[BlackNova]

As mentioned in a previous post, I had the Nova out to the track for the first time, and it ran very well. I managed to get a 13.80 @ 101.5 out of it in spite of constant tranny problems - it would not go to third except for two of the ten runs I made. Those times it did go were in Drive, and the shift occurred immediately after it went to second. It has a B&M Shift Kit, Pro-Matic 2 shifter, and all adjustable stuff is right on (kickdown, shift cable). It will go to third in conditions other than WOT. I can feel that shift on the way back to the pit area. The modulator has good vacuum thru a hard line, and I turned it out four turns from the bottom as a start point. Any help will be very much appreciated. Thanks in advance.

 

[Big Dave]

Governor weights or spring change made on the last rebuild? Line pressure?

Big Dave

 

[BlackNova]

Governor springs were changed. I used the ones in the shift kit. Line pressure is something I'll have to test. What should the pressure be and under what conditions?

 

[Big Dave]

I have experienced similar problems which could seem to be shift related. Both were the result of bad vacuum modulators or incorrectly installed shift kits. It would appear that either one, or both in combination could throw off the base-line pressure referenced by the detent valve. The line pressure is controlled by the vacuum modulator and the front pump (the pump makes the pressure, and the vacuum modulator applies it) which determines your shift points. You could have it incorrectly adjusted if it is an adjustable version, or it could be leaking (check for AFT leaking out at the vacuum port).

But your tranny most likely has a governor issue. Up-shifting trouble is the classic symptom. The governor controlled line pressure has a direct relationship to the output shaft, or road speed. The faster the out put shaft turns, the more the governor increases the line pressure (the pump puts out about 200 psi but not all of that is applied to the clutches). The governor must overcome the modulator pressure to shift. If the governor is stuck or sticking (a problem with older varnish filled trannies that have not been thoroughly cleaned prior to rebuild), it will cause a late, or delayed shift. The tranny sees a lower line pressure, and thinks the road speed is not that high, so it does not shift like it should. But, when you lift off the throttle, the modulator pressure eases up, and the low governor pressure now can overcome it.

Big Dave

 

[BlackNova]

I have experienced similar problems which could seem to be shift related. Both were the result of bad vacuum modulators or incorrectly installed shift kits. It would appear that either one, or both in combination could throw off the base-line pressure referenced by the detent valve. The line pressure is controlled by the vacuum modulator and the front pump (the pump makes the pressure, and the vacuum modulator applies it) which determines your shift points. You could have it incorrectly adjusted if it is an adjustable version, or it could be leaking (check for AFT leaking out at the vacuum port).

But your tranny most likely has a governor issue. Up-shifting trouble is the classic symptom. The governor controlled line pressure has a direct relationship to the output shaft, or road speed. The faster the out put shaft turns, the more the governor increases the line pressure (the pump puts out about 200 psi but not all of that is applied to the clutches). The governor must overcome the modulator pressure to shift. If the governor is stuck or sticking (a problem with older varnish filled trannies that have not been thoroughly cleaned prior to rebuild), it will cause a late, or delayed shift. The tranny sees a lower line pressure, and thinks the road speed is not that high, so it does not shift like it should. But, when you lift off the throttle, the modulator pressure eases up, and the low governor pressure now can overcome it.

Big Dave

Big Dave - Thanks for the tranny lesson. I'll check out the governor and the modulator. Hopefully I can ID a simple problem. Is there a preferred modulator starting point with respect to how many turns out from the bottom. When I installed the new modulator, I bottomed the screw, then backed it out four full turns. Enough? Too much? Thanks.

 

[Big Dave]

As to the initial modulator settings, I do not know. I would have tried it as received, and then adjusted it to match your manifold vacuum situation (cams effect what vacuum your motor can make; which is why an adjustable vacuum can is used).

If all else fails read the installation instructions that came with the part.

Big Dave