t-350 Transmission problems!

[moose]

Greetings All,

I need help with my trans. It will not up shift on its own. If I drop it into low I can get it to manually shift thru all three gears. At a stop it will down shift by its self. Vac line is hooked to modulator.

A little recent history.

My starter and flexplate self destructed. Do to the narrow x member in my chassis I cannot drop the trans. I split the trand and pulled the engine to replace the damaged flexplate. I unhooked the torque converter and slid it back into the trans . I then removed the engine. I never removed the torque converter from the trans. I hooked everything back together and when I test drove it it will not shift.

Prior to splitting the engine/trans before the strater took a crap it ran well and shifted great.

Engine is a 350 Chevy and the trans in a turbo 350.  Worked before not now What did I screw up??

Thanks
Tim

[Crosley.In.AZ]

as i posted to the HAMB..

check vacuum hook up to the modulator

[phat46]

Maybe you pinched the vac line to the modulator when you reinstalled the engine.

[moose]

I will double check it but I only have a short length of rubber tube at te h Modulator it it was connected. I will remove and replace it to be sure...

[wayne petty]

do you have a hand vacuum pump... the kind used for brake bleeding..

unhook the vacuum hose that feeds the tube to the modulator...

test that with the hand vacuum pump...   0 leaks.,..  it should hold perfect vacuum...    no leaks...

pull the hose off at the modulator...    pump the vacuum pump.. it should NOT build any vacuum... this proves the line is not crushed...
 you should also hook your vacuum pump or gauge to the end of the hose where you unplugged it from the modulator.. see if you get a nice strong vacuum signal...

sometimes the vacuum hose at the modulator swells with the atf and heat...   i have found the vacuum fitting into the back of the intake manifold filled with carbon ... blocking the flow..

the steel tube that hooks the modulator to vacuum is steel.. as it has to transmit rapid vacuum changes instantly...

i had another customer with a car... quit shifting... i did not have the time of the space to run his car up to swap the modulator..  seems the trans shop i sent him to swapped the modulator .. and still no shifting.. they quoted him a reman transmission.. he brought it back to me..
and waited..   late that evening..  when i got to it.. i found the smog guy had redone the vacuum hoses on the top of the motor..   hooked the modulator line to a green plastic tee...   when i tested the modulator for vacuum signal... i got a really slow response.. almost none...  seems the green vacuum tee.. was installed directly over the exhaust crossover.. the heat from that softened the plastic.. the vacuum then collapsed the bottom of the tee.. blocking the vacuum..  causing a no shift...

i am not telling you to do anything.. just explaining the theory...

it is just the first thing to check when transmissions stop shifting..

one more thing... i am taking that the kick down cable is hooked up and properly adjusted...  not stretched out to the max and locked in position...

[moose]

Well it shifts again. Never trust visually checking a vac hose. Thanks to Wayne joging my memory I dug out the mitivac. Forgot I had it. Not holding vacuuum. Replaced the tube at teh modulatoer and shifting fine again...Kzoo here I come.

Could someone please explain what the modulator function is.

Thanks for all the suggestions

Tim

[wayne petty]

the modulator modulates one of the line pressures in the transmission..

changes the shift points in relation to engine load.....

what...

under load with the throttle open wider.. there is reduced  manifold vacuum...  so the modulator is not retracted as far...  the line pressure  is held higher to keep the transmission from upshifting...

when the load goes away...  the manifold vacuum increases... this pulls on the diaphragm harder.. allows the line pressure to drop more.. so the transmission will shift..


if you ever looked inside the small end hole of vacuum modulators... you will see a small screw head.... takes a flat blade screw driver... like the tiny green ones electronics stores sell...  some people just use a ground chunk of welding rod or coat hanger...   you can adjust the screw a quarter turn at a time to change the shift points to match the manifold vacuum that cam and manifold swap has changed...   i forget which way to go...

hope this answers your question...

oh... and different weights for the governors are available for some applications.. really changes the shift points..

i am glad your transmission is shifting now...

for a buck or buck and a half..   transmission parts suppliers have little modulator Elbow vacuum hose sections.. that should withstand the ATF..

i am about to climb under my 70 ranchero... swap a new modulator in..  its leaking.. into the vacuum line...  the engine blows billows of light blue smoke when i start it and while its running.. the warmer ... the larger the cloud...

i drove it to the gas station just around the corner to air up the tires... got BOO's !!!! from people on the sidewalk...