3406B Jake

works fine at idle, does that mean it stalls the engine if you switch it on hi ? the jake system should power the clutch switch, then the fuel pump switch and then go to the dash switch. if it won't energize when the truck is in motion, one of the switches must be staying open. hook a 12v test light to one of the wires at the jake head spacer, with the engine off, key on and jake switch on hi the test light should be on, if so push the clutch pedal down until the light just goes off, this should be about 1/2'' of pedal movement,(Assuming this is a Pete) or 1/2 of the free travel, gently let the pedal up to see if the light comes back on, if it does or is sporadic find out why (poor pedal travel or adjustment ??) if the clutch pedal switch seems to work repeatedly and smoothly check for binding throttle linkage or turn the switch in 1 turn and see if that helps.

 

it could be either, if the linkage from the pedal is binding just a bit it won't let the governor go to low idle and the jakes won't come on, but a just close enough switch setting can give the same symptoms. I have had my floor mat slide under the back of the throttle pedal and cause erratic jakeing, Another thing to check is the hand throttle linkage connection, a bit of hang up there can cause jake issues