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Topic: Advise on modulator valve?

in Forum: C3 Driveline Components


Advise on modulator valve?

Posted: 7/31/06 7:01am Message 1 of 6
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Edgewood, KY - USA
Joined: 10/26/2003
Posts: 339
Vette(s): 1981, Black/Black, Engine bumped to about 400HP.Ext/int mostly stock.

Hi guys,

Had the auto tranny rebuilt this winter, then I went in and added a Transgo shift kit and changed the weights on the governor for a later shift.  You may remember a previous post of mine where the gear stripped on the govenor.  Well, I could not find any obvious reason for the gear to strip.  The govenor was NOT siezed in the bore.  In fact, the governor lands and the bore look great.  So, being a little paranoid, I just ordered a new govenor, instead of replacing the gear.  This has solved that problem, but the tranny was now up-shifting at very low RPM, as in about 1500 - 2000 RPM.  The shifts were also pretty soft, which shocks me, since I installed the shift kit.  Just playing around, I disconnected the modulator valve, just to isolate how the govenor operates independently of it.  With this setup, I have exactly what I'd like during spurts of spirited driving: upshifts at 4500 RPM and it shifts fast and very firm.  Of course, without a modulator valve, that's that way it shifts ALL the time - even in casual driving, it won't upshift until 4500 RPM.  Do I need a different modulator valve that is calibrated very differently or what do you guys think?  If needed, any suggested part #s would be great.

Thanks!




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Advise on modulator valve?

Posted: 7/31/06 7:47am Message 2 of 6
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Duncanville, TX - USA
Joined: 11/8/2003
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Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas
John, if you look into the fitting that the vaccuum line goes on(on the mod.), you should see a small screw. Turn that screw clockwise to make the shifts later/firmer. You can use a small pocket screwdriver to do this, then reconnect the vac. line.
The adjustment screw will only go so far, but it will make a big difference. You really don't want to drive it without the vac. hooked up very much. This maxes out the line pressure, and can damage the pump gears/pump body, as well as other parts.

If, by chance your mod. does not have the screw in it, then someone has removed it, or it is designed for use in a diesel application. If you need to replace the modulator, check with a local trans shop first.(Ask for a "dual red stripe" mod.) The parts they have are better than what you can get at most parts houses.

There are several modulators that will actually fit the 350 trans, but they won't all work right in it. The 350/400 trans are the same. Some people have used the mods for the older 300 trans...they don't werk very well at all, but they do fit. Make sure they know you need one for a 350.


Joel Adams
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Advise on modulator valve?

Posted: 7/31/06 1:23pm Message 3 of 6
Former Member
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Edgewood, KY - USA
Joined: 10/26/2003
Posts: 339
Vette(s): 1981, Black/Black, Engine bumped to about 400HP.Ext/int mostly stock.
Thanks Adam.  I knew about the adjustability, but thought I was once told it wouldn't make a significant difference (which it would need to in my case.)  But, I'll give it a try this week and see what I can get out of it. Thanks again!.


Advise on modulator valve?

Posted: 7/31/06 1:35pm Message 4 of 6
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Duncanville, TX - USA
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Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas
John, I forgot to ask, when you installed the T-go shift kit, did you put the spring(usually a small,pink spring) behind the mod. valve in the case? If so, remove it. It is really only needed if you have a low vac. situation, such as a race engine, or a really radical cam. If you use a spring behind the valve on a normal, street driven car, it can make the shifts earlier than they need to be. Just another idea...

THIS is what I mean...Adams' Apple38929.5742476852


Joel Adams
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Advise on modulator valve?

Posted: 7/31/06 3:59pm Message 5 of 6
Former Member
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Edgewood, KY - USA
Joined: 10/26/2003
Posts: 339
Vette(s): 1981, Black/Black, Engine bumped to about 400HP.Ext/int mostly stock.

Adam,

The shift kit I used only dealt with the valve body, nothing with the modulator, so I havent had the modulator out.  I'm running with whatever the Tranny rebuild shop installed there.  Before I got your last response, I tightened the modulator adjustment all the way and tested the car.  It did increase the WOT shift point to 3500 RPM and the shifts are firmer, but I'd really like to get to 4500 - 5000, so later this week I will see if it has the spring, remove if it does, and retest.  I'll let you know how it goes.

Thanks and good luck with the job search!




Advise on modulator valve?

Posted: 7/31/06 4:20pm Message 6 of 6
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Duncanville, TX - USA
Joined: 11/8/2003
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Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas
John, in order to raise the WOT shift points, you may need to tighten the adjustment on the kickdown cable, at the carb. I don't happen to have any pics/diagrams for a 350, tho.
The kickdown cable should have a slotted end that slips onto a stud/peg on the carb linkage, under the cable for the accelerator. If you follow that cable back to the bracket, you will see an adjustment spot behind the bracket. If you installed the shift kit yourself, and still have the instructions for it, it will show how to adjust the cable. If not, I will look and see if I have them, and post them for you. Also, the service manual, if you have one, should give the adjustment procedure.

As far as changing the weights/springs on the gov., that will effect the shifts for all driving styles, whether hot & heavy, or "granny". Sometimes the trade-off is acceptable, sometimes it's not.Adams' Apple38929.6828125


Joel Adams
C3VR Lifetime Member #56    

My Link


(click for Texas-sized view!)
             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

in Forum: C3 Driveline Components


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