Series 60 - Jake Brake Logic

[Oonrahnjay]

    OK, asking for a friend; I don't have full details but here goes.

    Friend has an MCI with a Series 60/Allison replacement power train.  Here's the problem --

1)  Driving a few weeks ago, and both cruise control and Jake went out.  A little digging and he found a blown fuse and loose wire in the cruise circuit; he fixed that and the cruise has worked perfectly since then.

2)  Still no luck with the Jakes and he's unsure how to troubleshoot.   So, a few questions ... 

  a)   How does the control circuit on the dashboard work?  He has a Jake brake master switch and warning light on the dash.  When that is turned off, it disables the Jake brake.  Is this a 12V (or 24V) switch circuit?  Is it just a simple switched feed to the ECM/engine computer?

  b)  The computer must sense a road speed of greater than 20 MPH and idle-position accelerator pedal (with the dash switch on, of course) for the Jake brake to engage.   He has checked and the accelerator pedal shows a range of 5V to 0V throughout its range to the computer; engine starts and runs normally and responds to the accelerator in a normal manner.  His speedometer works and his Silverleaf shows a continuous speed signal from the wheel sensors - but since the Jake brake quit working, his speedometer has read about 5 MPH slower than the GPS and it's never done that before.

   We think that the actuation is simple.  Is it correct that there is a 12V (or 24V) fused feed to the actuator circuit on the cylinder head, then the circuit goes through the actuator solenoids and out to a relay?   Also is it correct that that relay is triggered by a signal from the computer (a ground or "go low" signal) and when the relay is triggered by the engine computer, it grounds the solenoid circuit, actuating the Jake brake operation?

   We'd appreciate any help anyone can give on this one.  My friend does do computers so we'll have to funnel things through this discussion.

Thanks,  Bruce H, NC