I have been noticing a problem. At first it was bad and I did some work on the ****** (new gaskets and seals to stop some leaks) but I also got a new vacuum modulator. I'm not sure as to how to set it except I believe turning the pin clockwise makes it shift later and counter-clockwise... earlier. If this is backwards let me know. Anyways, my problem is when I slow down it seems to be shifting sparatically back and forth between 1st and 2nd. When I go up a hill, or even on a straight away, when it hits second gear it kind of boggs out or stalls. It REALLY seems as though it is shifting into second gear too soon. If it shifts to second gear too soon and the rpms are too low will it cause this bogg down or stall? should I try adjusting the modulator more before I ***ume the gear is burnt out?
thought you could run a hot wire to a switch down to the ****** and make the 400 turbo shift different?
Does this do this only in D range? Have you tried it in L2 and observed the same issues? This may narrow it down to what range it jumps around. Sounds like either a valve body or the 1st gear one way clutch is sticking. Governor may also be an issue, but it would show up on 2-3 shift too.
Where is the vacuum source that goes to the modulator coming from? You want it coming from the intake manifold itself or the right port on the carburetor. You want full manifold vacuum, not ported vacuum. This might cause some weird shifting while driving.
Check your modulator bracket. It may be installed backwards. Also make sure that you have only 1 o-ring on the modulator. Sometimes you remove the modulator and the o-ring stays in the case. Then the new modulator gets installed with a new o-ring while the old one is still in the case. Double o-rings will push the modulator out and cause a drop in pressure.
On a TH400 that should only affect the kickdown for p***ing gear I think. TH400s used a switch on the throttle linkage instead of a kickdown cable for this function. The vacuum modulator definitely affects shift timing. If it is adjustable I think you'll need to use an allen wrench down in the fitting where the vacuum hose connects. Try giving it a turn or two in one direction or the other and see if it makes any change. I'm not sure but I think you'll want to turn it clockwise to delay the upshifts.
Be sure the vacuum line from the intake manifold and the modulator is in good shape. Another thing you might want to check is the governor. I have seen springs break on the weights. It's on the right side of the trans towards the rear,4 bolts hold the cover on and once the cover is removed the governor slides out. Good Luck
Thanks for the responses. I checked the governor and it's not sticking. Everything runs fine except second gear at low rpms. I guess I'll try adjusting the modulator to shift later and see if that works... I REALLY hope second gear isn't toast.
My t-bucket was missing the hose and the vacuum modulator was Maul adjusted. So got a new hose, new fitting on the manifold, turned in the screw in and out. When I had found the center point on that adjustment screw, I drove it for a week adjusting 1/4's until I had a smooth ride. In the begining i was standing on the brake to keep it at the light. Now it sits and loudly rup - rups. Pisses off the Pious Prius owners.
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