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Author Topic: Starting (cranking) Problem  (Read 2323 times)
ceieio
MCI 7 DD 8V71, HT740
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« on: July 15, 2007, 10:36:42 PM »

My bus has developed a cranking problem.

First the bus:  73 MCI MC7, 8V71, HT740, ~18K on out of  frame for both engine and trans.

After I run for a while (can be as little as 5 miles, but enough to get to operating temp) the bus will sometimes not start, specifically, no electrical response occurs to pressing the start button. If you jumper the starter solenoid, the bus cranks right over.

The bus never fails to crank when the engine is cold.

At first I thought the cranking issue might be temperature related, but now I am not sure. 

The reason I thought it might be temp related is that I can start the bus here at home and drive it around some, shut it off and hit the starter again and it will crank right up.  Puttering around here at home will never get it up to temp.

The bus was used as a home base for a recent cancer fund raiser.  When I brought it home (8-10 miles) it was up to temp by the dash gauge.  I shut it off and tried a restart - no crank.  I checked the mechanical gauge in the engine room and it read 190.  I went about my business and went back later to try a start.  The mechanical gauge read 140 at this time and it still would not crank.  I didn't go back to try again until the next day, when the engine was at ambient.  I hit the starter and it cranked right up.  I have cold started it many times since then and it fires right up.

What has be wondering about my temp related suspicions now is twofold.  1) 140 degrees is not that hot, and 2) I don't see anything in the starting circuit that would suggest that heat would prevent a start.  I see the fuel pressure cut out switch that prevents cranking when fuel psi is above 8 lbs, but no other start circuit disabling device.

Thoughts?  It is a bit of a nuisance to trouble shoot as the problem only occurs at operating temperature.

Thanks,

Craig - MC7 Oregon
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Craig MC7 - Oregon USA
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« Reply #1 on: July 15, 2007, 11:01:03 PM »

Craig, any chance you tried the rear start button when it wouldn't crank? You mentioned that you can jump the solenoid. Is that the big one on the starter, or a magnetic switch somewhere? I think you need to rule out if it's the "little" wiring feeding the mag. switch or the big wiring feeding the starter and grounds.

On my old 4106 I had intermittent start problems, and only when it was hot... and even then, sometimes would crank, sometimes it wouldn't. The problem turned out to be the big #0000 starter solenoid to batt. wire was internally corroded. When I took it off, there was a big gash in it and lots of green, flaky stuff. After I replaced the wire and cleaned up all of the other connections, I never had a start problem again.

You could have a bad solenoid, or a few other things. You'll have to rule them out. Be careful with the big wires... they don't call it welding cable for nuthin'!  Wink

HTH,
Brian B.

p.s. I use a big spool of 8ga wire with alligator clips on each end for troubleshooting elec. issues. To isolate the big wires, you can use a heavy-duty set of jumper cables. You also might want to load test the batts to rule them out.
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Brian Brown
4108-216 w/ V730
Longmont, CO
ceieio
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« Reply #2 on: July 15, 2007, 11:20:21 PM »

Brian - thanks for the reply.  Yes, I did try the rear start switch and it did not do anything.  When I jumpered the solenoid, I used jumper cables.  The MC7 Has a heavy 24v Lug in the engine bay and I connected that to the terminal on the starter solenoid to energize the starter.  Doing this lights the DD right off, per normal.  The batteries are just aft of the front wheel curbside.

The above leads me to believe that the batteries are OK, and that the power is good to the engine bay.  There are plenty of cables etc. in the engine room that could have gone bad though.  I'll take your advice and start rooting around in the bay isolating wires to see what comes up.  There does not appear to be corrosion on the cables externally, but I did not consider internal problems... I guess I need to get if hot first.

Thanks for the pointer!
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Craig MC7 - Oregon USA
Jerry32
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« Reply #3 on: July 16, 2007, 04:13:03 AM »

I would check that fuel pressure sw also when it's not cranking. It could be hangingg up or bad contact. Jerry
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1988 MCI 102A3 8V92TA 740
donnreeves
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« Reply #4 on: July 16, 2007, 04:30:32 AM »

Tracing electrical problems can be tedious. The best method I have found is to follow the flow. Start at the start switch. Is there power there? There is usually a relay before the starter. Does the power go through it? Then the neutral safty switch and the fuel pressure switch you mentioned.If the power stops at any of these points, bypass it and see if that soves the problem.  Donn
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skihor
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« Reply #5 on: July 16, 2007, 06:22:01 AM »

I had a similar problem in an old S/S. It turned out to be a bad ground connection from the Batteries.

Don & Sheila
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ceieio
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« Reply #6 on: July 16, 2007, 09:09:17 AM »

Thanks again all!  I am headed out to poke and prod.  I will post any results that I have.  If any other thought pop up, Sing out as I can use all the ideas I can get!

