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Alternator Wiring

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I am wondering what the norm is for wiring an alternator in a race/track car scenario.

I no longer have the standard gauges, and don't intend adding a "battery" light. Instead making using of low voltage alarms on my digital dashboard from CAN.

My alternator as 4 connections

1 - Ring terminal To Battery

2 - Signal

3 - Switched Ignition

4 - Warning Light (Gnd?)

My alternator and battery have a kill switch between them, meaning when my car is stored the two are not connected.

What would you advise?


To avoid incorrect advice, exactly what alternator are you intending to use? Some use the warning light as the exciter circuit earth I mean power source.

Fair question.

I am pretty certain it's a Denso unit, fitted a 1993 Toyota Starlet. Will double check this weekend.


G'day Tim.

Is your kill switch an emergency one, or just for storage? If its an emergency kill, you'll want to ensure your alternator is wired to the battery side of the switch, as if its used while the engine is running, you'll disconnect the battery from the circuit, but not the alternator. The alternator will keep supplying the rest of the system, and give a decent load dump spike that can be pretty damaging :-/.

For the rest of your alternator wiring, you'll want the signal wire going to the non-battery side of your kill switch, this is the reference voltage the regulator in the alternator looks at to determine its current output.

The switched ignition is most likely the supply to the rotor field winding, you'll need this.

The warning light you probably wont need... But sometimes these are actually the supply to the rotor field winding, although thats a bit old-school.... Can you get the FSM wiring diagrams for the vehicle? Or something with a similar alternator?

Thanks for the response Zac.

Currently my kill switch is wired as follows: (Larger)

I think in my mind I had the alternator being connected to the opposite side of the switch to the battery, in a similar fashion to an older style FIA manual switch. But you've got me questioning that now. The "Spike Suppressor", is a large resistor sat behind a TVS diode, I guess the theory is that when the battery is removed from the circuit any spike is sent to ground via the large resistor rather than into the cars electronics.

The part labelled B200 in the above diagram is a V23132 TE Relay.

Thinking about it, the car should still stop if I wire the alternator to the battery side of the TE relay, negating the need of the spike suppressor, as I imagine the battery then does that job? Hmm.

Here is a simplified diagram of the factory chassis harness of my car (Larger)

I was unable to find time to get over to the car this weekend, hope to do so during the week.

A bit different to motorsport type wiring. But DG transport wiring will often use an electronic battery isolator that incorporates an additional set of contacts that kill the alternator field circuit. The alternator would typically be modified so there were 2 additional wires leaving the alternator that would be run through the isolator unit to complete the field circuit. The isolator itself would be wired to open circuit the main battery feeds directly (both positive and negative), and the 3rd set of contacts kill the alternator field. I think the alternator contacts also open monetarily before the main contacts when the unit is tripped, to give the alternator a bit of time to collapse the field. That way the alternator cannot generate power without the field circuit, so its out of the picture. And then the battery main contacts open and kill everything else. The isolator is wired to the E-stops around and inside the vehicle, and also a roll-over switch. Sometimes they will also have a manual switch on the dash to kill all un-nessisary power (including the alternator) but the engine can still run. That way the vehicle can be driven in/ out the dangerous area.

The 'negative cut trigger' I assume heads to an ecu input or similar to tell it to shut things down if either of the kill switches are pressed? The surge protection unit should take care of a load dump spike, so the pictured setup should be all good :-).

The negative cut trigger forms part of the Ignition switch circuit, so if either stop button is pressed it cuts ignition. (ECU, Coils, Fuel)

I've provisioned two wires for my alternator, although a part of me believes that one spliced to the Ignition and Battery sense connectors at the alternator would work as well, they're both 12v feeds after all.

Something I was reminded of when looking for some info' for the OP, if you take the alternator voltage senser (reference voltage) from the output, it will be less than the voltage at the battery due to the drop through the cable from the alt' output to the battery; some alternators can potentially supply high levels of current, so output and cable length should be taken into account when figuring the guage wiring to use.

NOTE, some vehicles have rubber or plastic insulators for the alternators to reduce vibration damage and these usually NEED a separate earth for the alternator body to the engine and it also has to be of a sufficiently heavy guage - I had one where I was replacing the alternator and the earth had got hot enough to melt the insulation on it, so I doubled it up with a spare link I had, something to bear in mind.

Just backing up Gord on this. Its best to run a separate sense wire to the alternator from the vehicle side of your kill switch. The sense circuit in the alternator draws very little current, so there is a very small drop along the sensor wire, this the sense circuit sees the voltage your Battery, ECU, etc will be seeing. Makes for a more reliable charging system. If you run a single sire down that powers both the rotor field winding, and splices off to the sense input, you might see a higher output current, and thus voltage than you expect.

I've struck the isolated alternator issues before also! I remember asking the mechanical guy several times if the alternator was mounted metal to metal, and was always told it was... After chasing my tail for a day on charging system issues I relented and spannered it off myself, to find some nice delrin isolation bushes in place. Made up a nice earth strap to connect the alternator body to the block, and amazingly all the problems disappeared!

Thanks for that.

I Stumbled across this as well:

I'll be sure not to cut corners. Earlier I suggested I'd provisioned two wires for my alternator, that is 2 wire plus the small gauge charge cable to the battery. I think I have my bases covered now, and a better understanding of why I need to do things this way.

Wow, that's a really thorough link! Going to bookmark that one for future questions on the topic :-).

The whole Wiki is pretty useful for Toyota tinkerers:

Including a file which contains every Toyota connector part number from 1991 to 2005!