John Deere M cut out relay

John Deere M

New User
Hi, I am new to this sight, bit anyways... I have a 1950 John Deere M, and just put a new wire harness in it. After hooking everything up according to the wire diagram that was provided, the only problem is that the cut out relay has voltage going down to it no mater if the key is on or off. Any ideas of what it could be??
 
A cutout relay is SUPPOSED to have battery voltage to it's battery side at all times.

The GENerator side is disconnected from the battery at shutdown by the relay inside the cutout.
 
The relay is normally open.

There should be power to the battery side all the time.

The other side of the relay is wired to the generator. That side should not have power unless the engine is running and the generator is turning fast enough to produce a charge.

That is how the cut out works, it closes only when the generator is producing enough voltage to close the contact, then the generator is in circuit, charging the battery. When the generator slows or stops, the contacts open, dropping the generator out of circuit to prevent it from drawing charge back out of the battery.
 
The "Cut out Relay" is also known but seldom heard of as the "Reverse Current Relay". It is a normally open relay only closed by the
current produced by the generator. Once the generator is producing current and closes the relay contact it lets the rest of the current go
thru and charge the battery.
If this was just a wire from the generator to the battery without a relay and the generator would stop, the battery would push current
back thru the generator and drain the battery.
When the generator produces enough current that current goes thru a coil with a steel core and turns that steel core into an electro
magnet and pulls the contacts shut to connect the generator output to the battery. Thus charging begins.
If these contacts would stick closed which does happen current would flow from the battery to the generator and drain the battery. If
you look closely at this cutout relay coil you will see a small wire with many wraps on top of a much larger coil underneath the small
wire. The small wire is used to pull the relay contacts shut and the high current goes thru the large wire coil then thru the contacts to charge the battery.
Once the generator stops producing current the battery voltage is higher than generator voltage. Current then reverses thru the large wire
in the coil and cancels out the magnetic field that was or is produced in the small wire coil thus spring tension then opens the contacts.
If you watch your amp meter as you shut off the engine you can see the amp meter go from charging then to discharge then zero out. That is
the current reversing then the contacts open.
On the older riding lawn mowers that had starter generators the generator always turned the same direction during charging and
starting so the only difference was the direction of the current going thru the generator. The starter button on these little riding mowers
sent current straight thru the generator making it a motor. Once the engine started and you let up on the starter button current stopped
and generator charging current started flowing the opposite direction. The starter button was actually a second manually operated cutout
relay wired in parallel with the cutout relay inside the voltage regulator. Hope all this makes sense. Many new cutout relays on these
older tractors such as AC tractors are all electronic.
 
"It is a normally open relay only closed by the current produced by the generator."

VOLTAGE would be a more accurate term.
 
John Deere M, you state "the only problem is that the cut out relay has voltage going down to it no mater if the key is on or off. Any ideas of what it could be?? " Seems some cornfusion below so I will state this:


First of all the BAT side of the Cutout Relay SHOULD ALWAYS HAVE BATTERY VOLTAGE if alls well and wired correct and regardless if key was on
or off!!!!!!!!!

The Cutout Relay is a set of NORMALLY OPEN contacts located between its BAT INPUT and GEN OUTPUT terminals.

When the tractor isn't running (or no voltage from genny) the relay is OPEN so its then that there's NO VOLTAGE on the GEN side but still on
the BAT side.

When the tractor is running at sufficient RPM and iffffffffff the Generator and Relay are all working and wired properly THE RELAY CLOSES
so THEN there should be voltage on BOTH the BAT and GEN terminals

HOWEVER if the relay were to stick and remain closed upon shutdown (should open on shutdown) again there would be voltage on BOTH the BAT and GEN terminals and the genny would get hot and draw current and eventually discharge the battery

Hope this helps

John T
 
It "kicks out" and returns to its normal open when the genny stops producing voltage due to BOTH

the facts 1) There's no longer any current flowing through the relays small shunt winding to overcome spring pressure which holds it open

2) PLUS momentary reverse discharge current from battery to genny through the large coil winding helps force it open.

NOTE this is NOT a perfect detailed perfect technical explanation but intended to answer the question in basic layman terms to the poster so don't anyone have a calf

John T
 
I disagree with #2 as a magnet always attracts unless it is another magnet. The reverse current cancels out the magnetism & the spring opens the points. If the points stick closed, the reverse current will actually pull them tightly together again.
 
The North/South magnetic field direction depends on the winding orientation of the big coil (not the shunt winding) of wire. It can push or pull one direction or the other. There's two coils, the smaller lower current shunt winding and the bigger coil of wire activated by momentary reverse current (and resultant magnetic field) when the relay returns to open.

John T
 
Thanks, I understand how it works but mine is not opening up when the tractor is shut down. I either have to disconnect the battery or pull the points a part. Is the cut out relay my problem?
 
I'm trying to get my 1948 M up and running after sitting for 20 years. The cut out relay is totally rusted out but is exactly like the one in your picture. The original is obsolete so can someone give me a part number so I can order a replacement
 

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