I’m dealing with the Massey 850/860 and 8560/8570 combines as examples, but this column is intended to show how to follow any schematic.
It isn’t as difficult as many folks think. I hope that by following along with the illustrations on the opposite page, readers might gain confidence in working with electrical systems.
This is how a functioning relay works. One of the pins will have a wire coming to it that is live all of the time. It is labelled one, B or 30. It might come from the live post on the starter solenoid.
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Another pin will have a wire attached to it that is switched. It will have a three or a C.
If there are any safety switches, such as an ignition switch, they will be on this pin.
When the switch is turned on, it sends a current to this post on the relay. Inside the relay will be a coil that pulls a contact across from the original live wire to another post that is hooked to whatever job is being done.
That is also how the relay can be tested. Put one live wire on the No. 1 or B pin. Put a second live wire to the No. 3 or C pin. No. 2 should be live. No. 4 is the ground. The main relay on the 860 combine is an example of this type of relay.
In the starting circuit of the 8570, illustrated as the engine start and cab power circuits, there is a live wire running from the big post on the starter to the B pin on the relay and then to the B post on the ignition switch. This is always live.
When the ignition is turned to “start,” it sends a current to the thresher clutch switch, which has two sets of contacts. When the thresher is in neutral it is open. That closes the two contact points for this wire from the start point on the ignition switch.
People have had trouble with this. The clutch stopped working and this switch was at fault, so to keep themselves going, farmers have hot wired this switch. It makes the clutch work, but the combine can’t be started after that.
After the clutch switch, which is closed when the clutch pedal is depressed, the current moves on to the unload switch, which has to be disengaged for it to be closed.
It is easy to check these switches with an ohmmeter. When they are closed, it should read nearly zero and when open it should read infinity.
After the current has gone through the three switches, it goes to the C pin on the starter relay. This connects pin B to pin S and the wire from pin S goes to the starter solenoid and starts the combine.
But hold it. We haven’t got any fuel there yet. This same wire, coming from the S pin on the starter relay, also goes to B on the three pin connector and via a white wire to the fuel solenoid on the engine. This engages the pull-in winding that draws about 75 amps and opens the fuel to the engine.
Shutdown circuit
That same fuel solenoid can be found on the emergency shutdown circuit in the top right hand corner of the figure. The middle wire coming to it is the one we just discussed. It operates the solenoid.
However, since it is now dead, we must get energy from somewhere else. Note the ignition switch is now on I, which is live when the combine is running. It sends a current through the fuel shut-off relay to the other pin on the fuel solenoid, which draws about one amp. This keeps the combine running.
If the coolant temperature rose to 220 F, the coolant temperature switch would close and the battery current would flow from 86 to 85 and the coil would pull the lever from 30 to 87a off the connection. There would be no power to the fuel solenoid, the engine would die and the warning light would come on.
So you see that anything that shorted out the wire anywhere along its length would make the light flicker and shut the engine down. This usually happens at a connector.
It’s hard to get into those connectors but the problem seems to correct itself when using that special connection grease available from the dealer. To keep the combine running, you pushed the over ride button. That opened up the circuit so the relay wouldn’t receive a ground for pin 85.
Do’s and don’ts
On most machinery negative is the ground. Backward hook-ups ruin alternator diodes. Don’t run the engine with batteries disconnected because it produces high, damaging voltage.
Also, don’t run the engine with the output wire on the alternator disconnected.
Prevent battery hook-up sparks by disconnecting the ground strap first.
Reverse procedure when installing the battery.
Guenter is a retired Massey Ferguson service manager.
