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Author Topic: 8V92 won't start  (Read 3990 times)
Charley Davidson
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« Reply #15 on: March 03, 2011, 08:45:37 PM »

How long you been sitting? If you don't have fuel pressure the starter will not work, check all the simple easy to get to stuff.
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chart1
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« Reply #16 on: March 03, 2011, 08:51:01 PM »

How does fuel pressure have anything to do with the starter turning over?
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1976 MCI 8
8v71/740auto
8" roof raise
Charley Davidson
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« Reply #17 on: March 03, 2011, 09:02:29 PM »

How does fuel pressure have anything to do with the starter turning over?

There's a switch at the front fuel filter (front meaning the one towards the front of the bus) that will not let electricity to the starter if there is no fuel pressure
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thomasinnv
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« Reply #18 on: March 03, 2011, 10:23:10 PM »

chart1, I could be wrong here but I think you have it backwards.  the fuel pressure sensor dis-engages the starter when fuel pressure IS present to keep the starter from grinding teeth once the engine starts.  this is how the start button override works when you hold the button down to move the bus off the road when the emergency shut down will not allow it to keep running on its own.  I suppose however that a FAULTY fuel pressure sensor could cause the starter to not engage.
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There are three kinds of people in this world....those that make things happen, those that watch things happen, and those that just wonder what the heck is happening. Which one are you?

1977 MCI Crusader MC-8
8V71N/740
95% converted (they're never really done, are they?)
chart1
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« Reply #19 on: March 04, 2011, 06:30:47 AM »

Really confused!!
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1976 MCI 8
8v71/740auto
8" roof raise
Merlin
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« Reply #20 on: March 04, 2011, 06:47:48 AM »

Thanks friends for the replies and suggestions.  Here is a brief listing of the info that I did not initially provide:

Engine is an 8V92T (silver, non-DDEC)
Allison 5sp automatic
1985 Prevost
Parked at Rainbow Plantation RV Park, near Summerdale, AL
Engine has been idle for 7 days
There are no electrical wires to either fuel filter housing meaning no pressure cut-off switch
Good voltage (25 VDC) present at right lug of starter solenoid when master battery switch is ON
Transmission is in neutral as indicated by the little light on the shifter control panel
Starter solenoid wiring looks to be in good condition, all wires are fairly new and no corrosion is evident

Today I have a diesel mechanic coming by that is familiar with "the old stuff" like two stokes.  After being tossed from one phone operator to another for half of yesterday, I finally was able to speak to this fellow who is a hold-over from the old days when diesel engines actually sounded good.

The suggestions on this board (all of them) are very appreciated.  All are common sense and armed with that advice I will sort through the easy stuff myself before the old diesel mechanic arrives.  Last resort is of course a big hammer applied to the solenoid to wake it up, or possibly totally destroy it.  In that case the diesel mechanic will have something expensive to add to his bill.

A side note here is the timing of this "event".  A dozen admirers were watching us methodically (looking like professional RVers) ready the bus for pulling out of the park.  Wife was all decked out in her baseball cap, wrap-around sun glasses, and two-way headset ready to guide me out of the parking spot.  Dead silence ... as the audience listened for the vaunted big diesel sound ... still dead silence.  Geesh, can't an embarrassing moment like this happen with no witnesses?
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GP
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« Reply #21 on: March 04, 2011, 07:02:12 AM »

Merlin,
The front/rear start toggle switch in the rear electrical box above the engine is notorious for failure, but that is usually after it has been disturbed. Just a thought.
And it seems this kind of thing only happens with a bunch of looky lews around.
George
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George & Debi
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papatony
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« Reply #22 on: March 04, 2011, 08:02:00 AM »

    I would get some one to start it while you are at the solenoid to see if there is power coming from the neutral switch on the tyranny
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Charley Davidson
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« Reply #23 on: March 04, 2011, 08:53:29 AM »

I stand corrected on that switch, it keeps the starter from engaging and running while the bus is running/has fuel pressure bad case of memory failure, too bad I don't have a switch that keeps my mouth from engaging when my brain is not running Roll Eyes
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Len Silva
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« Reply #24 on: March 04, 2011, 09:13:41 AM »

I hadn't posted a response because you said you had help on the way.  However, just in case that doesn't work out, you can try this.

If you can safely reach the solenoid with a screwdriver, you can short from the battery terminal to the small coil terminal.  Be extremely careful here as the bus may start right up.  Be sure that the trans is in neutral, the brakes are set, and that you can do this without getting tangled up in belts or fans.
This will bypass any safety switches in your system, so be very careful not to get hurt.

You know that you have good battery at the solenoid, so that if it turns over at all using this method, you have likely ruled out the starter or solenoid.  That leave an electrical problem which is very hard to solve from a distance with a good schematic.
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Merlin
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« Reply #25 on: March 04, 2011, 01:58:45 PM »

Success !!

We back tracked from the batteries ... starter ... and wiring in the rear junction box.  All good.  For whatever reason the bus would now start from the rear starter switch.  Can't understand why because we did nothing to change things except for many "switch flippings" and wire tracing.  Anyhow it started on the typical 1/4 turn.

Now the tracing was focused on the front junction panel and the run/start relay.  It is good.  Wire from front to rear beginning at this junction box is continuous.  Good signal on the tester.  OK ... now the wire from front starter to the front relay ... also good.

Beginning to sweat at this point because we were running out of things to trace.  Final is the thing we should have checked first, and that is the front starter switch itself.  Position 1 is good, and momentary position 2 does not pass the test. 

Embarrassing, eh?  Now I can phone Prevost to get a new keyed start switch or just go to NAPA and pick up an exact replacement which is the same as a garden tractor keyed start switch.  Why can't a Prevost bus have something more impressive than a Briggs & Stratton key?

End of saga.  If only all bus problems were so easy to solve although when old bessy fails to wake up it feels like a major problem.  Could have been worse.  We could have been at a truck stop fuel island when it failed to start.

Thanks again for all the words of advice.

Merlin & Cyndy
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Charley Davidson
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« Reply #26 on: March 04, 2011, 02:57:31 PM »

  Why can't a Prevost bus have something more impressive than a Briggs & Stratton key?
Merlin & Cyndy

I bet they charge for it as if were the key to Fort Knox gold vault

glad it was simple, it usually is as was my problem.
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gumpy
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« Reply #27 on: March 04, 2011, 03:22:06 PM »


...  A dozen admirers were watching us methodically (looking like professional RVers) ready the bus for pulling out of the park.  

Well there's your problem!!!  Roll Eyes
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Craig Shepard
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gumpy
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« Reply #28 on: March 04, 2011, 03:26:38 PM »

I stand corrected on that switch, it keeps the starter from engaging and running while the bus is running/has fuel pressure bad case of memory failure, too bad I don't have a switch that keeps my mouth from engaging when my brain is not running Roll Eyes

I was going to correct you on this, but I see you realized your mistake already. 

It's a common misconception that the fuel pressure switch keeps the bus from starting. It's true, that it disengages the started AFTER the bus starts and builds
fuel pressure. It's part of the safety shutdown system, and is what allows the driver to override the shutdown by holding in the starter button. It's also necessary with a properly functioning
safety shutdown system because without it, the bus will shut down until oil pressure is built up. That's why you have to hold the button in until the oil pressure light goes out on the dash. Of course
that only works if you have air pressure in the tanks.

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Craig Shepard
Located in Minnesquito

http://bus.gumpydog.com - "Some Assembly Required"
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