I thought I might as well ask on here. I have a switch on the dash that is supposed to increase idle speed in 150 RPM increments, 3 settings I think?? It doesn't work like that but does appear to do something. If I toggle the switch down a few times rapidly, the engines slows maybe 50-75 RPM but runs a little rough and rattily. If I toggle it up a few times the engine speeds up the same amount and runs smoother. I just leave there and it idles about 825 RPM. I read something about cruise control needing to be on on other trucks for the RPM switch to work but it doesn't mention it in the manual. Truck is a 1994 Ford L9000. It also has a Jake brake that doesn't work. I just use it on my land but maybe it's something fairly easy to fix??
That is the Increment-Decrement switch, I believe that is what Cummins called it. Its purpose was to try and find a less rough idle speed.
The early Cummins Celect engine had a ECM output that either powered or grounded a relay that would when activated would supply power to the Jake switches in the dash. Memory is failing here. The wiring from the dash switches to the Jake heads is supplied by the truck manufacturer. There will be a clutch switch and brake application switch as well. Can you bump up the idle with the cruise switches?
I never tried but will in the next couple of days.