Went to start the bus for it's monthly run and ran into a strange issue.
Starts and idles great, as soon as I flip on the high idle it jumps up to high idle.
After idling a bit it starts jumping in and out of high idle randomly, just like someone was flipping the switch off and on. If I turn off the high idle it idles fine again and if I press the accelerator to hold it at 800 RPM it will hold it just fine.
Tested out the switch and it seems fine, anything else I should be looking for?
Is your 1995 a DDEC? If so, Fast Idle is activated by two small resistors in the ECM's PTO circuit that the accelerator pedal is on. I have a bad connection to one or both of my resistors, so sometimes my Fast Idle isn't more than a hundred or two RPMs higher, not the 500 RPM higher it normally is. Maybe the same is happening with you? If it's DDEC, Fast Idle is an electronic issue. I can look at my schematic this evening to see the values of those resistors.
And if you don't have DDEC, I dunno!
John
Yes, it's DDEC, not sure if it's II or III.
That's interesting because my high idle is only about 200 RPM higher than standard idle, I wonder if I have a similar issue?
Ok, so looking at the schematic, looks like there was an option between 750 or 950RPM, I must have the 750 RPM option.
Now to track down that resistor pack and see if it has a bad connection.
On the parking brake valve there is a pressure switch when they go bad it will give you the same symptoms too
Isn't technology grand? Years ago we just pulled out on the hand throttle. Seemed to work OK. About 700 rpm cold turned into about 900 rpm while warming up. :)
Thanks! I'll check that out.
Quote from: luvrbus on June 08, 2017, 04:29:07 PM
On the parking brake valve there is a pressure switch when they go bad it will give you the same symptoms too
Another vote for parking brake switch. Next time it falls down out of high idle, give the parking brake Knob a good tug. If high idle comes back on, that's your problem. My old MCI 9 had that issue the entire time I owned it.
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