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

 

Did you happen to resolve this issue? I have the same problem going on right now. When u jab it it’ll work and it works intermittently when driving also. Thank you!

 

The potential problems are mentioned above. Given the age of the trucks that B models came in problems with the Jakes are usually related to linkage wear or switch adjustment.

What year and model truck do you have?

 

Okay thank you. It’s a 1993 peterbilt 379. 3406b. I’m thinking it’s my clutch linkage maybe not adjusted correctly. Do you know how you adjust the switch by chance? Thank you

 

I have an 89 379 and 89 378. Assuming that things aren't different on yours, there is a clutch switch that is under the floorboard below the clutch pedal. The switch is a spring affair that rides against the linkage. You should be able to reach in there with the truck shut off, move the linkage a bit by hand, and hear whether or not the switch is clicking. That would be the first thing to check.

Next, the throttle switch, which also adjusts low idle, is where I would check.
I circled it in blue and it is taped over in the photo, but you should have two wires going to this switch. With the key on but the engine off you should have power to both poles with the throttle all the way off. You can move the throttle linkage and power should quit at one of the poles. It is just a continuity switch. If you do not have power to either go back to the clutch switch and make sure the power is on both sides of it.

I assume it will be one of these two. You may need to add spring pressure to the throttle return, or check to make sure the throttle pedal is returning all the way unobstructed like Longhood mentioned. You should be able to chase it out with a simple test light. You mentioned that it works part time, so power/fuse issue should not be the problem.

 

did you figure it out?