Possible Transmission Problem
Thread Starter
Team Owner
Joined: Feb 2006
Posts: 24,460
Likes: 7
From: sw Ohio
Possible Transmission Problem
I was out driving my '85 vette down the highway today and noticed something a bit odd. When in 4th gear overdrive, if I let off the gas the car feels like its engine braking and does so until my speed drops to about 50mph or so. I dont feel anything wrong while giving it gas at all. Its an automatic and the tranny was rebuilt about 10K miles ago by the previous owner. It shifts very smoothly through all gears. Any ideas on what could be wrong?
Team Owner
Joined: Jul 2003
Posts: 24,650
Likes: 8
Sounds like a bad Torque Converter Lockup sensor or something. Could be normal.
It is normal for the engine to try and slow the car down at high speed and you let off the gas.
The engine is trying to idle, and the tranny is still spinning really fast. The torque converter is designed so it isnt all that bad. But try it by letting of the gas, and then pushing it just slightly, so the engine is able to breath. It will slow down, but very slowly.
Imagine it like trying to run the engine at high RPM with the butterflies snapped shut...not possible, so it drags the engine down, thusly, slowing the car down, until it gets to a speed it can run at comfortably.
Try driving it, Run it steadily at 50mph, and while stil holding your foot on the gas, press the brake just slightly with the other foot. You should see the RPMS jump up slightly for a second, then go back down as you let off the brake. If it doesnt, you have a fualty Torque convertor Clutch.
The torque converter clutch allows the TC to lockup at speed for increased fuel economy. On an 85, it can lockup in any gear it sees fi. or so Ive read..
It is normal for the engine to try and slow the car down at high speed and you let off the gas.
The engine is trying to idle, and the tranny is still spinning really fast. The torque converter is designed so it isnt all that bad. But try it by letting of the gas, and then pushing it just slightly, so the engine is able to breath. It will slow down, but very slowly.
Imagine it like trying to run the engine at high RPM with the butterflies snapped shut...not possible, so it drags the engine down, thusly, slowing the car down, until it gets to a speed it can run at comfortably.
Try driving it, Run it steadily at 50mph, and while stil holding your foot on the gas, press the brake just slightly with the other foot. You should see the RPMS jump up slightly for a second, then go back down as you let off the brake. If it doesnt, you have a fualty Torque convertor Clutch.
The torque converter clutch allows the TC to lockup at speed for increased fuel economy. On an 85, it can lockup in any gear it sees fi. or so Ive read..
Thread Starter
Team Owner
Joined: Feb 2006
Posts: 24,460
Likes: 7
From: sw Ohio
I had a feeling it was either the TC or something to do with the overdrive. When I just lightly touch the gas it coasts normally. I just found it odd going downhill at highway speeds and still slowing down. I figured that I would ask since I dont know much about transmissions, except that an '85 has a 2000rpm stall converter. I'll try out your advice to see if its faulty, thanks guys.
Team Owner
Joined: Jul 2003
Posts: 24,650
Likes: 8
Ive always been told that on the actual TC, if it takes off hard, and doesnt make any noises, and doesnt move the car @ idle..the actual TC is fine.
The TCC is known to reak all sorts of havoc, including stalling, shuddering....but they do give you better fuel mileage!
The TCC is known to reak all sorts of havoc, including stalling, shuddering....but they do give you better fuel mileage!
Thread Starter
Team Owner
Joined: Feb 2006
Posts: 24,460
Likes: 7
From: sw Ohio
Well on the way home I tried testing the TC. I couldnt get any increase in rpms when tapping the brake at 50mph, however at 45mph tapping the brakes made the rpms increase from 1200 to 1300. I'm still thinking about taking it to have it looked at though.
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
tcc
if touching the brake pedal makes the engine go up in RPM, the tcc is disengaging. watch the rpm in 4th locked up, keep the car at like 60 mph, left foot steady on the accelerator, tap the brake with your left foot. if the RPM changes, it's ok. if when you let off the gas, and the car does engine braking, it's ok. if you touch the brake on deceleration, and the car kinda seems to coast, that's normal mine does that (86 coupe)
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
Thread Starter
Team Owner
Joined: Feb 2006
Posts: 24,460
Likes: 7
From: sw Ohio
Originally Posted by coupeguy2001
if touching the brake pedal makes the engine go up in RPM, the tcc is disengaging. watch the rpm in 4th locked up, keep the car at like 60 mph, left foot steady on the accelerator, tap the brake with your left foot. if the RPM changes, it's ok. if when you let off the gas, and the car does engine braking, it's ok. if you touch the brake on deceleration, and the car kinda seems to coast, that's normal mine does that (86 coupe)
