Erratic Detroit 60 Series Idle

Hope this helps anyone with a similar problem...
Had a daunting intermittent issue where the idle on my (late 1996 build) 60 Series would "stutter" from the normal 700 rpm up to 1000 rpm the normal high idle speed. I would not characterize this as a galloping idle, which is a different issue that I also experienced in a different life. With the current problem I would also experience a periodic uncommanded high idle for several minutes (which became interesting at stop lights or backing into a parking space). A local Detroit service center focussed on the engine EEC by swapping in a new EEC to see if the problem would go away. Of course, the intermittent problem never occurred during this trouble-shooting effort. The high idle system is apparently quite complex, passing through a number of inter-related components and circuitry.
Since I had a trip planned for Nashville last month, I scheduled service with Prevost in Goodlettesville, TN to work this issue. Fortunately, as they were moving the coach into the shop, the gremlin struck. After a laptop analysis and some brainstorming by the old-timer's they discovered the fast idle relay (R22) had a bad ground. They cut wire 953 from terminal 87A and wired it directly to a chassis ground stud. Problem FIXED!
Kudos to the guys at Prevost.

It is amazing how those little problems can cause so much frustration. Good to know.

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Originally Posted by dogbone

Hope this helps anyone with a similar problem...
Had a daunting intermittent issue where the idle on my (late 1996 build) 60 Series would "stutter" from the normal 700 rpm up to 1000 rpm the normal high idle speed. I would not characterize this as a galloping idle, which is a different issue that I also experienced in a different life. With the current problem I would also experience a periodic uncommanded high idle for several minutes (which became interesting at stop lights or backing into a parking space). A local Detroit service center focussed on the engine EEC by swapping in a new EEC to see if the problem would go away. Of course, the intermittent problem never occurred during this trouble-shooting effort. The high idle system is apparently quite complex, passing through a number of inter-related components and circuitry.
Since I had a trip planned for Nashville last month, I scheduled service with Prevost in Goodlettesville, TN to work this issue. Fortunately, as they were moving the coach into the shop, the gremlin struck. After a laptop analysis and some brainstorming by the old-timer's they discovered the fast idle relay (R22) had a bad ground. They cut wire 953 from terminal 87A and wired it directly to a chassis ground stud. Problem FIXED!
Kudos to the guys at Prevost.

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dogbone,

Did the erratic idle occur only when the engine was cold?

Mine misses while it is first warming up on a cold day. If I let it idle at 700 rpm it doesn't miss at all, but if I use the high idle feature it misses.

If you experience a high idle, well above 1,000 RPM you might suspect the throttle position sensor. Mine would occasionally do this and thankfully stored the alarm. This is a simple 5 minute fix. Buy your replacement sensor from Prevost. There price is about $100 less than Detroit's.

Gil, Many owners don't think about checking with Prevost, the Converter and/or Detroit when purchasing parts. I replaced all the shocks on my 93 Marathon a few years ago, and the convertor wanted almost $200 each. I got the original shocks from Prevost for $35 each. It does pay to shop around!

I never recall the high idle going above 1000 RPMs when I had the bus, and there were no codes the last time I checked, but I suppose that is one of those things that can go bad from "non-use" maybe even faster than "normal-use".

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Originally Posted by Dale Farley

Gil, Many owners don't think about checking with Prevost, the Converter and/or Detroit when purchasing parts. I replaced all the shocks on my 93 Marathon a few years ago, and the convertor wanted almost $200 each. I got the original shocks from Prevost for $35 each. It does pay to shop around!

I never recall the high idle going above 1000 RPMs when I had the bus, and there were no codes the last time I checked, but I suppose that is one of those things that can go bad from "non-use" maybe even faster than "normal-use".

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Gil probably has the right response for this case. Diesels' idle shouldn't change without a distinct malfunction. As an example, we just fired up our Bluebird a week ago after 5 years 2 months. Once the manual prime pump was used to restore injector pump supply pressure, it fired up and ran as though it had run the day before, whether at idle or higher throttle. Dying idle and then stopping will occur, however, if the filters clog. My two cents.

My situation occurred generally when warm when,perhaps, thermal expansion caused the intermittent failed ground.

While researching an intermittent fast idle problem I came across this thread. I checked the ground as recommended and found all to be good. What I did eventually find was that the relay used to control the fast idle had been replaced with a 24 vDC relay on a 12v circuit. All the other relays in the control box are 24v as evidenced by wire color and the parts manual. The parts manual did not show that relay as 12 or 24 but the wire color (yellow) indicates a 12v circuit. Red wires are 24v.

Robert,

Great find. A technician might suspect a bad relay and replace it, but most probably would not have checked it's voltage. Prevost was pretty good at labeling the socket base for their relays with number and voltage. Unfortunately, the labeling is sometimes difficult to see.

If anyone is reading through this thread for the first time, I have updated information on the throttle position sensor. I replaced one a couple of weeks ago and ordered the part from Prevost. The replacement TPS has a different electrical connector. So, it is supplied with a new cable as well. That cable replaces the cable from the fuel pedal mounted TPS to its connection point in the front spare tire bay. This makes the 5 minute change out much longer. I wonder now if the direct replacement I installed a few years ago was more expensive because of its model.