In an attempt to figure out just what’s causing the gauges to go nuts, I looked into how the exhaust brake is hooked up. That’s because according to PacBrake’s documentation, the exhaust brake is supposed to be hooked up through the ECM.
Well, this one isn’t. This one wasn’t set up at all according to PacBrake’s instructions. There’s no air tank, no relays … it’s a very simple, direct connection from the compressor to the exhaust brake, and the compressor power is switched by the switch under the dash. That’s it.
How it’s supposed to be is, the compressor should be hot all the time, keeping the (missing) air tank pressurized, via a pressure switch. There’s supposed to be a switch on the gear shift that activates the exhaust brake solenoid. Both the pressure switch and the solenoid should be wired up through two relays.
Hooking it up the way it’s supposed to be lets you leave the exhaust brake on all the time (though you can turn it off with the shifter switch), and it will automatically turn off once the engine hits 900 RPM and back on when it drops below. As it is right now, I have to remember to turn it on when I’m decelerating down a ramp, and then back off again — otherwise the EGTs go through the roof. Also, I imagine the compressor has to stay on constantly, rather than cycling as it would with a pressure tank.
Well, the good news is this can’t possibly be affecting how the ECM works, so this is one fewer cause of the problem.