Best regards,
Craig - MC7 Oregon
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Craig MC7 - Oregon USA
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« Reply #7 on: July 16, 2007, 09:25:11 AM »

Craig, here's what I'd do to right away rule out the "little" wiring.  There should just be one "small" wire feeding your solenoid. Maybe a 10ga or so. Once you light it off, remove that wire and fish it to somewhere more convenient for testing. Drive her to heat her up, stop, and then put a test light on that wire and hit the rear start switch. If you got power there, it rules out any of the wiring in-between. Then attach it back to the solenoid (careful, it's hot in there!), then try to fire it off. If it doesn't crank, you have a bad solenoid. If it does crank... I guess you didn't heat it up enough (or Murphy is p*ssed  Cheesy).

And if you didn't have power at the small solenoid wire, your solenoid is likely good, and start working backwards in the "little" wiring to the magnetic switch, overrides, etc.

HTH,
Brian B.

p.s. if you have to replace the solenoid, I'd go ahead and replace the starter with it. The whole thing should be less than $200, and cheap insurance against a side-of-the-road experience sometime down the line.
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Brian Brown
4108-216 w/ V730
Longmont, CO
ceieio
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« Reply #8 on: July 16, 2007, 09:35:34 PM »

The little wire wins it!  I pulled the cover on the rear junction box (MC7 has this streetside inside the rear access door).  The starter relay is newer than most items in the box.  The "tiny" lead that Brian mentions was so loose the nut was about to fall off the stud.  The big leads were also a little loose as well.  I think the temp is a red herring and vibration was the key.  I tightened all this up and buttoned the box back up.  I will take the bus out sometime this week and will see what happens.  I suspect that this will cure the problem.

Thanks all for your help and thoughts on the problem.  I am betting that the problem is licked, but will post back in a few days to confirm.

Craig - MC7 Oregon
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Craig MC7 - Oregon USA
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« Reply #9 on: July 16, 2007, 10:58:39 PM »

Cool, Craig. I hope that's all it is... and it probably is, if the wire was that loose.

Keep us posted! And happy to help,
Brian B.
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Brian Brown
4108-216 w/ V730
Longmont, CO
ceieio
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« Reply #10 on: July 26, 2007, 11:50:35 AM »

and now, the rest of the story...

We piled in the Bus and headed south to Crater Lake, Klamath Falls, and the Lava Beds National Monument in California.  Lots of sauntering around as well (Bend, Sisters, and lots of small towns).

Everything seemed fine until I shut the bus off at Crater Lake.  After the 7000+ foot climb the engine bay was good and warm.  We walked about a bit at the Lodge area and after 90 minutes or so decided to move around the rim to some hiking areas that we like.  Well, the bus wouldn't crank. 

The last suspect on my list was the fuel pressure cut out switch that disables the starter relay when fuel pressure reached >= 8 psi (which should mean that the engine is running).  This device uses is small gauge wires that closes the circuit to ground.  I put a pocket knife blade across the terminals while my son was holding the start button down and the bus cranked right up.

For a field repair, I moved all the wires on to a single terminal creating a permanent connection to ground.  The bus gave us no more starting problems after that.

Now that we are home, I need to figure out where to buy the fuel pressure switch to fix the bus back to design.

Is this a NAPA kind of thing?

I can get the MCI part number out of my book, but who knows if the auto parts stores can cross reference that to current manufacturers and part numbers.

Thoughts?

Craig - MC7 Oregon
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Craig MC7 - Oregon USA
Stan
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« Reply #11 on: July 26, 2007, 12:44:43 PM »

If you know that it is 8 PSI then just get a normally closed 8 PSI switch  (switch opens at 8 PSI) with the correct fitting (Usually 1/8" NPT). It should be available from NAPA or other large parts houses but maybe not in stock and you have to wait for delivery. Since you are running, it is not a big problem.
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Len Silva
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« Reply #12 on: July 26, 2007, 02:10:04 PM »

Typically, the only function of that switch is starter protection.  If you are careful to never try to start while the engine is running there should be no problem.

I guess some folks have also used that circuit to operate a solenoid to connect house and start batteries.

Len
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ceieio
MCI 7 DD 8V71, HT740
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« Reply #13 on: July 27, 2007, 11:12:37 AM »

Thanks folks.  I'll go visit NAPA today as I am planning on playing hooky this afternoon.
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Craig MC7 - Oregon USA
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« Reply #14 on: July 27, 2007, 02:56:15 PM »

If NAPA doesn't have it.....try
http://busfixx.tripod.com/busservice/id8.html
scroll down towards the bottom of the page, they say they have several different types, reasonable price.

Chris
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Chris & Cheryl Christensen
Ex-Bus Owners
Eagle, Idaho
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