Hey folks have another head scratcher for ya'll. I'm working with a 96 Freightliner FLD Cummins N-14 Celect. When parked the High Idle (pto idle) wont increase the RPM's. This truck used to be able to bump the idle up to 900 RPM's while parked. It used to work regardless of the parking brake set or released. However if you roll / coast the truck slow (walking speed) the High idle will work but as soon as the wheels stop the idle drops back down. Even if you put it in 1st gear and slow roll you can hit the cruse and idle it up to 900 RPM's. Throw it in neutral the rpm stay at 900 until the wheels stop moving then drops back down to a low idle. Cruse control works fine. Clutch switch and brake pedal will knock the cruse off so I assume they are working. ECM checks out okay as well. To say the least I am puzzled and came to you folks for help. Hopefully there is someone out there who has experienced this before that can point me to the right direction.
I think your right on the money. Thanks for your help. I was thinking no way it's the speed sensor because cruise works and the speedo works. However I finally got a code pulled from it and it's showing 241 Vehicle Speed Sensor.
I finally solved my non working high idle issue after weeks and weeks of research. I changed all the air switches and the vehicle speed sensor on my 1998 FLD 120 with the 12.7 Detroit with no help. What I finally determined was that the high idle circuit passes through the hood tilt switch and is disabled when the hood is opened. If that switch is bad or the wiring to the switch is bad and that circuit is open, your high idle will not work even though your cruise control will not be affected. I was able to bypass my hood tilt switch by unplugging it from the harness and used a large paper clip as a jumper to close the circuit. My high idle started working immediately. Hopes this will help someone with this same issue.
It’s a small square box mounted inside the hood near the drivers side headlight box and plugs into the harness that tells the ecm the hood is open, therefore disabling the high idle feature of the cruise control system.
