Too Late shift problem

als804

Member
41
14
8
Location
kimcheon, ROK
Actually these days my shift points are seriously late.

used to(some times a little early sometime a little late)
1->2 at near 2400rpm at near 23mph
2->3 at near 2500rpm at near 40mph.

these days( can shift at the exact same speed):
1->2 at 3400rpm at 33mph
2->3 at 3200rpm at 52mph

these days shift is too late.More than 3000rpm makes me feel hard and painful!
i checked all the vacuum lines but nothing problem found out.
some dude get it connected directly between vpump and v modulator of transmission.

you guys can you notice the problem?
 

TechnoWeenie

Well-known member
Steel Soldiers Supporter
1,138
381
83
Location
Nova Laboratories, WA
That Turbo 400 uses the vacuum pressure to determine shift points. The vacuum regulator is there for a reason.

Also, if the vacuum pump is hooked up directly to the modulator, you should get super early shifts.

The lines, if not replaced, are almost 40 years old at this point. Replace the lines... It's possible the lines were going bad, and the previous owner hooked it up directly to the modulator as a way to mitigate pressure loss on a leaky hose.

tm9-2320-289-20 197.jpgtm9-2320-289-20 198.jpg
 

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Prepper_Pig

New member
4
2
3
Location
Arkansas
I have the same problem after that crazy cold snap we had back in February. I couldn't find any problems with the lines (save a worn rubber boot on the line into the modulator) so I replaced the modulator and boot but my issue was not resolved. :(
 

Sganderson

Member
38
45
18
Location
South Carolina
I had the same issue, changed all of my lines because they were dry rotted.. There are two sizes. You need a bigger one that fits directly onto the vacuum pump and an adapter to step it down to a smaller. I discovered someone in error had placed the lines on backwards at the vacuum valve on the injection pump (which caused my transmission to shift late) There are two ports , Remember top goes to transmission modulator and bottom goes to the pump behind the engine.
 

als804

Member
41
14
8
Location
kimcheon, ROK
somebody told the vacumm pump should make -20inchHg Vacuum pressure, but mine couln't make very very weak vacuum. thanks to this nice forum, i got found the problem that is a vacuum pump problem. see number 3 picture shows a crack on kind of rubber surface inside the vacuum pump. somebody knows who sell the repair kit?


1.jpg
3.jpg
5.jpg
 

chevymike

Well-known member
Steel Soldiers Supporter
498
286
63
Location
San Diego, CA
When waiting new pump, you just can connect vacuum lines together and drive. Gear changes will be wery soft and un-noticeable.
Actually that does not work. If not vacuum available at the trans modulator (the part the hose plugs into on the trans), it will only shift at high RPM and very harshly. This is usually the tell tail sign you have a vacuum problem (anywhere from the pump, lines, modulator, vacuum switch).
 

vanaisa

Member
37
23
8
Location
Tallinn, Estonia
Chevymike, it works great - maybe i expaining it wrong (sorry, english is not my language) but when i had problems with my weak vacuum pump, my oldtimer transmission repair guy advised to discard this vacuum thingy on IP (again, cant say what it is, in my language it is vaakumkorrektor :) )

And it worked great,, drove about 2000mil before got new vacuum pump.
 
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