According to the schematics, That wire is hooked to only 3 things: the Idiot light, the relay coil, and the alternator. Unless you have the wire shorting to another voltage source, that voltage is coming is coming from the idiot light...as expected.
As mentioned, a typical idiot light circuit is 12V IGN-->Idiot light--> alternator. When the alternator is not charging, the alternator control wire sinks current thru the light, turning it on. When the alternator is charging, the control wire goes up to 12V, so the Light has 12V on both sides and is off (no current flow thru light).
In this case a relay coil is added "sampling" the circuit. With the alternator disconnected, the circuit is 12V IGN--> light-->control wire--> relay coil--> ground. The voltage on the control wire is "in the middle"....dependent the internal resistances of the Light and the relay coil. If your relay coil is 120 ohms, and the light is 65 ohms, your voltage would be 8.3 between the two. The relay is apparently pulling enough current thru the light by itself to turn it on --is it dim??
TO test if the 8.3V power is coming from the light, pull Fuse F058: 3A fuse for A074 (indicator module) (Late model combines may have different fuse #s). The Light should go out and the voltage should disappear.
My remaining question is if you hook up the control wire to the alternator, key ON, but motor NOT running (or belt off), is the voltage on the control wire lower that 8.3V? Ie--does the alternator actually pull that wire down to ground when not charging. You need to test this.
As long as the alternator pulls the wire low, I think using a SS relay should still work. This should not change the basic function of the operation we discussed---the alternator should pull the the control wire low (turning on the Light) or push it high (turning on the Relay). What would not work is the light would not come on if the control wire is disconnected.
Whats really confusing me is your last statement....pulling the relay turns the light ON with the alternator wire disconnected.... if that is true, then either my theory of operation is all wrong, the schematic is wrong, or .... something is really strange. Or the A074 indicator light module is was smarter than I thought..
Last edited by Jeff-C-IL; 12-13-2018 at 07:03 PM.