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Message |
Craig (Ceieio)
Registered Member
Username: Ceieio
Post Number: 291
Registered: 12-2004
Posted From: 206.212.230.165
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 16, 2007 - 1:38 am: | |
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 |
John MC9 (John_mc9)
Registered Member
Username: John_mc9
Post Number: 441
Registered: 7-2006
Posted From: 68.205.165.5
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 16, 2007 - 2:34 am: | |
I'm known to be good at providing dumb suggestions......
Why don't you put some cardboard (or a towel) in front of the
radiators while it sits running in your yard, and see if it's a
temperature related problem? Running "down the road" can
add a "vibration related problem" to the mix.
If you don't narrow it down you're going to go nutzier than me,
well ahead of your time!
(TrUsT mE WiTh tHaT) |
Ben B. Wolff (Ben_mc7)
Registered Member
Username: Ben_mc7
Post Number: 5
Registered: 4-2007
Posted From: 24.73.4.21
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 16, 2007 - 8:32 am: | |
How old is the starter? It is possible that age has caused the unit to lose strength when it is heat soaked. I have a 1972 MC7 myself, when I first got it, the engine barely turned over, but always started. I changed the location of the batteries, switched to series 31? size, changed from 24V to 12V and now the starter spins like mad. |
marvin pack (Gomer)
Registered Member
Username: Gomer
Post Number: 58
Registered: 3-2007
Posted From: 76.4.143.219
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 16, 2007 - 8:47 am: | |
I don't think you are nutzzz john however, doesn't the mci's have a overheat sensor that will not let the thang start when overheated? It could be defective and maybe you could jump it out to try. |
George M. Todd (George_mc6)
Registered Member
Username: George_mc6
Post Number: 228
Registered: 8-2006
Posted From: 76.168.69.233
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 16, 2007 - 10:59 am: | |
Good morning Craig,
I have read your post carefully, and offer the following:
1. It is NOT the starter, as you have told us it will start reliably when you jump the solenoid.
2. It is not the overheat protective circuit for the following two reasons. One, you have NOT told us that the hot engine light comes on, the buzzer sounds, and the engine shuts down while driving. Two, this circuit is disabled while starting, along with the low oil pressure shutdown, otherwise the engine wouldn't start because it has no oil pressure stopped! ALL of that protection disables the Skinner valve, so if it was bad, the engine would CRANK, but not START.
Now, to positive suggestions, in the start circuit there is a start RELAY in addition to the start SOLENOID. The relay deals with the protective circuit, and the fuel pressure switch. First thought when this happens is to put the bus in NEUTRAL, turn on the master, go to the rear, and try to start it from the rear start switch. The result will narrow the search for the problem. Obviously, a start from the rear will tell you to look forward, no start tells you to get help for listening for clicks. The start relay is in the battery switch box on mine, I'm not sure where yours is, read your schematic and find it, have somebody listen while you operate a start switch. I assume you have a neutral safety switch? If its just a little out of adjustment, bingo, and more wires in the start circuit.
At this point, you will need a test light, or a voltmeter, to find the problem, which is probably a loose wire, or a loose ground terminal in the start circuit. Possibly the start relay, hold the start switch down and have someone else tap the relay and see what happens.
A 12V starter on 24V WILL "spin like mad," it just won't last long.
Good luck hunting, and please write back with what you find.
George
(Message edited by George Mc6 on July 16, 2007) |
Craig (Ceieio)
Registered Member
Username: Ceieio
Post Number: 292
Registered: 12-2004
Posted From: 206.212.230.165
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 16, 2007 - 12:01 pm: | |
Thank you all for the suggestions.
John - I will try the cardboard... If nothing else I will make a short driving loop. No place to turn the bus around once I pull out of the driveway so I would have to travel a 10 mile loop or so, she should be warm by then if the cardboard doesn't do it. (also, it is too late for me to worry about going nuts... I have a bus after all!)
George - More data I left out of the original post - I tried the rear start switch with the same results as the front, no crank (no clicking either as I recall).
There are no lights and buzzers on, temp gauges front and rear appear to be in normal operating ranges. Oil is full, engine has good pressure when running (but as you say, this should not impact cranking)
I don't know the age of the starter, but it cranks strong (when it cranks at all).
