My alternator seems to have two field terminals or field wire posts, rather than the usual single field terminal or field post. It looks like there is a jumper wire connecting the field terminals together. I am used to seeing only the single field wire, a ground wire, and the large battery wire or output wire. Why would there be two field terminals?
Technical Editor:
Jumper wire between two field connections? The older (Prestolite-Ford D0FF and similar) alternators only have one field wire, which provides field current. The alternator housing provides an internal ground path for the field circuit. The newer alternators have an ‘isolated’ field circuit, with two insulated field terminals adjacent to one another (F1 and F2). The field circuit has no internal connection to the alternator case.
There is sometimes a grounding jumper soldered or crimped to a shielding braid around the positive field wire. Sometimes that outer braid is connected to ground at the regulator, sometimes at the alternator, and sometimes at both ends. Grounding both ends is usually not a good idea. If anything increases the resistance of the main bonding strap between engine and airframe, any wire shield that is connected at both ends will try to carry current. You should see what happens to a sixteen or eighteen gauge wire when the starter tries to put a couple of hundred amps through the shielding braid! Even if the bonding strap remains fully intact, return current from any engine electrical devices, such as the alternator, will be shared across all available electrical paths to the airframe. This can create noise in the DC buss; the exact opposite of what the field wire shield is supposed to accomplish.
If you have the newer style isolated-field alternator, with the two adjacent terminals marked F1 and F2, the positive field wire goes on one of these. The other field post gets a grounding wire. Someone may have set up your wiring so that the field wire shielding braid pigtail is providing the ground connection for the alternator field circuit. This presumes that the braid is also grounded at the regulator end or the firewall (not to the alternator or the engine). Otherwise there has to be a separate ground wire for the other (negative) field connection. In either case, the purpose of the isolated field ground is to prevent other engine electrical loads from using it as a return path from engine to airframe. Regardless of the form taken by the ground, there cannot literally be a jumper between F1 and F2 (positive field voltage going to both posts), or the alternator won’t function.
If there are two pigtails coming from what appears to be a single wire, one has to be a ground to the outer wire shield. You can check this with a meter. The center lead should have positive voltage with the Master and Alternator switches on. The other lead should measure almost zero resistance to ground.