I recently had to swap out a bad trans from a 2000 silverado for one with about 100k miles on it. The “new” trans shifts fine and acts normal mostly, except that in 4th gear it has unusually high rpm starting around 55-60mph. It’s not a crazy difference but it is noticeable, with 60mph normally being around 1600-1700rpm now it’s at nearly 2000. Normally it would only reach 2000 at about 70mph. Is this a bad sign/can it be fixed? Another thing to note is that the trans is slightly overfilled at about 1/4” above the full line. Not sure if this matters though as I’ve heard you can slightly overfill it.

 

First I would look for a TCC apply. That would explain the raise of RPM's at highway speeds. Tapping the brake pedal slightly (enough to turn on the brake lights but not enough to apply the brakes) at 45mph should unlock TCC. Release of the brake should allow it to re-apply.
If you feel nothing with tapping the pedal. You will need to see if it is being commanded on with a scan tool. I have had several customers call back and ask about and extra shift about 45-50. It is usually caused by fixing a bad TCC solenoid they did not know they had. Your issue is likely to be just the opposite. You have a bad TCC on/off solenoid now and the old unit it was still good,
It's highly likely to be a fairly simple fix. Once diagnosed a simple universal solenoid change could fix it.
I like the Fitzall K77929U by TeckPak

https://www.teckpak-fitzall.com/spgm.cfm?id=K77929U

 

thanks for the reply. I tried unlocking and didn’t notice any differences. Also checked and there were no transmission codes. I’m wondering ,because the previous transmission was rebuilt ,if it had different gear ratios. Could something like this be the cause?

 

No they all have the same 4th gear ratio.
Let me simplify what Tranzman said:
When your torque converter locks, it drops the engine RPM by about 200-300 RPM. Since you are reporting 200-300 higher RPM, he has deduced that your torque converter is not locking. This could be due to a (usually) relatively minor transmission problem as he explained. IIRC, if the O-ring on the input shaft was bad or missing, there won't be enough pressure to lock the torque converter.

 

It’s highly likely the o-ring was bad. Is it going to cause temperature issues if I don’t fix the TC, or is it mostly a mileage thing on the 4l60e?

 

Because 4th gear is an overdrive ratio at .7:1. Without lock-up the transmission will heat up at approximately 1 degree per mile. so once at a full temperature it will take 65-70 miles and the transmission temperature will be at 260. These temps will burn up the transmission in short order.
You can drive forever in M3 range without lock up.
1st) I would verify the apply command and see if the PCM is requesting lock-up. If it is you will need to test and repair as required. It is possible for it to have a bad o-ring on the input shaft. It could be as simple as a bad TCC solenoid or a solenoid o-ring

 

Damn I remember the days of people roasting their THM700-R4 Transmissions in 4th-Gear when the Torque-Converter was not locking-up.

 

that is interesting info…why do the temps climb in overdrive and not in direct? How does the lockup (or lack of) play into increased temps BECAUSE your in O/D? I always assumed lockup was an efficiency/mpg thing, not really a “necessity” to be operational. I get its always slipping to a degree, but thats in every gear, not just OD.

 

Because overdrive or 4th gear is an overdrive ratio. The .7:1 ratio causes the converter to stall more or be less efficient. For example in a 1.1 ratio a 1600 stall converter when maintaining 60mph will be 95% efficient but the same converter in a .7:1 ratio will only be 80-85% efficient. This efficiency loss creates heat. The factory designed cooler-flow cannot overcome the additional heat that is built up by that loss. The OE design is built to make the best of both worlds and by using a lock-up plate it gives an overall efficiency of about 98%. That's why manual transmissions are more fuel efficient for a long time, they have full time lock up in all gears.
Now you modify things & can increase your cooler flow to overcome that, but that is a whole other Oprah show.

 

Awesome info. I knew there was a slip/loss. Never knew a non-functional lockup could actually overheat a trans, that is good to know. My experience is turbo trans and manuals. Learn somethin all the time, thanks!

 

This could be a good place to get into how the ATF actually enters and exits the Torque-Converter and Cooler Circuit...

How the Flow changes comparing Lock-Up to Un-Locked operation...

As well as how the TCC Apply Valve-Train can be modified (For example TransGo Valve-Train for Non-Lock-Up Torque-Converters) when using a Torque-Converter that does not have a Lock-Up Clutch... but can still have the Cooler Circuit maintain acceptable Temps.

 

We're listening.

 

What do you mean by this? I’m new to transmission terminology. What would it tell me?

 

With a bi-directional scan tool you can monitor whether or not the PCM is commanding TCC on or off. if it is commanding TCC on and don't feel it apply you will need to test the TCC on/off solenoid for proper operation. You can test it for proper 20-30 ohms rating. I test them for being open & closed.
I use my test box for that. It can be pined out with a suitable test connector and pins "E" & "T" of the 20 pin case connector. pin "E" is +12V and pin "T" is grounded by the PCM. I have had PCM's that are bad and will not unlock once commanded. We tested that by cutting the wire and running it to a ground through a toggle switch. The toggle switch allowed us to turn it on or off manually. When we hooked the wire back the issue returned. The PCM would show that TCC was released but actually did not. A new PCM fixed it.
It may be just as easy to replace the solenoid for $35-$40 (solenoid, pan gasket and some fluid). You should be able to re-use most of the fluid if drained to a clean pan & ran through a paint strainer.
All this is based on that you believe that the other transmission had TCC.

 

I would actually love to do it...
But it would be a larger project than I have time for right now.

I think I will make it the next project that I will do...
But also, all you guys have access to a free Online Manual in the Hyperlink below:
Hyperlink

Many of you are more than capable of doing this also!