So I went ahead and got a replacement pigtail from APE: same problem. I figure there are two possibilities:

1) the replacement is also bad.

2) the switched voltage is incorrect. Can someone verify that the behavior I'm seeing at the harness is correct? The TPS switches on an 8v signal?

Also, does anyone know the maximum recommended voltage of the solenoid?

 

I get the same voltages, but the 8v throttle open is on the yellow wire. 12v on the green wire regardless of throttle position.

 

Problem solved, for now...

Yeah, that's actually what I get (I mixed up the colors in the previous post). So it appears that the new harness I was sold is also bad. Thanks for checking this.

I found a workaround: I went to Fry's and bought a high-current transistor (NTE11) and put the yellow wire on the base so it selectively applies the 12V line to the solenoid. It works perfectly for now. My only concern is that without knowing the full specs, I have no idea whether I'm applying the right amount of voltage to the solenoid. The solenoid measures 10 ohms, so it's currently getting 1.2A. I want to make sure this isn't going to burn it out.

I took apart the circuitry on my original harness and it looks like an amplification circuit (i.e., increasing the current from the 8V line and using the 12V line just to power the circuit), rather than the gated circuit that I built (i.e., the 8V line activates the switch, allowing the 12V line to reach the solenoid).

If anyone has any advice on this, that would be much appreciated. But for now, I've put 100 miles on it and it seems to be functioning fine (though I'm a bit nervous that it was way too easy to replace this full circuit with a single transistor).

 

The ecu uses a transistor to do this