Resistor

will this resistor work to go inbetween the alt. and the and ignition switch? or is there something i can use in place of this?

http://cgi.ebay.com/5W-10-ohm-10-Wirewound-Power-Sandblock-Resistor-12-/380235075508?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item5887c6cfb4

No, not if your 2840 shuts off with the key-switch. 10 ohms isn't near enough resistance to prevent alternator feedback from keeping the injection pump going after you turn it "off."
75 ohms is often used in gas engines and a diesel will need something higher if it has electric shut-off.

The shut-off in the injection pump only draws around 1 amp and can work at 7 volts.

Do yourself a favor and spend a $3 at Radio Shack for two diodes. IN504. Radio Shack # 276-1144 Rated 3 amps. One works fine, but two hooked in parallel gives you reduncancy. Lets power TO the alternator to excite it, but lets NO power back to screw things up.

If you need a charge-indicator light to work, then just wire through that indicator bulb. It will BE your resistor and hopefully, enough to prevent the engine from running when you turn it off.

Gas engines are easier since an ignition coil needs more power to stay running. The solenoid in the injection pump for the diesel can keep going on much less.

Just use Ohm's law if in doubt. Backfeed from the alternator will be 14 volts. You have to cut it down at least 5 volts to shut let the pump shut-off.

I forgot to mention one thing. Most later Deere's like your 2840 have a key-switch with IGN and ACC terminals. So, you don't need any resistor or diode. You can put all the tractor electrics on the IGN, and put the alternator exciter on the ACC and both will be isolated when you shut it off and both energized when the key is "on."

its not for the 2840 its for a farmall H with a delco 10si alternator sorry for being unclear