Hard shifting in Reverse from Park, but not from Neutral to Park...

Hard shifting in Reverse from Park, but not from Neutral to Park...

Jim
Jim

September 2nd, 2011, 1:35 am #1

Just had my 1955 FOM tranny rebuilt and everything seemed great, but car shifts extremely hard going from Park to Reverse, but shifts smooth from Neutral to Reverse. Tranny shop thought it was valve body, but ordered a replacement and the exact same thing happens. We then spend a whole afternoon trying to adjust the kick-down rod from the accelerator linkage down to the tranny, but no difference. Shop now wants to put a pressure relief valve internally, to reduce the fluid pressure when shifting from Park to Reverse... Not sure if that's even possible...Any idea's on what's wrong here???
Quote
Share

Joined: July 17th, 2011, 7:07 pm

September 2nd, 2011, 4:37 am #2


A few things contribute to harsh reverse engagement.

High/Reverse clutch pack and reverse band concerns...

1. Wrong steel plates in the high/reverse clutch pack. The proper steel plates are "dished" and give a "cushion" effect when the clutch is applied.

2. Too tight clearance in the high/reverse clutch pack.

3. Rear (low/reverse) band adjustment too loose.

Here's how this works....

If the clutch clearance is too tight.... and the rear band too loose....

...when the shifter is moved to reverse, the high/reverse clutch applies first, before the low/reverse band gets a chance to apply.

This gets the mass of the gear train spinning inside the trans... and then the low/rev band applies, and has to stop that spinning mass. Result? WHAM! it hits hard in reverse.

If the low/rev band applies first, the gear train is held stationary, and then the cushion effect of having dished plates in the high/rev clutch pack can do its job, and give a gentle apply.

Common mistake is to adjust the low/reverse band to "factory" specs. Wrong, its too loose.

I usually adjust the rear band by "feel" and never have a problem with harsh reverse engagement.

Lacking the "feel" its probably safe to adjust the rear band like this....

Loosen the lock nut, and then screw in the adjusting screw, just until its snug. Don't torque on it. Just snug is enough.

Then back out the screw approx 3/4 of a turn. Hold the screw at that point and tighten the lock nut. Factory spec is 1.5 turns out from snug.

If you are nervous about it, back it out 1 full turn and lock it there.

If the builder put flat steels in the high/reverse clutch, or got the clearance too tight, the trans will have to be taken apart to put the dished steel in, and/or loosen the clearance.

Unfortunately the dished steels are getting hard to come by. The last one I rebuilt, I had a hard time getting them.

The dished steels are around, but might be a special order and take some time to get. (if you can find them)

The flat steels are still easily available, so if your trans needed new high/reverse steels, they may have substituted the flat ones.

The "internal pressure relief valve" sounds like snake oil to me... I've never heard of that in my 30+ years of building trannys.

Don't overlook idle being too high, that can also cause harsh reverse.
Last edited by galaxiex on September 2nd, 2011, 4:42 am, edited 1 time in total.
Quote
Like
Share

Joined: July 17th, 2011, 7:07 pm

September 2nd, 2011, 5:10 am #3


The other simple possibility that occurs to me...

The linkage is binding and when the shifter is moved from park to rev it "drags" the kickdown lever a bit and increases the pressure in the trans.

When going from neutral to rev its moving the other way... so the pressure increase doesn't happen.

Get someone to work the shifter and you crawl under and watch the linkage. The kickdown should not move at all when the shifter is moved.

BTW that "kickdown" linkage is more that just kickdown... its properly called TV or throttle valve pressure.

The TV function is to increase pressure in the trans as the gas pedal is pushed. If that pressure increase doesn't happen, the trans gets damaged.

On later trannys the TV function is controlled by the modulator, and the kickdown is just that, only a kickdown, no other function.
Last edited by galaxiex on September 2nd, 2011, 5:13 am, edited 1 time in total.
Quote
Like
Share

Joined: November 18th, 2004, 4:20 am

September 2nd, 2011, 10:25 am #4

Just had my 1955 FOM tranny rebuilt and everything seemed great, but car shifts extremely hard going from Park to Reverse, but shifts smooth from Neutral to Reverse. Tranny shop thought it was valve body, but ordered a replacement and the exact same thing happens. We then spend a whole afternoon trying to adjust the kick-down rod from the accelerator linkage down to the tranny, but no difference. Shop now wants to put a pressure relief valve internally, to reduce the fluid pressure when shifting from Park to Reverse... Not sure if that's even possible...Any idea's on what's wrong here???
"Shop now wants to put a pressure relief valve internally, to reduce the fluid pressure when shifting from Park to Reverse... Not sure if that's even possible..."


Other than cutting a spring to shorten it or install a shorter one there is no other way.
By so doing the pressure to the rest of the transmission will suffer resulting in slip.

Like galaxie said, direct steels must be dished to compensate for higher pressure needed in Reverse.

A clutch pack either too tight or too loose and a loose band will add to a harsh shift.

I like galaxies band adjustment method, old school technology, been there done that numerous times.

I have seen the detent ball/cylinder in the case get a groove from the rooster comb and cause the linkage to stick when pulled out of Park, resulting in a sudden application.

This is a very simple transmission and usually work quite well for the application. No bells and whistles.
Quote
Like
Share

Jim
Jim

September 2nd, 2011, 5:43 pm #5

The other simple possibility that occurs to me...

The linkage is binding and when the shifter is moved from park to rev it "drags" the kickdown lever a bit and increases the pressure in the trans.

