Alternator Warning Light

I am trying to understand how to set up a MoTeC to receive an alternator warning signal.

I will be removing the factory cluster and running a M150 with C127.

The alternator is Mitsubishi 140a with a 2-wire connector. One wire goes to battery ground +ve, the other wire connects to the warning lamp in the factory instrument cluster. The factory diagram shows the other side of the warning lamp in the factory cluster is connected to 12v through the ignition switch and also has a 80 ohm resistor wired in parallel.

I was hoping to connect the wire that goes from alternator to the warning lamp, directly to the M150. Can I use a universal digital input for this, and how would I set it up to work correctly?

I've included some pictures of the factory wiring diagram, along with how I plan to wire it if that's not clear.

Hi Tim,

First off I would recommend you take a second look at the diagram you attached. The voltage sense circuit of the alternator is wired to bat +ve not bat -ve.

The load circuit of the alternator can't be wired to the M1, none of the inputs or outputs have the appropriate hardware.

The load circuit of an alternator is specifically designed to use an incandescent bulb.

Hey Scott

Yes sorry, the sense circuit goes to bat +ve.

However the L (lamp) circuit actually goes to an LED in the cluster, and also triggers the LCD display to show a 'Check Alternator' message. I've attached an updated diagram. With this additional context, is there anything I could do to make it work?

You would need to scope the X1-13 pin currently going to the instrument cluster.

As it's an LED it might just be a warning lamp and not actually involved in the alternator control.

If this signal is just 0V key on engine off and rises to bat +ve once the alternator is charging you may be able to wire it to an input of the C127.

To be 100% certain you would want to temporarily disconnnect the intrument cluster globe from the alternator to be sure it will still charge.

On older alternators the field coil and globe are the in the same circuit, current has to flow through the globe and rotor field otherwise the alternator will never work.

Hey Scott

Yep sorry for the confusion. The wire I'm referring to does not control the alternator. I think it's exactly how you explained it, 0V when engine is off and then raises to 12-14V. I think it's known as an exciter wire, and it's also used to supply the initial power to the alternator (as well as illuminate the warning lamp).

I found this in the documentation:

"With the ignition switch in the ON position, current is supplied via the warning lamp to the L terminal of the regulator. This allows current to flow (within the regulator) from the generator B+ terminal to the brushes and rotor winding."

So I think I need the M150 or C127 to provide 12v to this wire (and I'll probably also need to wire in a 80 ohm resistor), and I'll also need the M150 or C127 to detect when this circuit is open (when there is no ground i.e. the alternator is working properly) or closed (i.e. when the alternator detects a fault and grounds the circuit). Not sure what pin would be best to use for this, or if it's actually possible.

I was actually suggesting the opposite. Based on the diagrams it looked like the LED wasn't used for field excitation.

If the load circuit of the alternator is actually used for field excitation like you're saying the documentation suggests then none of the IO on the M150 or C127 is really suitable for this purpose.