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Electrical Discussion of preventative/corrective maintenance and other technical issues regarding your coach's electrical system.

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  #1  
Old 09-05-2019
ripple_john ripple_john is offline
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Default WB40 Cruise/High Idle/Speedometer on the fritz

1988 WB40 with 8V92 DDEC1... has started and run flawlessly over the the last 5,000 miles since purchased last fall (after spending a small fortune at WW Williams). Recently did some wiring repair myself around the back engine hatch to replace some failed plugs that were causing blinker issues... that problem solved... but...

Over the last 5,000 miles the speedmeter occassionally doesn't work when first starting the coach (but usually does)... but always starts working by at least the 10 minute mark of running down the road. So, sometimes speedometer is flaky to start, but always starts working eventually. Seemed like rare occurence prior to my blinker work... more common after... but still returns to working all the time.

The high-idle has always worked up until recently... occassionally would blip up or down throttle, but always settled out at nice high idle. That is, until recently (OK... after I finished blinker repairs... errrr). Now it doesn't seem to work at all, no matter what.

The cruise control worked flawlessly. Then post blinker repair it didn't work. Didn't work at all for 500 miles. Then... out of the blue, I pressed the old cruise control button and it started working! The cruise worked last time I drove both when speedmeter wasn't working and still again after the speedmeter started working. Ah!

Blinkers work all the time...

Is it all related? Or, have I just gone crazy? Any suggestions on where to start?

Nothing here stops bus from starting, driving, and enjoying. But, would like to fix it... and ideally fix it myself...
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1988 WB40
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  #2  
Old 09-05-2019
Alfa Alfa is offline
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The high idle may be an air pressure switch in the first bay, street side. Our 88 WB Canadian brother, Duncan, has been fighting his high idle, so he probably has the most DDEC 1 high idle experience.

Diego
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Last edited by Alfa; 09-05-2019 at 06:02 PM. Reason: spelling
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  #3  
Old 09-05-2019
79Ranger 79Ranger is offline
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From what I have traced out using the schematic:


The ignition switch will energize the fast idle relay that is in the genny control compartment which is energized as soon as the ignition switch is turned on.


The wires are 504 and 560 at the relay contacts (not the coil).


560 then goes to the pressure switch that is on the far left of the 3 that are mounted in that same genny compartment.


510 is the wire that comes from the ECM that goes to the other side of the pressure switch.


504 and 529 go to the cannon plug under the transmission keypad area which then go to the fast idle switch.


529 then goes to the center of the fast idle resistors and then the 2 other wires from the fast idle resistors 452 and 527 go to the ecm.


The issue I have been having is intermittent fast idle PLUS it deciding to go to fast idle while I am driving which then throws a code 12 which is PTO high, which makes sense as the DDEC 1 is a very basic ECM.


I have replaced the pressure switch, cleaned all connectors and checked everything I can think of, all looked good.


After that I tried the high idle probably a dozen times and it worked flawlessly...however after the last time, it suddenly shot up to 1200 rpm (fast idle) and the switch was ineffective...which is most bothersome.


For the moment I have removed the fast idle resisitors as to eliminate that input from the ECM, I have not had a chance to look any more into it.


I'm heading out on a 2000 mile trip right quick so I guess I will find out then.


The only thing that makes sense to me is a broken/shorting or intermittent wire that goes from the front to the ECM.


As far as the cruise control, it looks from the schematic to be a seperate entity that goes from the 3 switches to a cannon plug under the transmission keypad and then goes straight back to the ECM.


There is a pickup on one of the rear drums, might want to look at that connection as well.


If I were a betting guy, I suspect something might have got knocked/moved/dislocated while you were working on it as I can't see any correlation on the schematic for the cruise or fast idle control.
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Alberta
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Last edited by 79Ranger; 09-05-2019 at 11:39 PM. Reason: fat fingers spelling
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Old 09-05-2019
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Randy Dupree Randy Dupree is offline
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Duncan,thanks and keep plugging along,you will get it.
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Old 09-05-2019
79Ranger 79Ranger is offline
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Quote:
Originally Posted by Randy Dupree View Post
Duncan,thanks and keep plugging along,you will get it.

Thanks Randy. Once I get back and have some time I will be digging into it deeper.


Not to derail the thread but I want to mention that the way Detroit says to clear any historic codes in the ECM is to disconnect the battery.


After I got the high PTO warning I could not get the codes to clear by a battery disconnection. I tried 2 or 3 times with long periods (2 weeks) with the batteries all disconnected and the charger disconnected.


One of the things I did was to remove the ECM out of the back so I could inspect the connector and wires......when I reconnected everything the codes cleared...weird.


I picked up a Pro Link scanner and plan on keeping it plugged in like a reader for my upcoming trip to see if anything shows up there.
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  #6  
Old 09-06-2019
ripple_john ripple_john is offline
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Duncan - excellent info. I did quite a bit of poking around in the generator compartment while diagnosing the blinker issue... including removal and checking the directional control box. I'll have to go out and check, but I recall the pressure switch on the far left looks to have been replaced with newer unit sometime in the last few years (the other 2 look original). So... it appears I've got a spot to start looking for the hi idle issues and could be that was caused by me - or some poking around pushed something over the edge. Thanks much. I'll keep you posted.
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