40mt starter ccw counterclockwise left hand lh delco 50mt solenoid selanoid fun!

[busfan]

OK..

Got a 40mt CCW and have a couple questions.

Once upon a bus inspection we took a large wrench and shorted the solenoid terminals..The starter spun but solenoid did not engage, since that first spin nothing just bigger sparks.

Three questions, Does shorting the solenoid terminals cause harm to the starter motor internally? Where should I be looking for the short when I open up the starter? Is an 8 brush more powerful than a 4 brush?

I have researched and read the two most relevant posts below, the starter is a 12v ccw Delco 40mt(unknown brush count 4 or on a 51' gm 4907 straight 6-71..

TomC:
You'll just have to compare the two starters and make sure they are both for a V drive.  Count the gear teeth on both and just put them side by side.  If the new one has a shorter motor, I would pass on that, since sometimes the 6V had a smaller starter than the 8V. You can also try jumping the positive motor post directly with a screwdriver that you don't care about since it might get welded. Hook up jumper cables with the negative to the big cable out the motor, and with the screwdriver clamped in the positive jumper, try tapping the other big bolt going into the starter that is connected to the starter solenoid by a big strap.  If you get no response, motor is burned out.  If you get big sparks, motor is shorted out.  If you get some sparks and the motor spins up, then you have a bad solenoid.  But from what you said about slow turning, it is probably the motor.  Replacement time.  Good Luck, TomC

littlehouse:
Hello Henry
I don't know if this will help but I'll try.
The way the system works is the main power [elec.]  comes from the battery with the big cable to a large nut & bolt
on the back of the starter. Power [elec.] also comes from the starter button, or key, in a smaller wire to the back of
the starter. This unit on the starter that the wires are hooked to is called a selanoid, it is an electical magnet that
when power from the start button, or key, makes it into a strong magnet it pulls some kind of lever system to shove
the bendex gear into the flywheel. It also at the same time pulls a large copper washer against the head of the bolt
that large wire from the battery is hooked to. the elec. goes through washer to a bolt just above the starter at the
bottom of the selanoid. This bolt is hooked to a wire or strap that takes the power[elec.]  into the starter.
Sometimes this copper washer gets a burn spot that wont make a good elec. connection so even if the bendex goes
into the flywheel the starter still wont run. Now this washer and what ever system moves the bendex are inside the
selanoid so you cann't see them from outside but they're ez to fix if this is the problem. If you hook power up to the
big bolt that goes to the starter[below the bolt with the wire from battery] the starter shoud run with out trying to                                   
start the  eng.,  the starter will just spin by it's self if it's okay. So if you take an OLD wench or screw driver; something
you don't mind if it get a burn spot in; and you make contact from the bolt with the battery wire to the bolt with the
strap or wire going into the starter only, the starter shoud spin. Remember you cann't get shocked from DC but you CAN
get a bad burn.
I don't know if this helps at all or just muddys the water. If I'm wrong I'm sure someone will let me know.
  Lots of luck
Ray with the littlehouse in Kent Wa.


Thanks!

[bevans6]

Just behind the two large high current terminals on the starter solenoid is a large copper disc that is the actual switch inside the solenoid.  Jumping across the outer terminals is electrically the same as when the copper disc closes, so no harm is done inside the motor that wouldn't be done if the solenoid was operating properly.  The difference is that only the motor is energized, so it just spins.  The solenoid does two things when it is energized - one is it closes that high current switch with the copper disc and the second is it shoves the pinion out to engage with the ring gear.  Unless you already know how to rebuild a starter motor I wouldn't bother trying to fix it, I would just take it to a rebuilder.

[brmax]

Yes it can heat the windings and damage them if continued.
Its probably worth chatting about; the outcome of any electrical continuity checks done at the solenoid during any past regular start attempts.
Just an fyi, the solenoid replacement requires a spec. adjustment of the linkage that moves the bendix gear to the flywheel.


Good day and glad to help

Floyd

[Oonrahnjay]

... fyi, the solenoid replacement requires a spec. adjustment of the linkage that moves the bendix gear to the flywheel.
Floyd 

      Good point, Floyd.  If not done right, the teeth don't fully engage and you're almost certain to get premature wear on the teeth on the bendix and the flywheel.

[luvrbus]

The solenoid won't engage unless the small start wire is jumped along with the large battery terminals ,? was it jumped ,you roll the dice spinning a starter without a load BTDT,why a 50MT that is a huge starter for a 6-71,a 37MT or 41MT is the norm      

[busfan]

Thanks for the informative replies guys!

Bevans where$ the fun in that?  $35 for the rebuild kit but I guess it does matter what's wrong with it(brmax pointed to armature) also I noticed a bunch of springs in the kit that are not shown on the 42mt youtube rebuild..

BRmax, looking at youtube video's the windings in the armature are encased in epoxy correct and will be peeling up if unusable correct?

Oorah thanks for the emphasis, I will make a mental note / paint note of the approx threads/ grease line.

Luvr it is a 40mt just saw I might be able to use a 50mt CCW, looks like 42mt ccw is my best bet buying new/rebuilt thou.

Strange it spun once and now just bigger sparks, guess she had one left..(pun intended)

[brmax]

Is there electric at the ignition/start wire when button is pushed ( or key start app).

Yes I did say continued shorting accross would be detrimental to the starter and its windings.

We need to establish the beginning issues and electric checks done. Versus whats listed here so far, I may have missed it but, it sounds like it could use taking to a rebuilder.


Floyd

[luvrbus]

Are there any parts around to rebuild a 40MT,the 42MT replaced it years ago, the 40MT is obsolete starter

[busfan]

Floyd,

Thanks Floyd, yes the solenoid extends on button push, it definitely could use a a trip to a ex lineman rebuilder, so friggin heavy!

Lovr, there are a couple Kit's on ebay and amazon!

I have appointed myself re-builder and will video the process and post on youtube and link back..

This starter looks like it may have had 700 miles on it, bendix cavity looked brand new, and I found 15$K worth of engine rebuild receipts from the late 80's in the bus paperwork box..  

Gotta be something on the commutator / amateur I'm thinking.. Quick lathe shave?! I'm a theoretical youtube starter pro now. Thanks again y'all for the insight information!

https://www.youtube.com/watch?v=MkBLwUG_AdU

https://www.youtube.com/watch?v=-3zX-V_Xjqo

.pdf breakdown of starter  https://www.jnelectric.com/publications/printableunits/Delco%2040MT.pdf

[busfan]

Well I was about to tear into the 40mt but having finally digested the current flow I decided to test her one more time and she was alive!  Jeez..

Turns out the bus was wired from the front to shut the engine down not start it up!

Lacking a helper and just hearing the click I thought the starter solenoid was engaging and it was actually the fuel shutoff solenoid / and the relay..

So I got the bright idea that if I took my screwdriver to one of the remaining relay's something else might happen   Bang whoa what was that..(initially the starter engaging and scared me and I backed off).. Then with a little more enthusiasm I pushed the relay closed for a half second and rrrroom! bog rrrooom bog rrrrrrrrrrrrrooooooommm then I was thinking Oh no! this is going to (might run away) and I held the fuel cutoff solenoid down.

Before I tried to start I did make sure the rack was free.. I also was going to change the fuel filter but the fuel kept pouring out and I just sealed her back up so the revs might have been the lean air working it's way thru?  Anything else I should do before starting her up again?  I have a gasket rebuild kit for everything.