The bus bus is a factory auto, and all interlocks still function (fast idle shuts off if you release the park brake, fast idle will not engage if tranny is not in neutral, engine will not crank if tranny is gear, and so on. I am rather amazed by all that. :-))
Another tidbit: to jumper the solenoid, I use jumper cables. The MC7 has a large (high current) 24V lug in the engine bay. I connect the jumper cable to the lug, the to the terminal on the starter solenoid. Since the bus is warm, it is running in the usual 1/4 turn.
I conclude from this that 1) the batteries are up to the task (plus they always crank it cold), 2) the grounds are intact (I only jump +24v), 3) the starter motor is fine, 4) the problem is likely in the engine bay area, and 5) I dislike having my head in the engine bay when the engine starts!
The book shows a starter relay located in the rear junction box. The drawing looks like a old Ford starter relay. I will take a gander at that too when the start circuit acts up again.
Thanks you all for the suggestions. "Talking" out problems like this helps organize the approach.
Also, I will keep the faith and post back what I find. It may take a while to have results, but it build a better archive if we all "close the loop" on these problem here in the forum.
Thanks again,
Craig - MC7 Oregon |
Don Evans (Doninwa)
Registered Member
Username: Doninwa
Post Number: 56
Registered: 1-2007
Posted From: 65.61.96.82
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 16, 2007 - 12:38 pm: | |
Craig,
Just to clarify, when you jump it with a jumper cable to a terminal on the solenoid, are you applying power to the activation terminal or are you jumping start current to the output terminal of the solenoid bypassing the contacts in the solenoid?
Sounds to me like you have a winding in a relay/solenoid that is broken. When heat soaked it expands and opens and when cool contracts to close the circuit again. I have had that happen a couple times with ignition coils. I would take a close look at the start relay. A heat gun or hair dryer and a can of ether will provide quick heat and cold. Just be sure the ether has completely evaporated before applying current. Safer cooling spray can also be found at Radio Shack.
Good luck
(Message edited by doninwa on July 16, 2007) |
niles steckbauer (Niles500)
Registered Member
Username: Niles500
Post Number: 758
Registered: 11-2004
Posted From: 24.129.157.205
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 16, 2007 - 2:51 pm: | |
Sounds like Don has it right - "something" is expanding when heated via the heat soak from the mill or less possibly an electricaly generated heat - and thanks for promising to provide 'the rest of the story' |
Gus Causbie (Gusc)
Registered Member
Username: Gusc
Post Number: 399
Registered: 11-2005
Posted From: 206.40.238.72
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 16, 2007 - 6:06 pm: | |
Craig,
This very same thing is on a long thread at MAK.
Since you have the same problem with the rear start switch that means the problem is in the relay/solenoid or primary cable circuits. It is not with the front start switch. It could be in the heavy duty battery cables from the rear lug to the starter since you bypassed them when you used the jumper cable.
These large cables are very bad to corrode internally and at the connectors. You also could have some badly corroded connections from the lug to the solenoid. Probably not, but possible.
However, my money is on the relay/solenoid circuit or the internal parts of one of these two. You also bypassed these with your jumper.
Solenoid points can become badly burned/pitted. Corrosion becomes more resistive when hot.
Relays don't carry large current so a small resistance (Corrosion) there can put it out of commision. You can also have a poor connection at the relay. You can have a short in the relay windings which might only happen when hot. (The coil wires expand and touch when hot). This same type of short could also happen in the solenoid since it and the relay are both electromagnets.
I disagree with Don about the wires, but we agree on the location of your problem.
Wire will expand with heat and close a break, it actually becomes longer. The wire will shrink when cold and open the break. This is especially true of wire in a coil.
(Message edited by gusc on July 16, 2007) |
Ben B. Wolff (Ben_mc7)
Registered Member
Username: Ben_mc7
Post Number: 6
Registered: 4-2007
Posted From: 24.73.4.21
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 16, 2007 - 6:09 pm: | |
Just to correct, my system has been converted to 12V from 24V, it is not a 12V starter connected to a 24V system. |
john w. roan (Chessie4905)
Registered Member
Username: Chessie4905
Post Number: 641
Registered: 10-2003
Posted From: 71.58.117.21
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 16, 2007 - 9:41 pm: | |
Have you checked for proper voltage at the starter when it won't crank. Also check if there is amperage available if you have the proper voltage. Could be a problem in the starter/ solenoid as they are the only things to get warm with the engine consistantly. |
Craig (Ceieio)
Registered Member
Username: Ceieio
Post Number: 293
Registered: 12-2004
Posted From: 206.212.230.165
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, July 17, 2007 - 1:05 am: | |
Well, those that put their money on the relay solenoid have the winning tickets! I pulled the rear junction box and checked the starter relay.
