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Old 08-27-2006, 02:10 PM
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Blower motor T-400 late shifting

Searched the DB, this issue has been beat to death. I've got a SBC 383, 8-71, 2-BG 750 blower carbs and T-400 w/ a transgo manual/auto kit.

I'm getting my vacum at the carb mounting plate, under a 1" spacer, it runs 17.5 in/hg @ idle, 22.5 at 1500 rpm and 13.5 @ 2500. Everything is new. This thing will not shift 1-2 or 2-3 until 4200 rpm manually or automatically. One oddity, in reverse it seems to hang or drag a little wanting more throttle to move at first then all is fine. When I blip the throttle vacum drops but of course immediately comes back up. I tried pulling vacum at both sides of the carb base but get fuel from both.

I spoke to Transgo, checked for constant voltage at the detent, none, even disconnected it. It's the old style, enclosed switch, coils not visible and I'm using the metal gasket as directed. I've tried two new modulator valves, no difference. Adjusted them out, same thing. I haven't tried running it without the modulator hooked up as I hear it creates high pressures, not good. I haven't gone back to the governor as it was free when I installed it.

I'm considering a B&M governor recalibration kit or trying to make this thing full manual if that would help, but really don't want that, plus don't know how to do it without buying another shift kit.

Any help? Thanks, Larry

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Old 08-27-2006, 06:16 PM
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Pull the actual modulator valve out behind the vacuum modulator and be sure it isn't binding.
If all seems OK, pull the governor out and check the gear and weights.

You've already covered the detent, but you might replace the solenoid for giggles.
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Old 08-27-2006, 06:18 PM
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Also,
The superchargers and vacuum modulator don't work well together. The carb base will show vacuum even when the motor is making alot of power under heavier throttle.
The best setup to retain the auto function is to use a mechanical modulator.
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Old 08-27-2006, 06:55 PM
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Thanks Jake, I can see that with my gauge running it.

What I don't understand is why won't it shift when I tell it to manually?? I'm obviously moving the shift valve into position for the 2nd and 3rd gear shift.

If I were to make whatever changes needed to go full manual, what do I do with the modulator and how about the high pressures not connecting it?

I'll look around for a "mechanical" modulator, I'm curious how they work - any leads to a manufacturer?

Thanks, Larry
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Old 08-27-2006, 07:56 PM
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I am building something very close to what you have. I am going with the full manual valve body. Doing away with all the modulators and stuff, just have to shift it.
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Old 08-27-2006, 08:00 PM
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Yeah, I'm thinking in that directin as well. Do you know about the issue of high line pressures eliminating the modulator?

Crosley - can you tell me the fix for my shifting problem or how to make this full manual, correctly? Please see my earlier posts.

Thanks, Larry
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Old 08-27-2006, 08:16 PM
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Larry,
For anyone including Tony to tell you how to fix the shifting problem, they would have to diagnose it. There are 3 major factors that will cause a late shift. One is the modulator. If the modulator or valve is seeing low vacuum (shouldn't be in your case) it will cause the trans to stay in downshifted (modulated downshift) mode.
This is why I told you to check the actual valve itself, it could be binding in the bore causing higher line pressures.

Next is the governor, the governor detects roadspeed by being driven off the output shaft. If the governor gear is worn or broken, the governor cannot sense road speed and will cause the shift valves to stay in the downshifted position. Same deal if the weights are sticking.

Last is the detent solenoid. If the detent solenoid is sticking it could cause it to stay in the downshifted mode even with no input electrically.

You will have to do some footwork yourself and get back to us on the findings and one fo the builders here can keep you on the right track. We don't have a way to determine the cause without either us or you looking into the trans. We can only tell you what can cause it, and there is usually more than one possibility.

If you go to full manual and eliminate the modulator you will have full line pressure at all times. This isn't necessarily a bad thing in a boosted application but will cause more pump and sealing ring wear.
If you prefer the auto shifting you need to figure out what's wrong, swapping valve bodies may "fix" it but doesn't determine the cause.

You can get the mechanical modulators from GM or from TechPak, I installed one on a TH400 I built for a turbo'd application a few weeks ago.

It uses a 700-R4 style TV cable to sense throttle position replacing the vacuum modulator with a mechanical one.

It won't shift when you command it manually because the "auto" mode is staying downshifted, so you have to get above the rpm where the auto mode will allow it to shift.

It would only take a half hour to check the modulator valve for freedom of movement and the governor for gear condition and weight movement. This is where I would start.
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Old 09-01-2006, 08:59 AM
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A few items:

I no longer moderate on this board for about 2 months now. I visit mayb once a week. No need to explain , it is what it is.

your tranny shifts at 4200 rpm every time?? This is near the default RPM shift point of a solenoid detent. That makes me curious.

I would plug off the detent solenoid feed hole.... plug the separator plate hole ( I use aluminum welding rod, peen it over with a hammer) You can plug the large hole or the tiny hole under the v-body it's self. Or place a small piece of gasket under the solenoid to block the hole. In a forced induction setup , you don't need the solenoid IMHO.

In my blown appications ( 6-71 small & big blocks) I always pulled vacuum from under the carbs. Tranny worked great. Crank the adjustment screw in fully is what I did...... YMMV

Check the simple stuff first as mentioned by Jake.

Next....... reverse bind up; could be several things. Could be a cross leak in the rear band servo feeding 2nd gear from a worn bore area. Few people seem to check these bores for wear from the sealing rings or a cracked piston.

Next... manual / auto matic v-body: kinda a misnamed item. You have full manual control ONLY on down shifts , NOT upshifts.

To convert to full manual forward shift pattern is easy I can show if needed.

Two items : you can leave the vac modulator on in full manual mode vacuum hooked or not.

IF you wish to remove the vac modulaotr and plug with common availible aluminum plug w/ O-ring you MUST mod the boost valve and plug a hole that feeds the boost valve in reverse. If you do not do this reverse pressure goes sky HIGH with a blocked modulator valve!
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Old 09-01-2006, 07:12 PM
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First, I'm sorry to hear you're not moderating any longer, whoever is dong it now has some very big shoes to fill.

I bought a full manual Transgo-S kit but have an issue with the rear servo modifications. I removed the inner spring, seat and e-clip and installed the six washers as directed. I cannot get it even close to meeting the mounting surface with the cover, so started removing washers. Long story short, to get the cover to mate I removed all but two (leaving the original washer). Now I need to remove two additional to allow for band clearance. Heck, that puts me back to the stock configuration only with no spring and seat! I even removed the O-ring to ensure I was pushing it all the way up.

Anyone installed one of these kits? Am I missing something?
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