Engine brake partly works

Probably not but there maybe a problem within the jake wiring harness!

 

It's usually the harness, there's a connector on the side of your engine that everyone has trouble with, the pins in it are frail. A quick diagnostic at a shop should show you where the problem is.

If you have a LOT of miles on the engine and the compression brakes are rarely used it might be a mechanical problem, could also be time to PM the compression brake, it's usually done as part of an overhead adjustment.

 

David, I assume this is a DDECIII engine.
There are three solenoids that each operate the Jakes on two cylinders.
Low is 3 & 4. Orange wire 562 from ECM terminal T3.
Medium is 1 & 2 and 5 & 6. Red wire 561 from ECM terminal S3.
High is all six cylinders.
The ECM monitors the resistance of the solenoids and will log a code 31 if the resistances are outside specs.
To request Jakes the dash switches ground out two input wires, usually wires 583 from K2 and 979 from K3. My guess would be that you have a break in one of these two wires somewhere between the ECM and the switches.
With the key on and switches off, you should have voltage on both wires(5v?).
Turn the switch on and the voltage will drop to 0.

 

Is this correct?
With both switches off there are wires on the back of both with 4.5v.
The wire on the back of the 2cyl switch drops to zero when the switch is turned on, but the wire on the 4cyl switch stays at 4.5v.
The wire on the back of the 4cyl switch drops to zero when the switch is turned on, but the wire on the 2cyl switch stays at 4.5v.
This would lead me to believe that the wiring is ok. You would need to connect a reader or laptop to confirm this.

The 2-pin connector for the Jake solenoids is just below the inlet manifold, above the starter. Usually about 6" from the rear of the block.
I doubt that this is your problem, as the ECM will log a code 31 if it detects high or low resistance in the solenoid circuits.

Don't you just love those trucks with the engine half in the cab. Short bonneted Western Star's are about the worst.