Ok, I used to go down the highway at 90mph at 2000 rpm's, I recently noticed that 90mph was at 2500 rpm's, and 2000 rpm's was about 83mph 7mph for 500 rpm's doesn't sound right for a 2.59 rear end in overdrive. When I let off the gas at 90 the tach quickly drops back down to the 2000 mark where it use to be. What could this be? I'm thinking a few lost fins on a torque converter maybe??? :cry I've also noticed a loss in high rpm power, but that could also be from some other stuff I'm doing with my car. But the low end still seems to be alright. Any help would be greatly appreciated.

 

:confused:

My '95 w/ 2.59:1 gear ratio turns an even 2000rpm at 75mph with OEM size Goodyear GS-C's mounted. This is in OD with torque converter locked up. When the pump in the transmission was going bad the converter would not lock up and it would turn about 2200 rpm for 75mph.

When I was reading your rpm levels in the post I was thinking you had a six-speed...




[Modified by Lone Ranger, 12:57 PM 11/8/2002]

 

Indeed it sounds like your TC may not be locking up; 2000rpm is close to the stall speed for a stock TC. Once the TC is locked up lost/bent fins shouldn't be an issue. Note that on an L98 the TC isn't locked up during the engine warm up period.

You can check the TC lockup command signal from the ECM on ALDL pin G to pin A with a test light or DMM. There should be no voltage when the lockup command is being sent to the lockup solenoid.

To test the condition of the TC, check the stall speed by doing a brake stand to see where the rear wheels break loose.

 

Do the 4L60's have a lock-up converter? If so I wasn't aware of this I'll have to try a break stand and see what happens, but my brakes aren't very good and I think the torque from the engine would pull the tires free before a lock up would occure.

But if the lockup is gone is there a way to fix it?

 

I'm sure the TC is still the lockup type.

If the lockup is gone you need to find out if the ECM is not sending the lockup signal or the TC isn't obeying that signal.

If the ECM isn't sending the command signal, there must be a faulty sensor (you might pull any codes); it the TC isn't obeying the signal either the signal isn't getting to the solenoid, the solenoid is failing or the TC clutch is shot (not likely I should think).

 

Well, the tires spin at about 1500 rpm. But won't the engine eventually make enough torque to spin the tires regardless of when the lock-up is set at. Kind of like not having the clutch pulled out all the way but still able to spin the tires.

So if it is the lock up in the converter that is bad would that mean a new converter or is it repairable?

 

I would think that if the TC had deteriorated (like bent fins) the stall point would rise. This you would see when doing the brake stand. With my current 2,000rpm TC I can brake stand up to about 1,900rpm; whereas with my stock 1,500rpm TC the tires borke loose at around 1,400rpm.

If the lockup solenoid is bad that should be replacable, if the TC clutch is bad it might warrent replacing the TC (I'm not sure if the clutch is rebuildable or replacable separately).

But first see if the ECM is even sending the lockup command.

 

Ok..I can see your confused. I'll give a little background on converters for you.

The torque converter works by coupling the engine to transmission rotation by fluid shear of forcing trans fluid through vanes inside the converter. At a certain RPM difference between the motor and transmission, the fluid shear force will overcome the rotating resistance of the trans/drivetrain and force the transmission input shaft to turn. This greatly increases torque transferred from the motor to the transmission, and the RPM at which it occurs is called stall speed.

What you have done is powerbrake the car to find the stall speed of your torque converter, of which yours stalls at 1397 RPM by design ( all vettes from 87 to the mid 90's stalled at 1397 RPM. )

The converter also has a clutch element built in, called the "Torque Converter Clutch" or TCC for short. It's purpose is to eliminate any RPM loss between the engine and transmission to improve economy. It is not a heavy duty clutch element, so it is only applied in a light load to coasting condition, where you spend a majority of time driving. Any heavy acceleration will force the computer to unlock the TCC element, to prevent damage to the converter clutch itself.

Doing your power brake will in no way tell you anything about the lock-up clutch, since it is not activated until you reach about 30 MPH at light load.

The most common symptoms of TCC failure is no lockup at all, or a rough chugging sensation when it is locked up, or a lugging the motor down sensation when slowing the car down if it fails to unlock.

How to tell if it is working is to do a staedy cruise at about 40 MPH, in overdrive, and while maintaining a steady gas pedal position, lightly press the brake pedal and notice the engine RPM flare up a few hundred RPM's, then release the brake pedal and notice the RPM drop again. Once you learn the feel of the TCC, you can determine if a TCC problem has occurred, but I can tell you , at 80-90 MPH, you will never feel the TCC clutch kick in or out, as the vehicle speed puts the engine significantly over stall speed of the converter in the first place, so the percentage of slip is soo small, it's impercievable to the driver.

You can smell the trans fluid and if any clutch is failing ( including the TCC ) you will smell a strong acrid odor. If it smells like new clean trans fluid, you have no converter clutch, or any other clutch for that matter, problem.


[Modified by BBA, 6:34 PM 11/8/2002]