Here you go Bud,
Essentially, you need to lift either the ground wire or the positive cable leading from the battery terminals and insert your meter set to read DC amps in series with the remainder of the tractor wiring.
READ YOUR METER INSTRUCTION MANUAL ON HOW TO SET THE METER FOR DC AMPERAGE AND WHICH METER SOCKETS YOU NEED TO INSERT YOUR MEASURING LEADS INTO TO MEASURE CURRENT.
The sockets for current are different than the sockets to measure voltage and ohms. This is true 99% of time on multimeters.
SET THE METER TO READ THE HIGHEST CURRENT POSSIBLE - USUALLY 10 AMPS ( some meters have a 20 amp scale). THIS IS DONE TO PROTECT THE METER FROM BLOWING AN INTERNAL FUSE IF THE CURRENT THRU THE METER EXCEEDS THE METER RANGE.
If the parasitic current is less than 1 amp, the meter may not register any reading on the 10 map scale. It is then safe to switch the meter to read on a lesser scale for DC amps - say 200 milliamp (.200 amps).
Hook up the meter leads in your tractor like in the diagram above. The parasitic current will actually be flowing thru the meter body.
Let's say for the sake of conversation, the parasitic drain is in the headlamp circuit.
We hook the meter up in the positive battery cable circuit, note the meter reading and it is .3 amps - so we have a 300 milliamp (.3amp) parasitic drain.
Now lets look at all the components current will flow thru in the above diagram.
Starter solenoid , ignition coil, and lighting circuit.
Start by removing the blue wire from the starter solenoid - no change in meter reading - no parasitic current flowing thru the solenoid. Replace the blue wire.
Lift the orange wire from the ignition coil - multimeter reading remains at .3 amps.
So no parasitic current is flowing thru the ignition coil. Replace the orange wire.
Now we come to the headlight / taillight circuit. Lift the Green wire from the headlight
Multimeter indication drops to zero amps - you found the circuit the parasitic current is flowing thru.
Now, if you have taillights parallel wired of headlights, disconnect the headlights. Still have .300 amps on the multimeter readout - well sir, you have unwanted current flow to the taillights. Time to take a visual look at the taillight wiring / taillight housing.
If you let me know what model tractor you have, maybe we can pull a wiring schematic and start to highlight the different current paths in that particular model.
Please say its not a 520H !!
Good question - thanks for starting a good thread.