Auto Trans Reverse Gear Thump

[Pepperbilly]

On a mission to eliminate the harsh shift into reverse gear when first selecting. Even before I restored this 412 wagon I remember this same thing happening years ago. Let me explain...

It is a worse thumping/clunking when the engine has warmed up. When the engine is started cold and I shift into reverse it does not exhibit this... it is actually very subdued, just like going into any of the forward gears. The transmission otherwise shifts and performs very nicely all other times. No slipping, just nice direct shifts. Maybe a bit abrupt but that is always how it's been since new. When the trans was out on the bench the fluids and internals were really clean, no burning smells or other abnormalities. New seals were installed and new fluid. The only thing that concerns me is the reverse gear harshness. Any ideas? Goldie is running so sweetly and with each gas fill up the fuel mileage is improving. Good stuff as this engine is breaking in.

Bill

[raygreenwood]

I eventually had this issue on every 411/412 I owned 2ith an automatic transmission.

My understanding is this:

When it's cold, both the fluid and the metals in the valve body are cold. Fluid viscosity is slightly higher and valve body piston movement is slightly more precise.

Once it's warmed up.....fluid thins slightly and metal expands.....and you get a pressure drop somewhere in the valve body. You also have to add in the thought process that all of the plungers/pistons/ball check valves etc.....are countered/controlled by springs. Add into this....some age related wear to the fluid pump.

If fluid pressure drops then the action of the springs may become excessive at some point in the stroke of the plungers.

This is why in the Bentley book there is a section on pressure testing with gauges and hoses while driving....hot, cold and revving to near stall with the brake on as diagnostic for the transmission valve body pressure.

It's interesting that there are several pressure ports but from memory only one or two are used. It's never explained what the unused pressure ports are used for....but it was probably for original construction tests.

The gist is that with age....usually a tweak to the main pressure needs to be done to help get smoother shifting.

This testing is a pain if you do not have the set up already ready to go. For diagnostic....you need a pair of long hoses or tubes and a pair of gauges so you can run them into the cabin to view them while you drive. Connecting the gauges to the ports while the engine and transmission are installed....is difficult. Last time I did this decades ago...I think I disconnected the right hand rear hanger bar and one upper mid mount bolt to get the transmission to lean over and down.

Once I got the gauges connected.....while the readings were not new car perfect....it did not really tell me any more than that the transmission is functioning "nominally".

That's a lot of what the testing is for.....to tell you if things are slipping or need a large adjustment or needs to be rebuilt.

I found that if your pressures are not indicated to be grossly low or high.....meaning its all functioning......and nothing needs to be replaced....and there is really only one pressure adjustment you can do without having to replace oil pump or valve body parts.......it's just as simple to drive and make minor adjustments to the vacuum modulator valve.

If adjusting the vacuum modulator has no effect.....make sure the vacuum modulator is functioning. If it is.....make sure it's getting proper vacuum. This is where things get interesting.

See.....as the engine wears with age....or gets modified....like a new cam....or gets re-tuned for performance or new parts....the vacuum signature can change.
In reality....most of the time, if all is well within the transmission.....what you are adjusting modulator/fluid pressure to correct....is changes in vacuum signature from the engine.

If your idle is higher than normal or lower than normal....you can get harsh shifting. It's both a fluid pressure change and a vacuum change.

The purpose of the modulator valve....is to keep fluid pressure .....nominal.....or at least at minimum requirements.....regardless of rpm.

From everything I can remember....the cause of this rough shift is low main pressure at low rpm or idle....most probably from a combination of basic mileage related wear to the pump and valve body bores.

I suggest first....adjusting the bands by the book. Then make sure your TB is operating properly and your idle is correct and stable. If you have to have idle not the same as stock....I would err Slightly higher than stock but not at the expense of vacuum. So in other words....try not to have the throttle propped open.

Then, if your modulator is functioning....meaning not leaking and holds vacuum....and a test with a hand pump shows that the rod inside moves....make sure it is installed correctly. There is a lock nut and an adjustment for it.

Once all of that is done....you need a long 4mm Allen wrench to adjust the modulator through the vacuum nipple.

So, start car.....warm it up completely. Then make sure idle and timing are correct. Put the car in reverse. If it's rough, put it in park, set the brake....turn it off for a second. Slide down underneath on the right side and pull off the vacuum hose. Insert Allen wrench, make an adjustment like 1/5th or 1/6th of a turn......clockwise I think.....to increase main pressure. Take notes for your adjustment. Re-install hose, start car....test....repeat.

Ray

[Pepperbilly]

Thanks for that detailed explanation Ray. When I had the trans out I did adjust the bands per recommended torque specs. So that has been done. Really, I am very happy with the trans performance outside of this reverse gear engagement. When the weather warms up around here I will be trying some of the things you have mentioned…I am also fighting a back issue right now so I have to take things slowly and carefully.

Bill