TH400 Modulator questions
 

So, last time I had my GTO out, one of the few drives I've made in it so far, it seems to shift pretty early.

It's got the original OEM modulator, which when tested with vacuum seems to operate as it should.

I was wondering, if I remember correctly, I think I put an extra fat, or possibly two o-rings on it because it was leaking a little bit. Would moving the modulator out (away from the trans) by the thickness of an additional o-ring effect the shift points? Seems like the vacuum pulls the plunger to the outside, so if the plunger was already moved out a tad by additional or fatter o-ring it would imitate a larger vacuum signal.

Also, would a tired spring on an old OEM modulator create shorter shifts?

I have an NOS Borg Warner unit on the way just in case.

Modulator
 

Agreed with two O rings I would say the valve would then think modulator is retracted
Farther into the body of modulator then it really is. That would mean early and softer shifts.

Gerry.

Yes, 2 O rings would make it shift slightly softer and earlier

Thanks gents.

Governor inner weights (and springs) control the upshift. Lighter weights and weaker springs for later upshifts.

Vac Mod has only a little effect on shift time, more like Line Pressure increase for shift firmness.

Was the 2nd Post but i see that my Post didnt stick.

Well the shifts are bit soft too, I've got the standard Trans-Go performance kit, the least "performance" of the bunch I think. The whole trans is fresh, I rebuilt the entire thing, and it probably doesn't have 30 miles on it. So there could be a whole slew of issues I guess :)

get an adjustable one... ? its only for part throttle shifts.. not wot as i understand it.. i use adjustable on my th400 , has allen key

The NOS BorgWarner unit I purchased as a backup is identical to the OEM unit with the exception that the adjustment is not spot welded like the OEM unit.

And yes my soft early shifts are at around 2/3 throttle.

OK, it's been awhile. Been playing more with non-modulator transmissions lately. Maybe I've forgotten something.

WHY would an extra O-ring on the modulator have ANY effect on it's operation? The O-ring(s) slide into the case, they don't change the position of the modulator within the case. The modulator retaining fork-and-bolt end up in the same place with two O-rings as they do with one O-ring.

I have put double O-rings on every modulator, and every TH400 filter extension since Fido's great-great Granddoggie was a pup.

On this same subject...I did an engine swap and the edelbrock performer only has 1 tap for vacuum at the rear. I need it for Brake booster AND I need vacuum for my TH400 modulator. How can I split that off or is there an alternate vacuum source for the smaller 1/4" line that leads to my transmission? I understand transmission vacuum cannot be sourced from the carburetor. Thx