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  Mustangsandmore Forum Archive
  '64 1/2 to '68 1/2 -- The Classic Mustang
  Vacuum Modulator...shift points

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Author Topic:   Vacuum Modulator...shift points
FASTFORD
Gearhead

Posts: 105
From: USA
Registered: Aug 2002

posted 09-16-2002 10:26 PM     Click Here to See the Profile for FASTFORD        Reply w/Quote
I read somewhere on this board that you can adjust your transmission by turning a screw in the vacuum modulator or something like that...I don't even know what a vacuum modulator is. Please explain how to adjust my tranny like this...

thanks

ice'68
Gearhead

Posts: 127
From: Fresno, CA
Registered: Apr 2002

posted 09-16-2002 11:17 PM     Click Here to See the Profile for ice'68        Reply w/Quote
Would this also require taking the transmission apart or just pulling it?

Moneymaker
Administrator

Posts: 29200
From: Lyons, IL, USA
Registered: May 99

posted 09-17-2002 01:18 AM     Click Here to See the Profile for Moneymaker        Reply w/Quote
Before we go through all of this what exactly is the problem you are having? All adjusting the modulator will do is increase or decrease the speeds that your trans will shift at in the drive position. You do not have to remove or dissassemble the trans to do this. It is a very simple operation that is all that needs to be done.

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Alex Denysenko
Co-Administrator and Moderator/ non 65-66 Mustang owner sensitivity co-ordinator

NHRA/IHRA/SRA/MCA member
NHRA and IHRA SS/LA National Record Holder '00, '01, & '02
Fleet of FoMoCo products including 88 ASC McLaren Mustang #28
Part time secret agent license #0089
Professional Manwhore
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Quote: "I never met a magazine mechanic I liked."

Dave Gibson
Moderator

Posts: 10769
From: Norfolk, Virginia, USA M&M#166 MCA#47921
Registered: Aug 99

posted 09-17-2002 06:37 AM     Click Here to See the Profile for Dave Gibson        Reply w/Quote
If you have a C-4 tranny, the modulator is located toward the right rear. It uses manifold vaccum pressure to help shift your tranny. Follow the vaccum tube from the back of your carb to the modulator. You can't miss it.

Dave & Terri

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Poison Ivy (Dave's)
'65 Fastback Modified 289 4V, C-4, dual exhaust, styled steel wheels, rally pac, A/C
'02 Explorer XLT, Black, 3.73:1 limited slip, class III tow package!

indyphil
Gearhead

Posts: 3394
From: Senoia, G.A. USA
Registered: Jul 2002

posted 09-17-2002 08:40 AM     Click Here to See the Profile for indyphil        Reply w/Quote
I am dumb when it comes to my C-4.
Lets say I put on some performance mods that make my manifold vacuum drop some. Like a new cam or something.

What does that do to the shift points? With perf mods you want the shift points to be higher so you can make the most of the RPM's and horsepower. Wouldnt be very good if it shifts on you at 4 - 1/2 grand all the time.

Moneymaker
Administrator

Posts: 29200
From: Lyons, IL, USA
Registered: May 99

posted 09-17-2002 12:12 PM     Click Here to See the Profile for Moneymaker        Reply w/Quote
Generally, the lower the vacuum, the higher the shift point will be.

------------------
Alex Denysenko
Co-Administrator and Moderator/ non 65-66 Mustang owner sensitivity co-ordinator

NHRA/IHRA/SRA/MCA member
NHRA and IHRA SS/LA National Record Holder '00, '01, & '02
Fleet of FoMoCo products including 88 ASC McLaren Mustang #28
Part time secret agent license #0089
Professional Manwhore
The Barry of BarrysGrrl

Quote: "I never met a magazine mechanic I liked."

Timo
Journeyman

Posts: 40
From: Toronto,Ontario,Canada
Registered: Feb 2001

posted 09-17-2002 12:30 PM     Click Here to See the Profile for Timo        Reply w/Quote
Let's just says for Sh$ts and giggles that I wanted to increase my shifts to 5000 rpm's. Can I do that by just adjusting the vacuum modulator. How do you adjust it.. is there a screw there or something. Inquiring minds want to know....

FASTFORD
Gearhead

Posts: 105
From: USA
Registered: Aug 2002

posted 09-17-2002 09:18 PM     Click Here to See the Profile for FASTFORD        Reply w/Quote
How much do I turn the screw? and is there anything else I would need to do (mabye drain some kind of fluid so I can take the vacuum hose off)?

Dave Gibson
Moderator

Posts: 10769
From: Norfolk, Virginia, USA M&M#166 MCA#47921
Registered: Aug 99

posted 09-17-2002 09:22 PM     Click Here to See the Profile for Dave Gibson        Reply w/Quote
You should not get any fluid from the vaccum hose when you remove it from the modulator. If you do, you should replace your shift modulator. There is a small slotted screw in the modulator. Turn it left to make the shift points quicker or right to make them longer. It does not take much and you will need to take a test drive or two or three to get it adjusted just the way you like it.

Dave & Terri

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Poison Ivy (Dave's)
'65 Fastback Modified 289 4V, C-4, dual exhaust, styled steel wheels, rally pac, A/C
'02 Explorer XLT, Black, 3.73:1 limited slip, class III tow package!

ice'68
Gearhead

Posts: 127
From: Fresno, CA
Registered: Apr 2002

posted 09-18-2002 12:07 AM     Click Here to See the Profile for ice'68        Reply w/Quote
So when people describe "tire chirping" shifts, is it the amount of torque that produces this effect, or playing with the Vacuum Modulator (on an Auto)?