The switch lead was so loose the nut was at the end of the threads on the stud. I tightened up the lead and checked the others; three of four leads were slightly loose to barely connected.
I think that will fix it, but the test will be in a couple of days when I take her out on the road.
thanks to all for the advice and help! I'll post back when I know for sure.
Craig - MC7 Oregon |
Craig (Ceieio)
Registered Member
Username: Ceieio
Post Number: 296
Registered: 12-2004
Posted From: 207.101.213.58
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, July 26, 2007 - 2:51 pm: | |
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 |
Gus Causbie (Gusc)
Registered Member
Username: Gusc
Post Number: 406
Registered: 11-2005
Posted From: 63.97.117.37
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, July 26, 2007 - 4:01 pm: | |
Craig,
This thing puzzles me? Are you sure this isn't an oil pressure switch. Usually an oil pressure cutoff won't let the starter continue cranking if the oil pressure does not reach a minimum press.
I can't imagine any reason for an excess fuel press safety switch? It's pretty hard to get excess fuel pressure with only the starter cranking the engine? The only possible reason I can see for it is in case the starter relay sticks but if the starter solenoid sticks cutting off the relay won't help any?
Some older trucks had two start buttons, one had to be held down until oil pressure reached a min press. If it was released too soon the starter would stop cranking. |
john w. roan (Chessie4905)
Registered Member
Username: Chessie4905
Post Number: 670
Registered: 10-2003
Posted From: 71.58.117.21
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, July 26, 2007 - 4:54 pm: | |
It MAY be a low fuel pressure relay...If you run out of fuel or filters plugged or fuel pump fails and fuel pressure doesn't reach at least 8 psi., the relay won't allow you to continue to crank the engine till problem with low fuel pressure is resolved. Some inexperienced coach operators might ruin the starter or run down the batteries otherwise. |
Craig (Ceieio)
Registered Member
Username: Ceieio
Post Number: 297
Registered: 12-2004
Posted From: 207.101.213.58
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, July 27, 2007 - 1:31 pm: | |
From reading the repair manual, schematics, and things of the like, this is what I conclude this circuit is all about (it is not spelled out that clearly in the repair books. It could be in an operators manual, but I don't have one of those!).
WARNING: Hobbyist Theory to follow!
When you activate the starter on the MC7 it overrides the low oil shutdown (good, 'cause if it isn't running, there is not much for oil pressure), and the starter button also overrides the high temp shutdown.
The fuel pressure switch is screwed into the casting for the two fuel filter mounts. It opens the starter relay connection to ground when fuel PSI reaches 8 lbs. (So sayeth the good book of MCI) which prevents further cranking of the starter.
My conclusion for this is that this is how an emergency override is implemented. If the bus shuts down for low oil or overheat while on the freeway or straddling the rail road tracks, the starter will crank the engine back up because it overrides the oil and temp shutdowns while the button is pressed. If you release the button, the engine will get shut down again.
So my guess is the operating plan after an automatic shutdown is to push and hold the starter button to keep the engine running long enough to get the bus to a safe location (like off the train tracks). The fuel pressure cutoff keeps the starter from going BOOM while doing this. The whole procedure is awkward enough to discourage the operator from trying to drive "to the next town" (thus completely toasting the engine) which would defeat the whole point of the shutdown in the first place. It could also help avoid some really nasty headlines for the bus company and MCI.
This is they only thing I could come up with that would explain why it is built this way, and I could be completely off base (heck, I usually am!)
Opinions?
Thanks,
Craig MC7 Oregon |
George M. Todd (George_mc6)
Registered Member
Username: George_mc6
Post Number: 231
Registered: 8-2006
Posted From: 207.231.81.129
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, July 27, 2007 - 10:05 pm: | |
Craig,
You are exactly correct, one end to the other of your above post. I don't know if the ability to run the engine with the starter button down was designed into it, or whether it just happened because of the necessity to start the bus with no oil pressure? Anyhow, its there, and that is exactly how it works. It also keeps deaf or indifferent drivers from grinding or over-revving the starter.
George MC6 Calif |
Gus Causbie (Gusc)
Registered Member
Username: Gusc
Post Number: 407
Registered: 11-2005
Posted From: 63.97.117.37
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, July 28, 2007 - 10:01 pm: | |
Craig,
This makes sense to me. Aren't Manuals wonderful? |
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