Ok here we go. This is the entire wiring harness of the 1872 / 2072
NO = normally open
NC = normally closed
From the engine connector.
>Yellow wire is magneto to M term on switch.
>White wire is alt charge.
>Orange is oil light to sender.
At ignition switch.
>B term Black is battery +(POS) from ammeter.
>L term Blue is lights.
>M term Yellow is magneto.
>R term White is charge from alt.
>S term Red is Switched ignition hot to PTO relay.
>G Ground/Green
At PTO relay.
>term 1 Red from S term switch. (Starter circuit).
>term 2 Red through brake switch to starter solenoid.
>term 3 Black through reverse relay and PTO.
>term 4 Brown through NC reverse switch, NO seat switch to reverse relay
>term 5 White from alt to reverse relay NO.
White wire from ALT Ties to OIL light,hour meter,PTO switch,reverse relay then to PTO on black when PTO relay closed.
Red from battery through fuse to ammeter becomes black to ignition switch B term.
Green wire is chassis ground to various points including engine block and G term ign switch
The diagram doesn't show it but the Ign switch has a ground strap and grounds through the case to the dash on Magnum engines. When my switch went bad last winter in the snow it grounded internally and I had to pull it out of the dash to make the tractor run till I got it replaced.
PTO switch
>OFF 1+2
>ON 3+4
>start 3+4+5
Ignition switch
>OFF M+G
>RUN with lights R+L+B
>RUN B+R
>START B+S
Frame, engine, dash tower, battery should all be grounded together. Should check as 0 on Ohm meter between any and all.
Hope this helps. I think I got it all.
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