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Old 11-17-2019, 04:51 PM   #1
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Join Date: Apr 2017
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08 Diplomat ISL 400 crank no start

2008 Monaco Diplomat with 400 ISL.
Ran fine 4 weeks ago ended a 3500 mile trip in NE Tennessee, ready to leave for Florida and all loaded but she will not start, spins over like normal but will not fire up. There had been several nights in the low 20's so I plugged in the block heater the night before. Thinking fuel might have gelled I bought both fuel filters, prefilled both with a mix of fresh diesel and 911 fuel treatment. Still no start. The Medallion gauge normally will display "engine not running" when I turn the key to on and do not start it, but now it is displaying "engine comm failure".
Took the ECM off and had Cummins Knoxville connect on the bench, they are able to communicate with it and determined there were no fault codes.
Pulled both ECM fuses and I do not get any voltage to the hot side of the fuses. I do not have schematics for my coach so am stuck.
Need assistance or suggestions.
Dan Brinson
2008 Monaco Diplomat 40PDQ
ISL 400
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Old 11-17-2019, 06:09 PM   #2
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Here's some reference threads you might try:

Replace your Ignition Key Switch. Is it "sloppy" when you turn the key?

Are you definitely chasing an electrical problem? If so, the question becomes:
1) Where does my ECM get it's power from? (Key switch)

2) Where could there be a short to ground? ...Usually shows some voltage, but you say you have none. So...

3) Where could there be an "open" circuit?

You might verify one of your solenoids did not blow. (Not sure if this will do anything, but it's good to "rule things out if you don't know where to start.")

And while you are looking at possible solenoid issues, might as well hold down the "BOOST" solenoid switch and see if you can start your coach. (Couldn't hurt, but probably not the problem.)

For future reference, attached is a solenoid diagram.

Also, if you have a fuel delivery problem... not enough fuel vacuum pressure... I would advice you to tighten your 3-Lift Pump Bolts. But you don't. Right? So keep this tread for future use and think about upgrading to a FASS lift pump in the future to avoid CAPS injection failure.

In this thread there are lots of tips on preventive maintenance. Including a $119 Scan Tool you can buy on Amazon. I carry one with me now so I can rule-in or rule-out probable clause.

Good luck with your situation and keep us posted.
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Old 11-17-2019, 06:28 PM   #3
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Searched for "Engine Com Failure"

Go to this thread for wiring diagrams and other comments about "Engine Com Failure" on an ISM engine. I know it's not your engine, but if you follow the "black wires" in the diagram you can see all the devices that your ECM communicates with...more than what you have, but many are the same.

In addition, this thread is saying your chassis batteries might be below the minimum voltage your ECM needs to operate, but if you are able to crank your engine I doubt that is your problem.

...or this thread that about the Water In Fuel (WIF) Sensor located on the bottom of you fuel filter shorted out! (Wouldn't that be great if this is the source of your problem!)

...Just pull the 2-wire plug off the filter and see what you get?

Good luck.

Sounds like something is either "open" or something is turning off your ECM power.

And by open you are looking for a loose connection or a dirty connection.

However, if you get no voltage to your ECM, none at all, then I have to wonder if your key switch is the problem or there is a blown fuse somewhere?

...That's why I say you might try "Jumping" your solenoids to back-feed power to your ECM... even if your BOOST switch doesn't seem to work. Couldn't hurt. Bad solenoids cause lots of strange problems.

Hopefully, a real mechanic can get you going today!
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Old 11-18-2019, 03:28 PM   #4
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Verified that the ECM was not getting battery power and ECM fuses were not blown.
I was able to remove the fuses and apply power directly to the ECM side of the ECM fuse holders. Turned the key to 'on' and got the usual message 'engine not running' , cycled the key about 15 times leaving it in 'on' position each time for 10 or more seconds. attempted to start and she fired right up. Hallelujah!!
Now to try and track down the reason it's not supplying power on it's own.
I will post final resolution when discovered and remedied.
Thanks for the help and support so far.
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Old 11-20-2019, 12:40 PM   #5
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Just thinking out loud....when you applied direct power to the ECM fuse....were you also using an alternative ground wire as well when it fired up?
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Old 11-20-2019, 06:48 PM   #6
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QUESTIONS: Auto-Rest Fuses and/or Relays That Power The ECM

This thread identifies "power posts" and auto-rest fuses in an older RV, but the principle is the same for all of us.

I'm not so much concerned about the power post as being a problem when trying to trace voltage to the ECM, but I can definitely see how a bad or failing auto-reset fuse would be.

So the "OP" might want to examine this if for nothing else for general knowledge. Here's the link:

Normally, I would say that you should also be looking for a poor ground while you are giving your electrical system a full-physical. However, in this case, I believe you said you were getting 0V to your ECM.

Questions For The Experts:

* Is there a auto-rest fuse in line to the ECM? And if so, will that create the "open voltage" in this case? Or could this fuse lower voltage or pass a dirty signal to the ECM if it's a part beginning to fail? And if so, where is this fuse located?

* Or maybe there a relay involved with passing power to the ECM? ...And does the key switch activate this relay so the relay can available battery power to the ECM? (That is to say, this relay switches whatever voltage the battery has.) If so, where is this relay located. ...Or can we trace it by looking for wire that goes to the battery.

* That said, if there is a wire that goes direct to the top of the engine battery (maybe a 10 or 12 gauge or so) which the ECM relies upon, maybe this 0V problem is due to a bad connection at the battery? I just don't know? And I don't know what some of those separate wires do... that are located on top of my battery. So I"m asking if one of these wire is for the ECM power? My guess is "No". My guess is that there is a relay next to the main power post located behind my engine batter bank... or maybe it's one of those fuse blocks mounted on the battery side wall.

What I am getting at is this: I don't know how the ECM receives power or how that power is switched by the key-ignition? Can someone describe this for all of us and verify there is or is not a auto-fuse or relay involved in this circuit? Thank you!
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