When going from neutral to rev its moving the other way... so the pressure increase doesn't happen.

Get someone to work the shifter and you crawl under and watch the linkage. The kickdown should not move at all when the shifter is moved.

BTW that "kickdown" linkage is more that just kickdown... its properly called TV or throttle valve pressure.

The TV function is to increase pressure in the trans as the gas pedal is pushed. If that pressure increase doesn't happen, the trans gets damaged.

On later trannys the TV function is controlled by the modulator, and the kickdown is just that, only a kickdown, no other function.
Thanks so much for the insight... I'm printing this off, and will bring it to the ship. I don't think it's the TV linkage moving, because we disconnected the TV linkage at the accelerator linkage and pulled it up all the way (thus reducing the pressure), while the mechanic moved the shifter. Still bangs as hard as when the TV linkage is in place. No difference... They're now going to have to open this thing up, and check to see they have the correct pack inside. Can't THANK YOU enough!!! Please keep a look out for any future questions... Jim...
Quote
Share

Jim
Jim

September 2nd, 2011, 5:45 pm #6

"Shop now wants to put a pressure relief valve internally, to reduce the fluid pressure when shifting from Park to Reverse... Not sure if that's even possible..."


Other than cutting a spring to shorten it or install a shorter one there is no other way.
By so doing the pressure to the rest of the transmission will suffer resulting in slip.

Like galaxie said, direct steels must be dished to compensate for higher pressure needed in Reverse.

A clutch pack either too tight or too loose and a loose band will add to a harsh shift.

I like galaxies band adjustment method, old school technology, been there done that numerous times.

I have seen the detent ball/cylinder in the case get a groove from the rooster comb and cause the linkage to stick when pulled out of Park, resulting in a sudden application.

This is a very simple transmission and usually work quite well for the application. No bells and whistles.
Thanks so much for the insight... I certainly don't want them messing anything else up in the transmission. I'll print this off and give it to them. Hopefully we can exhaust all other avenues before butchering the tranny... Thanks again... Jim.
Quote
Share

kevink1955
kevink1955

September 5th, 2011, 4:27 pm #7

Just had my 1955 FOM tranny rebuilt and everything seemed great, but car shifts extremely hard going from Park to Reverse, but shifts smooth from Neutral to Reverse. Tranny shop thought it was valve body, but ordered a replacement and the exact same thing happens. We then spend a whole afternoon trying to adjust the kick-down rod from the accelerator linkage down to the tranny, but no difference. Shop now wants to put a pressure relief valve internally, to reduce the fluid pressure when shifting from Park to Reverse... Not sure if that's even possible...Any idea's on what's wrong here???
That's one of the best explanitions I have seen, realy helps me understand what can go wrong with these trannys. I love this place
Quote
Share

Jim
Jim

September 7th, 2011, 10:42 pm #8

The other simple possibility that occurs to me...

The linkage is binding and when the shifter is moved from park to rev it "drags" the kickdown lever a bit and increases the pressure in the trans.

When going from neutral to rev its moving the other way... so the pressure increase doesn't happen.

Get someone to work the shifter and you crawl under and watch the linkage. The kickdown should not move at all when the shifter is moved.

BTW that "kickdown" linkage is more that just kickdown... its properly called TV or throttle valve pressure.

The TV function is to increase pressure in the trans as the gas pedal is pushed. If that pressure increase doesn't happen, the trans gets damaged.

On later trannys the TV function is controlled by the modulator, and the kickdown is just that, only a kickdown, no other function.
Just an update... The shop admittedly installed flat clutch plates in the reverse clutch assembly, but said they were the only one's available and that they've rebuilt other T-Bird tranny's with the flat plates as well, and haven't had a problem. I was able to acquire 6 NOS "dished" or "cupped" clutch plates, which are being sent to me as we speak. The shop first wants to replace the pressure regulator within the tranny, thinking it is bad. If it's not, I will give the new plates to them, and tell them to install them. CAN'T thank you guys enough for the heads up on the "dished" plates. Would have never known about that!!! Will keep you updated... Jim...
Quote
Share

Joined: November 18th, 2004, 4:20 am

September 9th, 2011, 12:46 pm #9

Many transmissions have dished plates made just for that purpose.

Others have bleed holes and or check valves in pistons to help alleviate the sudden surge of higher pressure needed for proper application.

Some transmissions have one beveled or wavy plate in a drum to ease application.

If the steel dished plates aren't installed with the dishes in the same direction you will get a squawk when the piston is applied. Don't ask how I know.

Good luck with your venture.

If pressures are reduced or modulated that will affect not only reverse application but forward as well.
Which could lead to premature failure.
Quote
Like
Share

Jim
Jim

September 10th, 2011, 2:02 am #10

So I received the supposed "dished" plates in the mail today... Turns out, what I thought to be "dished" or "cupped" is not what came to me in the mail... These plates look flat from the naked eye, but if you put them on a flat surface, you can notice a slight offset, but barely anything of substance. Are these correct??? I was thinking I would receive plates that had an offset of about 1/8 of an inch or more. Something noticeable... But then again, I'm not a tranny guy and don't know what I'm looking for. So, my question is... Do I have the right plates for the reverse clutch assembly? Are the "cupped" or "dished" plates supposed to be clearly noticeable with the naked eye? Or is the "dish" ever so slight that it's barely noticeable? I was told these were NOS, and they are the correct ones for the assembly. Thanks guys for the help...
Quote
Share