Hi Terry,

As Mick said the alt can cause this.
The second pin carries the load info and should be approx 50% duty cycle at idle (depends on a list of things) and 100% at full load (electrical load).
Yours is reading 100% at 0 volts and so the engine ECU has 'lifted' the revs to compensate.

Make sure you use a proper smart charge unit if you do fit one.

Hth Sean

Ps I think you could plug the replacement in prior to fitting and check for a duty cycle(earth the unit).