Moneymaker
Administrator

Posts: 29200
From: Lyons, IL, USA
Registered: May 99

posted 09-18-2002 12:26 AM     Click Here to See the Profile for Moneymaker        Reply w/Quote
Modulator has no effect on the shift firmness, only the timing of the shifts.

------------------
Alex Denysenko
Co-Administrator and Moderator/ non 65-66 Mustang owner sensitivity co-ordinator

NHRA/IHRA/SRA/MCA member
NHRA and IHRA SS/LA National Record Holder '00, '01, & '02
Fleet of FoMoCo products including 88 ASC McLaren Mustang #28
Part time secret agent license #0089
Professional Manwhore
The Barry of BarrysGrrl

Quote: "I never met a magazine mechanic I liked."

ice'68
Gearhead

Posts: 127
From: Fresno, CA
Registered: Apr 2002

posted 09-18-2002 12:32 AM     Click Here to See the Profile for ice'68        Reply w/Quote
Thnx for clearing that up Alex

I'm still trying to think of who told me that if you adjusted the vacuum modulator, you'd get a harder shift...

FASTFORD
Gearhead

Posts: 105
From: USA
Registered: Aug 2002

posted 01-06-2003 04:13 PM     Click Here to See the Profile for FASTFORD        Reply w/Quote
I looked on the back of my intake and the only thing I found was some kind of small brass looking box with one line going to the transmission and two other holes that are plugged...where is this screw to adjust shift points with?

Fastymz
Moderator

Posts: 22791
From: Reno Nv M&M #1240
Registered: Apr 2001

posted 01-06-2003 04:46 PM     Click Here to See the Profile for Fastymz        Reply w/Quote
quote:
Originally posted by FASTFORD:
I looked on the back of my intake and the only thing I found was some kind of small brass looking box with one line going to the transmission and two other holes that are plugged...where is this screw to adjust shift points with?

Follow the line to the trans thats where your
vacuum modulator.Pull that hose off you should see the screw when you pull the hose off.

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SCOOP
oddly obsessed with big scoops on little Mustangs

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Dave Gibson
Moderator

Posts: 10769
From: Norfolk, Virginia, USA M&M#166 MCA#47921
Registered: Aug 99

posted 01-06-2003 04:48 PM     Click Here to See the Profile for Dave Gibson        Reply w/Quote
Follow the line comming from your intake to the lower right rear of your transmission. There is a copper bell shaped thing, called a modulator that the line hooks up to. Disconect the line. You should see a nice little adjustable screw. Use a small flat tip screw driver to make adjustments, usually made by a quarter turn at a time.

Dave & Terri

------------------
Poison Ivy (Dave's)
'65 Fastback Modified 289 4V, C-4, dual exhaust, styled steel wheels, rally pac, A/C
'02 Explorer XLT, Black, 3.73:1 limited slip, class III tow package!

FASTFORD
Gearhead

Posts: 105
From: USA
Registered: Aug 2002

posted 01-06-2003 10:13 PM     Click Here to See the Profile for FASTFORD        Reply w/Quote
What are the shift points supposed to be on a c-4 behind a 4bbl 289? Like on full throttle and minimum throttle? What RPMs or Speed should it shift?
One more thing...Raising the shift points will result in a slightly harder shift b/c of higher RPMS right?

69stanger
Journeyman

Posts: 77
From: Northridge, CA
Registered: Aug 2002

posted 10-12-2003 04:30 PM     Click Here to See the Profile for 69stanger        Reply w/Quote
ttt

streetstang67
Gearhead

Posts: 397
From: SC, USA
Registered: Nov 2002

posted 10-13-2003 09:24 PM     Click Here to See the Profile for streetstang67        Reply w/Quote
good post...brought back from the dead

blzn1
Journeyman

Posts: 2
From: san diego
Registered: Nov 2002

posted 05-06-2004 01:18 AM     Click Here to See the Profile for blzn1        Reply w/Quote
having problems shifting, while in drive it wont shift, but if I downshift to 2nd it shifts or if I start in 1st then to 2nd it shifts fine, but in drive it wont shift, could it be my modulator? or amount of vacuum coming from the carb? forgot to tell what i have. 1968 cougar GT 390/c6 with edelbrock performer rpm, 4bbl holley. Any suggestions?
TIA

hardhat
Journeyman

Posts: 69
From: summerland, ca
Registered: Feb 2004

posted 05-06-2004 01:34 AM     Click Here to See the Profile for hardhat        Reply w/Quote
My kickdown doesn't kick down when I think it should. I've played with the screw on the carb but nothing.. Do I have a modulator and could this be somthing to look at? I have an FMX in mine. Thanks

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73' conv. 351C 2v mild cam, exhaust, fmx

whiteknight289
Gearhead

Posts: 1391
From: Wheaton, IL, USA
Registered: Mar 2004

posted 05-06-2004 08:04 AM     Click Here to See the Profile for whiteknight289        Reply w/Quote
You should have a "S" shaped tube a little smaller than a piece of conduit coming off the side of the carb that connects to a little arm on the shift lever of the transmission. At almost full throttle a lever on the side of the carb should push backwards on the tube, causing the tube to push down the lever on the side of the trans. If you can get the carb to push the rod sooner you will have a quicker kickdown.

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