1999 Peterbuilt 379 No Jake and No Cruise

I had a bad speed sensor once that caused the cruise to not work. Truck has 2 of them. I just happened to notice it one day. Also made the speedo and odo quit. I don’t think the jakes ever quit though. There’s an air switch for the brakes. I think it’s under the dash on a 379. It’s on the parking brake on my Freightliner. I know for sure it controls my brake lights. Not sure about jakes or cruise. Maybe another air switch? I really don’t know, just throwing that out there. A real Mechanic should know. Maybe a combination of 2 problems since it used to idle up. I’ve often had switches that occasionally don’t work. Have to hit them a few times. Whether it’s on/off or set switch.

 

My issue with this was a previous owner had tapped into a 'ground' which it is, but part of the circuit. Bump idle wouldn't work.

Get a 'Scan Gauge' -- it'll show you TPS actual reading without plugging in -

The whole circuit relies on clutch, brake sensors, the cruise and Jake switches, park brake sensor/switch.

Could be the TPS sensor is intermittently showing 1-5% , and needs changed - could be all of the above.

Mine is a1995 379.

 

Check your brake lights and see if they're shutting off when the brake pedal is released. If you have a brake pressure gauge it won't show zero which means your lights are still on.

That'll stop the cruise from working.
Could be effecting Jake's also. I don't remember mine working. I don't know why. Wasn't a problem in the summer but sometimes it acted up in the winter.

 

Possibly your clutch return spring? I had a '97 378 that suddenly had no cruise/no jakes. Return spring had rusted and finally broke and fell out.

Can test if this is the problem easily just by pulling your clutch pedal all the way up with your hand, then see if jakes/cruise come on.

I believe the clutch return spring on that rig is right below the cab. Hope that is your problem because it is an easy/cheap fix.

 

Floor mat or carpet up under brake pedal ?
Just enough to keep brake lights on and no cruise.
I had same issue because of floor mat. Glued a small piece of rubber onto mat so it could not slip under pedal.

 

Thats the mouse trap looking thing right under the driver side between the cab and frame rail. Their is a grease zert their that some how gets missed when greasing up the truck.

 

It'll be something dumb. You'll facepalm when you find it, and you better be back here to tell us what it was.

That dumb problem haunted me for 6 months till I figured it out. Someone tapped into a ground in that circuit that messed it up.

 

Had a similar issue with an '01 379 & 6nz, ended up being the clutch switch intermittently completing circuit.

Since you aren't getting any brake lights when not on the brake pedal I'd strike brakes for now.

What are the parameters set in the ECM for jakes min/max rpm&speed? how about min/max speed for cruise? does the ECM see current speed?