Jacobsen Tri-King 1900D suddenly stops

[Pilot J]

I have a 2002 Jacobsen Tri-King 1900D, which normally works just fine. But today, when I engage the cutter switch, the engine stops dead. The reels have just started to turn, but it's a sudden stop, as if the ignition has been turned off, although it has not.

The engine and forward and reverse movement works as it should, it's just when the cutter reel switch is turned on everything stops.

Any ideas?

Thanks

J