transmission Vacuum Modulator adj?
Thread Starter
Burning Brakes
Joined: Jul 2008
Posts: 876
Likes: 0
From: Canastota Ny
transmission Vacuum Modulator adj?
why im there job --- is it a good idea to change this out for maintenance reasons? and why / how are some adjustable? looking at napa they sell ones that are adjustable. do i want that?
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
I wouldn't replace it unless there was a failure. A failed modulator is not something that will put you on the side of the road. The trans will shift a bit differently, but it's not a critical failure. The adjustable modulators will affect the shift characteristics only slightly...a few mph either way depending on throttle. Hardly worth paying extra, but if it's the same price, knock yourself out.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,117
From: Crossville TN
Most original C3's had adjustable modulators..later years have THM350 which uses a throttle valve cable. If yours is functioning properly (light throttle has earlier, softer shifts; WOT has later, hard shifts), you should just leave it alone. If you need to replace it, get an original replacement, if you can find it. Most 'alternative' replacements do not have an adjustment built into them. Note: The amount of adjustment in a vacuum modulator is very small. Most folks wouldn't notice the difference with a full range of adjustment change.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
All THM 350's and 400"s use the vacuum modulator. The later 700-R4 uses the TV cable system. The 350 does have a cable, but it's for detent downshift only.
If you install a replacement modulator that is adjustable, you should not adjust it past 1-1/2 turns either direction from the factory setting. Any adjustment past that will stress the diaphragm in it and may cause it to fail prematurely.
If you install a replacement modulator that is adjustable, you should not adjust it past 1-1/2 turns either direction from the factory setting. Any adjustment past that will stress the diaphragm in it and may cause it to fail prematurely.
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
that modulator has about 15 full revolutions of adjustment. my kickdown was not working so well so i adjusted it in to about 5 turns from the middle setting. i then found if i was really on the gas and manually changed from 3rd to 2nd it would kick down to 1st automatically . as my car doesnt have huge amounts of vacum its a touchy setting to get right,in fact i couldnt get it right. now i have just wound it back 5 turns from the middle setting and if i want to hammer shift manually from 3rd to 2nd knowing i wont jump to 1st with the kickdown.even with all this adjusting it hasnt failed yet 
if i was in 3rd and punched it it would not always drop back to 2nd even though i felt it should have so now i like the manual shift better.
if i was in 3rd and punched it it would not always drop back to 2nd even though i felt it should have so now i like the manual shift better.
Last edited by gingerbreadman1977; May 12, 2009 at 09:46 PM.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
That's great! As I recall, Standard Motor products had an adjustable modulator that came with a sheet that said 1-1/2 turns off center was all their modulator would take without chance of failure. Sounds like you got either a different brand or got lucky.
