4l60e problem - stuck in 3rd(?)

My brother's 96 firebird suddenly started acting up today. When he started it, it was stuck in 3rd or possibly 4th. It would rev to 1500 and really lug and struggle to get moving.

At first I thought, "hey, it's probably electrical... I think they default to 3rd if power is disconnected"

Well the transmission is getting power. I checked the shift solenoid resistances, and all are good, ~23ohms. I tried grounding the solenoid wires at the PCM and got audible clicks from them, so I think they're working.

Next, I thought maybe the range switch was saying the wrong gear, but that checked out okay.

So I'm at a loss right now. I'm thinking maybe a shift solenoid "B" failure, causing it to stay in 4th gear when it should be in 1st. What other tests can I do to be positive, and how do you get to the shift solenoids? Or are there any other suggestions on what could be causing this? NO codes, btw, except P1670, which was there before this started today.

TIA,

Any "SES" lights? Have you had it scanned?

Frank
CPT

Yes, the SES light has been on since he bought it. :D

But the only thing related to the transmission is the P1670 "transmission component slipping" code. That has been there since he bought it, and was there before this started happening.

No new codes, surprisingly. Should I try to drive it a lot more to force it into getting some new codes? I'm really surprised there isn't any.

Are you sure that's no an 1870?

Frank
CPT

oops! you're right. Sorry, I was going from memory

Before you go swapping a bunch of stuff, you might want to have this thing ran with a scanner with data capabilities. My guess is, you'll see 3rd gear commanded, and you'll find it's in "limp mode". The 1870 is most commonly caused by the TCC regulator valve wearing the bore (in the valve body) out. Sonnax industries sells a valve kit to replace the worn one, but the valve body needs to be reamed to install it.

Frank
CPT

TCC regulator valve? As in for the torque converter lockup control?

Why would a TCC valve cause it to run in limp mode? Is there some logic to this that I'm missing?