th400 not shifting til 4500rpm

68SS502 · Oct 31, 2009

Hey guys, it's been awhile unfortunately...anyhow, I have a 68SS with a ZZ502 crate motor and a Rossler racing built TH400. I've been trying to troubleshoot a shifting problem I've been having, my car WILL NOT shift into the next gear(1st to 2nd, 2nd to 3rd) unless I hit 4500RPMs PERIOD. No matter what, full throttle, part throttle, it WON'T shift until it hits 4500RPM. I have replaced the vacuum modulator, both rubber lines(top and bottom) and even sent the governor back to Rossler to have him check it out. Still not upshifting until it its 4500, and as soon as it drops to 60MPH in 3rd it kicks down into 2nd gear. I dropped the pan tonight and check everything in there and all is tight.
Then I noticed that the kickdown wire goes to the switch panel I have mounted underneath the dash(the way it was when I bought it 9 years ago). Question is, where should the kickdown be wired to? And what am I missing that will let my baby return to normal shifting during normal driving??? It gets really aggravating when I'm trying to drive it sensibly through town yet it won't upshift until 4500RPM...

I've talked with Carl Rossler at Rossler transmission and he says it must be a vacuum issue but I have replaced the rubber lines up top and at the modulator. I haven't gauged it at the modulator but at idle you can feel vacuum with your finger, I have to think it has something to do with the kickdown being wired to a switch under my dash. Does the kickdown work on a constant 12v or only when the pedal is at WFO?

My Baby has been pretty much useless to me all summer cause she's undriveable everywhere except on the open road and I want to get her squared away so I can give her the kind of use she deserves!

Thanks to all in advance for any and all advice.

Mike
68SS502

Vette468 · Oct 31, 2009

Mike , I read this and I was like "Odam" , Ive been there.. with a fresh motor no less,, it was a while back ... ten years .. I remember fearing for my motor because it was NEW and my tranny,, "because it was MY tranny

... alls I can tell you is it ended up being a vacuum thing for me... too maybe try a new hardline to the tranny "vacuum" and keep the rubber to a minimum,,, good luk bud :thumbsup:

I didnt see anything wrong with mine but I think I just tried a different one and it started shifting...:yes:

wes69ss · Oct 31, 2009

disconnect the kick-down wire at the trans and drive it. if it still shifts late ,replace the solenoid at the valve-body. the solenoid should be about $10 at a trans shop. the kick-down wire goes to a switch at the carb. should have 12 volts going to the solenoid at wot only.

keithl1967 · Oct 31, 2009

X2...if the switch is "incorrect", it iwll mess withthe proper shifting...

I had this happen about 12 or 13 years ago...turns out thw switch went bad...take all of the kickdown wiring off the tranny and take it for a ride...I'll bet she'll shift perfectly, then you know you'llneed a new kickdown switch...

What's happening, is the transmission is "reading" that it shuld stay kicked down, and is only "upshifting" when it hits a "protectiono circuit" to cause the tranny to automaticcaly upshift for self-preservation...the way it was designed...

Also, the kickfdown shuld run up to a switch that is actualted by the throttle linkage on the carb, and shoudl not run to a switch inside the vehicle (if I read that correctly)...

Everett#2390 · Oct 31, 2009

You can unplug the vacuum line, cap off the source of vacuum, then hook up to the disconnected line a hand vacuum pump and pump down to 20" of vacuum.

Go for a ride and see/feel if shifting improves. If no improvement, exchange vac modulator, be sure the little rod is there between the v/mod and valve body. Grease will hold it inoplace when installing new v/mod. If there is improvement, not enough vacuum, or too stiff on v/mod for vacuum supplied from intake.

Solenoid gets ign power. Switch is closed at WOT for forced downshift. Trans will run fine without solenoid, just need to manually downshift when WOT is achieved.

I too, have a Rossler TH400 in my MCSS and it shifts smoothly at very light throttle, all in by 20 mph. Any more than 1/4 throttle, and 2nd gear chirp happens along with a hit in the kidneys at deeper throttle positions.

jakeshoe · Oct 31, 2009

It's probably not a vacuum problem.
It's also not the governor.

Unhook the kickdown wire from the trans and see if it fixes it.

If it does not, drop the pan, pull down the kickdown solenoid.

If you install a gasket on the solenoid that uses a rubber disc to seal, you will get the exact symptoms you described.

Fred Ficarra · Nov 1, 2009

X2

Try to find a swich to mount on your carb that looks like this;

68SS502 · Nov 2, 2009

Well, turns out that the kickdown wire for some reason was wired to one of the 3 toggle switches I have mounted on a panel under my dash(Ignition, electric fan and ? until today...) I've always got in my car, turned the 1st and 3rd switches on before starting, knowing that one was the ignition and not KNOWING what the 3rd one was. I left the 3rd switch down this evening and took her out for a test drive and drove and shifted perfectly

VERY happy that I can now use my baby whenever I want and not just for "higway trips" but also VERY

that I've wasted this whole summer troubleshooting things that tirned out to be a simple wire and not using my car all summer.

Thanks for the help though guys, I should've posted this months ago and I wouldn't have wasted the whole summer.

On an additional note, this is what I get for not installing my own tranny after I had it built...

Cheers!:beers:
Mike
68ZZ502

keithl1967 · Nov 2, 2009

Glad to hear you are back in business...enjoy the ride!

Fred Ficarra · Nov 3, 2009

Love happy endings but not ones that take so long. And your location Mike, is a bad one for driving your baby this time of year huh?

68SS502 · Nov 9, 2009

I'm just south of Pittsburgh so usually it's not the best time of year to have her out but if the weather stays like it was over the weekend(70 yesterday)and they're saying 70 and sunny today I'm gonna make up for lost time! Long as there's no salt, cinders or gravel on the road I'll keep driving her!:thumbsup:
Cheers!:beers:
Mike

th400 not shifting til 4500rpm

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