autobox changing up early?
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
autobox changing up early?
I adjusted my throttle cable today at the carb, took car for a spin, I found it changing up early, about 4000rpm, normaly I can get upto 6000+ rpm before it changes up with foot to floor, anyone any idea,s what the problem is? and also can someone explain what controls the upshifts at full throttle?, ive got a th350, with shift kit
cheers
dave
cheers
dave
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Who made the shift kit? Any of the 350/400 trannys see two things for upshift. Line pressure, and governor pressure. Few other things involved but those are the big players.
Make sure your detent cable is adjusted correctly. The vacuum modulator also plays, but at WOT there's no vacuum so it's really more for part throttle shifts.
If your shift kit was setup correctly- all the springs, holes and valves for a particular application, WOT shifts should be real tire-chirping.
Make sure your detent cable is adjusted correctly. The vacuum modulator also plays, but at WOT there's no vacuum so it's really more for part throttle shifts.
If your shift kit was setup correctly- all the springs, holes and valves for a particular application, WOT shifts should be real tire-chirping.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
I'm struggling with this one. If you had accidentally knocked off the vacuum supply to the vacuum modulator on the tranny, the shifts would come later [not sooner].... Unless you have a T.V. system on that tranny (throttle valve). Perhaps you messed up the adjustment on the T.V. linkage.
All automatic trannies set their shifts from a governor signal (pressure varies with RPM); but that shift signal gets modified by the vacuum modulator or throttle valve (you have one or the other of those systems), based on the amount of throttle you set. [The more throttle, the later the shift.] And since all you've adjusted is the throttle cable, I doubt that the governor is your problem--unless you are just unlucky and it failed at the same time by coincidence.
All automatic trannies set their shifts from a governor signal (pressure varies with RPM); but that shift signal gets modified by the vacuum modulator or throttle valve (you have one or the other of those systems), based on the amount of throttle you set. [The more throttle, the later the shift.] And since all you've adjusted is the throttle cable, I doubt that the governor is your problem--unless you are just unlucky and it failed at the same time by coincidence.
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
I'm struggling with this one. If you had accidentally knocked off the vacuum supply to the vacuum modulator on the tranny, the shifts would come later [not sooner].... Unless you have a T.V. system on that tranny (throttle valve). Perhaps you messed up the adjustment on the T.V. linkage.
All automatic trannies set their shifts from a governor signal (pressure varies with RPM); but that shift signal gets modified by the vacuum modulator or throttle valve (you have one or the other of those systems), based on the amount of throttle you set. [The more throttle, the later the shift.] And since all you've adjusted is the throttle cable, I doubt that the governor is your problem--unless you are just unlucky and it failed at the same time by coincidence.
All automatic trannies set their shifts from a governor signal (pressure varies with RPM); but that shift signal gets modified by the vacuum modulator or throttle valve (you have one or the other of those systems), based on the amount of throttle you set. [The more throttle, the later the shift.] And since all you've adjusted is the throttle cable, I doubt that the governor is your problem--unless you are just unlucky and it failed at the same time by coincidence.
I did change the jets in my carb(running to rich), but I dont see how that would make any differnce
So what can you fit or adjust to give me longer shifts at full throttle, ie my peak power is around 6700rpm or so(big cam, etc), so im looking to reach 7000rpm, before it shifts up.
cheers
dave
Last edited by corvettedave383; Nov 23, 2008 at 06:38 AM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
As I stated before, your symptoms are a bit confusing. Just working on the throttle cable shouldn't have altered the shift schedule. And changing the kickdown linkage would only cause a downshift to be earlier or later...not an upshift. You didn't say whether your tranny used a vacuum modulator or T.V. system to regulate shift schedule (based on throttle position); that is important to know. Therefore, I would assume that you have T.V. linkage and that it was also altered, such that the tranny no longer 'thinks' you are going W.O.T. when you do so. Do you have a Chevrolet Chassis Service Manual for your [year] car? If so, it should have some info in it about adjusting the T.V. linkage.
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
As I stated before, your symptoms are a bit confusing. Just working on the throttle cable shouldn't have altered the shift schedule. And changing the kickdown linkage would only cause a downshift to be earlier or later...not an upshift. You didn't say whether your tranny used a vacuum modulator or T.V. system to regulate shift schedule (based on throttle position); that is important to know. Therefore, I would assume that you have T.V. linkage and that it was also altered, such that the tranny no longer 'thinks' you are going W.O.T. when you do so. Do you have a Chevrolet Chassis Service Manual for your [year] car? If so, it should have some info in it about adjusting the T.V. linkage.
so I must have a vacuum modulator system
if say Ive adjusted the throttle cable incorrectly, and carb isnt opening the 2nd's fully, could that cause a issue?
cheers
dave
Last edited by corvettedave383; Nov 23, 2008 at 12:29 PM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Yes, it could. If your throttle is limited, the engine will be limited on power because of it. When the engine gets to that new 'max power point' it will just stay at that speed and intake manifold vacuum will increase...then the vacuum modulator will help it shift early. In essence, you aren't really putting the engine at wide-open-throttle....only partial throttle. And it will shift early, under those conditions. Get your throttle adjusted correctly so that the carb butterflies are fully open when your foot is to the floor (not just when your hand can open it fully under the hood). That adjustment may need to be made at the pedal linkage as well as at the carb.
