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Please help. Transmission occasionally going out of gear - speedo bounces

Started by jbrockminer, 13 September 2015, 07:22 PM

jbrockminer

I heard nothing at the transmission when pressing the kickdown button.

I did however do three road tests moving the pressure control rod forward (since I was already going that direction).  The last test was a total of 7 turns out, it didn't flare during the final test!

I want to resume testing tomorrow,  and see if these results hold up before getting too excited.

???

jbrockminer

My excitement witholding was warranted, the 'ol 300sd behaved as if I had adjusted nothing.
Early shifts, flaring, out of  4th gear too.

I tried turning the rod adjustment to an absurd 14 complete revolutions - no effect.

I'm at a loss.

jbrockminer

If you disconnect the pressure control rod at the throttle area,  is the rod,  if actuated by hand, supposed to move freely?

daantjie

Yes it should gently move in the fore aft motion but there must be a light tension on it as it works against the spring in the control pressure valve inside the valve body in the transmission.
Daniel
1977 450 SEL 6.9 - Astralsilber

jbrockminer

Performed a few "SMOKE TESTS"
Curious if any of my results mean anything to anybody.

Plugged off side port of VCV result: no difference.

Pulgged off top port (to control modulator) result: a "brisker" 2-3 shift ... still flares in 4th, drops to 3rd, out of gear, I let off throttle, then it's back in 4th.  This occurs around 50-60 mph.  Also speedometer erratic (drops off 15 mph or so and bounces around) preceeding gear flaring.

Plugged vac line to modulator,  as well as top port to VCV result: same as above.

I know this testing is not considered in good taste, but vacuum testing equipment is on its way, and I'm somewhere between frustrated and impatient.




jbrockminer

During a recent test, after adjusting the modulator a little (with no noticeable effect), I realized that the transmission does not flare or slip if the accelerator is being pushed generously.  If I lay off the throttle,  say in order to maintain 50 mph, it will go to neutral after 20 or so seconds, in which case I let off the throttle - it goes back to 4th. 

Big throttle - little to no vaccum signal sent to transmission = no problem

Light throttle - vacuum signal gets sent to transmission = problem

If the modulator is only there to receive information from the VCV, and that information contains details on shift quality (hard or soft), what might be causing the "flare" at light throttle position?


daantjie

Are your shifts firm right now under big throttle openings? If they are soft then you can crank the vac mod more clockwise. Under full throttle the shifts must be crisp and quite firm, much firmer than US made cars from the same era. There is a pin inside the vac mod which pushes into the valve body. Higher vacuum sucks the pin outwards, away from the transmission more than low vacuum. Firmer pin pressure inwards towards the transmission translates into firmer shifts which is what you want.
Daniel
1977 450 SEL 6.9 - Astralsilber

jbrockminer

Thank you for responding.

The shifts are indeed firm.

I have to wonder if the vac mod is working,  seeing as the vehicle behaves the sane whether or not it is hooked up.

However, even if it weren't functioning,  I'm not seeing how it could be responsible for the 4th gear slippage. There was a drop of fluid under the cap both times I made adjustments.

I read somewhere tonight that a  300td owner had a similar problem,  and that it was (I'll paraphrase) "a certain valve and spring that drops pressure in 4th gear had failed".

I don't want to go there yet!







coldwar79

I wanted to update this thread as I have acquired this car and am the new owner.

How often have you stumbled across a forum with little or no resolution and the thread dead ends? This one is for the archives.

The transmission output shaft nut was loose causing the back part of the transmission to loosen, releasing governor pressure and dropping gear. The Speedometer bounce is attributed to the ring gear on the tail cone becoming loose ( from the loose shaft nut.

Replaced nut and torqued to spec, staked and issue has been fixed.

Also, the downshift switch or wiring is indeed defective. The downshift solenoid was bad, so there was no 1st gear start on takeoff. Replaced solenoid and 1st gear starts are back. Downshifts are not back so more troubleshooting to follow regarding the switch under the pedal.




daantjie

No kick down can also be control pressure not adjusted properly.
Daniel
1977 450 SEL 6.9 - Astralsilber