C4 - Vacuum Modulator Valve - Vintage Mustang Forums

 
Thread Tools
post #1 of 9 (permalink) Old 01-10-2003, 03:58 AM Thread Starter
FAQ
Senior Member
 
Join Date: Jun 2002
Location: USA
Posts: 217
What does this valve do, and what would be signs of it going bad?

I don't know if this related, but most of the time when I mash the pedal, my C4 doesnít down shift when in 2nd or 3rd gear. I don't think it's related; so if you could comment on both topics I would appreciate it.

Thanks
FAQ is offline  
Sponsored Links
Advertisement
 
post #2 of 9 (permalink) Old 01-10-2003, 05:27 AM
Gone but never forgetten
 
johnpro's Avatar
 
Join Date: Jul 2000
Location: Roseville, California
Posts: 25,239
The modulator valve is connected to an intake manifold vaccum source, and is used to control the throttle valve in the transmission. The strenght of the vaccum tells the transmission how fast the engine is spinning, and the transmission uses this information in conjunction with the speed of the governer valve to determine when to shift the transmission. If it were going bad, your transmission would not shift well on up shifts OR down shifts.

Massing the pedal to the floor and having it down shift to 2nd is usually a function of the kickdown lever. At slower speeds, the throttle valve (remember ... controlled by the vaccum modulator) will kick it down at lower RPM's but will not kick it down at higher ones as it thinks your going fast enough to be in 3rd, so sees no need to make a shift. The kickdown arm is mounted on the side of the transmission where the shift linkage attaches (and where the neutral start switch is), and is a mechanically controlled device. In other words, you must make sure that your kick down linkage is all in tact, and adjusted correctly. When it is, stomping the pedal to the floor will pull on the kickdown arm, which in turn opens a valve in the valve body and causes the downshift.

HTH.


VMF Jedi Knight who's spirit will be with us forever.

Best Auto Insurance | Auto Protection Today | FREE Trade-In Quote
johnpro is offline  
post #3 of 9 (permalink) Old 01-10-2003, 11:30 AM
Senior Member
 
Join Date: Jun 2001
Location: Huntsville, AL
Posts: 203
Pull the vacuum line that attaches the modulator to the intake and look for tranny fluid.

In Thrust We Trust.
'69 Sportsroof
'94 F150 4.9l
'06 Mustang GT
fastback69 is offline  
post #4 of 9 (permalink) Old 01-10-2003, 01:44 PM
Senior Member
 
Join Date: Jul 2000
Location: Central California
Posts: 10,518
What does this valve do, and what would be signs of it going bad?

Controls throttle pressure, which interacts with governor pressure to control shift point and firmness in relation to engine load and throttle position. When it goes bad, shifts get late and harsh.

when I mash the pedal, my C4 doesnít down shift when in 2nd or 3rd gear.

Johnpro covered downshift linkage details; in the interim, you can do like I've always done (I don't use downshift linkages)....downshift manually when in a passing or heavy acceleration situation.

Good luck!

[color:white]camachinist

http://forums.vintage-mustang.com/signaturepics/xsigpic18756_2.gif.pagespeed.ic.3vzsIY6Mah.jpg
Visit my VMF lingo page
Visit the Ford engine torque specs page
Visit my MySpace Blog
camachinist is offline  
post #5 of 9 (permalink) Old 01-10-2003, 02:56 PM Thread Starter
FAQ
Senior Member
 
Join Date: Jun 2002
Location: USA
Posts: 217
Hmm, Shifts are harsh at times even with out much throttle. They could be late to. I seem to notice it when the engine/trans is cold, but seems to be fine when it warms up.

I wish there was another way to tell.
FAQ is offline  
post #6 of 9 (permalink) Old 01-10-2003, 04:44 PM
Senior Member
 
Join Date: Jul 2000
Location: Central California
Posts: 10,518
Disconnect vacuum line at modulator, plug line, and re-test, comparing to previous observations. Post results.
Also, note any ATF leakage from hose nipple on modulator...

Get back to us...

http://forums.vintage-mustang.com/signaturepics/xsigpic18756_2.gif.pagespeed.ic.3vzsIY6Mah.jpg
Visit my VMF lingo page
Visit the Ford engine torque specs page
Visit my MySpace Blog
camachinist is offline  
post #7 of 9 (permalink) Old 01-10-2003, 04:44 PM
Senior Member
 
Join Date: Mar 2001
Location: Santa Monica, California
Posts: 1,030
For a start, make sure the Type F fluid level isn't low.



Driving a stock Mustang is like driving a sofa. I'm Real Good at driving sofas!
66conv6 is online now  
post #8 of 9 (permalink) Old 01-10-2003, 06:46 PM Thread Starter
FAQ
Senior Member
 
Join Date: Jun 2002
Location: USA
Posts: 217
I wonít be under the car again till tomorrow at the soonest, but your saying to plug the vacuum line from the intake, drive the car, and compare?

Last time I was under there (last weekend), I noticed there was fluid around the modulator. There's fluid all over the place though; the pan gasket is cracked. Iím Going to replace it this weekend.
FAQ is offline  
post #9 of 9 (permalink) Old 01-10-2003, 08:45 PM
Senior Member
 
Join Date: Jul 2000
Location: Central California
Posts: 10,518
Remove rubber hose at modulator; plug hose (bolt works); test as indicated in my prior post.

Post back the results of your driving and visual inspections....

[color:white]camachinist

http://forums.vintage-mustang.com/signaturepics/xsigpic18756_2.gif.pagespeed.ic.3vzsIY6Mah.jpg
Visit my VMF lingo page
Visit the Ford engine torque specs page
Visit my MySpace Blog
camachinist is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Vintage Mustang Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in









Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are Off

 
For the best viewing experience please update your browser to Google Chrome