MCI MC12 6V92 High idle issue

jcdillin · June 08, 2017, 02:11:44 PM

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?

Iceni John · June 08, 2017, 02:47:37 PM

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

jcdillin · June 08, 2017, 04:13:05 PM

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?

jcdillin · June 08, 2017, 04:17:54 PM

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.

luvrbus · 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

HB of CJ · June 08, 2017, 04:50:11 PM

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.

jcdillin · June 08, 2017, 05:53:22 PM

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

Scott & Heather · June 08, 2017, 08:18:07 PM

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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