Transmission Retarder Lever location? Position?

[Stevie Wander]

Confirmed! It's the lever I thought it was, as described above. It's pretty obviously plumbed into the control palel, now I looked at it again.

With NO THROTTLE, and the lever in the UP position

Activate the transmission retarder via the switch on the transmission control panel, driver's left side. The indicator light will NOT come on until the retarder is acting on the transmission

The transmission retarder will engage if the BRAKE treadle is depressed OR

You can set the drag by lowering the lever partway or all the way. If the indicator light is on, the system is actively slowing the vehicle down.

For obvious reasons, you shouldn't hit the throttle when the transmission retarder operating, that would be like pushing the gas and the brake at the same time.
You can keep the system activated, with no drag set on the lever (indicator light off) and it will only assist the brakes. But don't forget to turn it off in the flats or you'll just be adding stress to the system every time you hit the brakes.

Keep an eye on the Transmission Retarder Temperature gauge, it heats up surprisingly quickly. I got mine up to about 260 degrees; the manual says 330 is tops, and that tracks with what's on the gauge.