuring the circumfence of my tires -> it's 223cm, which is almost exactly 87,76" - as shown in the shop manual for 7:60-15 tires.

I did today some pictures from the differential and the transmission - because I was hoping to find anything on the housings that tell me the ratios. See the pictures attached.

On the differential housing I found a number called "A25 50" - Google didn't know anything about that. And on my transmission there is a plate which says "D56 - 46365" - is this a 1956 dual range Hydramatic on my 1955 maybe?

I was already thinking how can I get the exact rear end ratio, I will do as you suggest below and tell the result here. I will also do another test drive and measure the exact GPS speed in both gears with my phone.

Thanks again for all the help - I really appreciate it!

[Bill Hanlon]

Originally Posted by Onestone

Thank you so much for your help!

I already thought the same way - how to determine which gear 2500rpm at 35mph is

So what I already did is measuring the circumfence of my tires -> it's 223cm, which is almost exactly 87,76" - as shown in the shop manual for 7:60-15 tires.

I did today some pictures from the differential and the transmission - because I was hoping to find anything on the housings that tell me the ratios. See the pictures attached.

On the differential housing I found a number called "A25 50" - Google didn't know anything about that.

And on my transmission there is a plate which says "D56 - 46365" - is this a 1956 dual range Hydramatic on my 1955 maybe?

The "A25 50" is actually a casting date for the diff housing. And the zero is not a number, but a mark in the casting that sort of looks like a flat head screw. There is another such mark at the front of the casting date. GM did this on many major cast parts like engine blocks, heads, transmission cases, etc. After 70 years they can be hard to read.

The remaining A255 decodes as ...
A = January, B = February, C = March, etc
25 is the 25th day of the month (if less than the 10th only one digit is used)
5 is the last digit of the year, 1955

Originally Posted by Onestone

And on my transmission there is a plate which says "D56 - 46365" - is this a 1956 dual range Hydramatic on my 1955 maybe?

Likely, but my Pontiac decodes only go thru 1955. The '55 would be a "P55", but remember in 1956 Pontiac used two automatic transmissions, the dual-range HydraMatic like you have and the Strato-Flight Hydra-Matic (also known as Controlled Coupling Hydra-Matic or Super Hydra-Matic). Those would have a different letter/number on the plate.

I've updated my spreadsheet with your tire size. When you get me the rear end ratio I'll load a new one.

[Jack Gifford]

Bill- the only failure I would expect of a hydro's clutches would be failure-to-engage. A failure-to-engage the front clutch would cause 2nd & 4th to act as neutrals. (Ask me how I know...)
Perhaps he should try driving it in 3rd up to a speed where it would try to shift into 4th and see what happens?

[Bill Hanlon]

I agree Jack, but how will he get it to a high enough speed in 1st gear to get it to drop into 3rd with a "neutral" 2nd gear? Is there a way to force it to start in 3rd?

[Jack Gifford]

Not sure Bill. When I experienced a failed front clutch I had no problem accelerating enough for it to do a 1st-to-3rd shift. One main factor in up-shifting is the governor which rotates via the output shaft, so maybe driving on a downhill stretch of road would help?
No, there's no way to start in 3rd.

[Onestone]

I checked the rear end ratio and did some test driving yesterday.

For the rear end ratio I did like Bill suggested with marks on tire and driveshaft. Result: a little more than 3 driveshaft turns wer equal to two wheel turns. So a rear end ratio of 3:2 ??? No way this could be correct.
After fighting the in my head, I remembered that if one wheel is blocked, the other wheel turns double the amount with these old simple differentials. So the correct rear end ratio should be 3,08 : 1.

To make the proof I jacked the car on both sides and let my daughter turn the other wheel simultaneously to me The result was exactly as expected - 1 wheel turn was equal to 3 and a tiny bit more driveshaft turns. So the ratio is 3,08 : 1 for sure.

Next I did a little test drive with my daughter and let her note some speeds and revs in the two gears my Pontiac is using. I used my timing light as tach (because my other tach is not exact) and took speeds by GPS with my phone, so both values should be very precise.

Here is the math I did (European units, sorry ):

• In lower gear: 1820 revs at 30 km/h
• The math: 1820 revs per minute * 60 = 109200 revs per hour / 3,08 = 35454 wheel turns per hour * 2,23 meters wheel circumference = 79,063 kilometers per hour / 2,63 trans ratio of the 2nd gear = matches exactly the 30 km/h I measured

I did this with a few other speeds / revs I measured in the lower gear, it always matches the transmission ratio of the 2nd gear according to
https://over-drive-magazine.com/2023...urtesy%20lamps

I did this for the higher gear too:

• In high gear: 1650 revs at 70 km/h
• The math: 1650 revs per minute * 60 = 99000 revs per hour / 3,08 = 32142,8 wheel turns per hour * 2,23 meters wheel circumference = 71,6 kilometers per hour, matches a trans ratio of 1 : 1, which is the 4th gear

Did this also with a few other speeds / revs in the higher gear again, always comes to the 1 : 1 ratio of the 4th gear.

@Bill Hanlon: does your sheet confirm these two gears too?

So right now I am pretty sure it's the 2nd and the 4th gear that are used only. Also upshifting is a bit hard / rough, but this would be just a band adjusting issue I guess... I also checked the ATF fluid, it looks pretty clean and its not too little, its above the mark on the dipstick.

I hope this helps with the further diagnosis...

[Bill Hanlon]

Does reverse work? Reason I'm asking is the troubleshooting guide I'm looking at has a "Misses 1st, 3rd and reverse" problem that is "front planetary gear locked up" (sounds familiar - this is what Jack was talking about - although neither of you have mentioned reverse). There is a test listed to check for this problem.

[Bill Hanlon]

I agree with your metric test results.

I've tweaked my .XLXS spreadsheet to only show 2nd and 4th gears, show MPH and KPH and to use your RPM observations for each gear. The KPH calculations come out like your observations. That verifies that you are using 2nd and 4th gears.

Screen grab attached below.

If you want the spreadsheet send an email to [email protected].

[Onestone]

Reverse is working without any problems. So in total whats good:
- 2nd, 4th, R and N are working fine
- upshift from 2nd to 4th happens (I think it happens where the 2nd to 3rd upshift should be)

Whats not good:
- 1st and 3rd are missing completely. No upshift into these two, also no downshifts into these
- tested: also no downshift into 1st when rolling in LO (2nd) and flooring the pedal