Alternator exciter wire

I recently installed a new alternator in my 87 XJ6. This is a 3wire 100amp AC Delco. I installed the same type alternator about 10 years ago using the old air pump bracket and it served me well, but it finally died. My problem is that the exciter wire remains live even with the ignition off. After an hour or so the alternator gets hot near the plugs. I have unplugged the exciter If I have to start the car, plug it in, then unplug it
once the engine is running) What is the most likely cause(s) of this? Any ideas would be appreciated.


When plugged into the alternator? Or unplugged?


You are running current through the field coils when the engine is turned off. Either the internal voltage regulator is non functional, or your replacement is set up for an external regulator. The wire energizing the field coils should only be hot when the ignition switch is in the “run” position. If you had an external voltage regulator, it would monitor the battery voltage and adjust the current to the field coils, which would be proportional to the voltage output. With an internal regulator, you just need to provide 12 volts to the field terminal when the engine is running.

Thanks. It’s hot when unplugged and remains hot when I plug it in…both times the ignition is OFF

If it is thermally hot with the field coil (rotor) excitation unplugged then the only way current can flow through it is if you have a defective diode on one leg of the stator. Diodes act as one-way valves, collecting 3-phase AC and converting it to DC. Normally they fail open, no current in either direction, but if it were to fail closed for some reason, it would feed DC current from the battery into the stator.

Odd (I assume you mean 12V by “hot”), The wire IIRC is supposed to be connected to the indicator light, the other side of which is connected to ignition-switched 12V. So, when measured with a high impedance voltmeter, you should get 12V transmitted through the light, but only with ignition on.

Try grounding the excitation connector. There are two possible outcomes:

  1. The alternator indicator light lights up, whether the ignition is on or off.
  2. The light does not come on. You might get a spark or blow a fuse, or maybe not.

If the light comes on, even with the ignition off, there is a short providing 12V to the other side of the light, bypassing the ignition switch.

If the light doesn’t come on, you’re getting 12V directly to the exciter wire. And the 12V is NOT coming from switched ignition. IMHO. [There’s a third possibility that the light is burned out, but current is flowing through a resistor in parallel with it. Not likely but would confuse the diagnosis.]

As Mike says, Robert; you have defective diode(s)…

With the exciter wire detached from there is no ground, and the current through the wire’s warning lamp is too low, some 1/2A, to heat the alternator. Besides, the lamp would be lit if current goes through it - and if the bulb is blown; even if a bypass resistor is fitted, the current is too low to eat the alternator. And fourthly; there is no power at the ign switch to the exciter with the ign ‘off’ - if it was, the waning lamp would be lit…:slight_smile:

Standard diode check; disconnect a battery clamp from its battery post, and measure current flow (with test lamp or ammeter) between the clamp and the post. With ign ‘off’, you should read some 15 - 35 mA - a blown diode would fully light the test lamp, and ammeter would read several Amperes…

xj6 85 Sov Europe (UK/NZ)

I think you’re right, Frank. I misread the OP, thinking that it was the wire that stayed “hot” meaning that it had 12V present. But the post mentions actual heating of the alternator. As you say, that wouldn’t be possible with the very small current supplied via the indicator light.