Alternator Exciter resistor value?

[Demlotcrew]

Gents,

Does anyone know the clusters exciter resistor value?

Thanks

Andrew

[Demlotcrew]

I took a set of clocks apart and it looks like the resistor is 10ohms.

[Brianmoooore]

The lit resistance of the bulb is 120 ohms, so 10 ohms sounds very low - it would be dissipating around 15W when the bulb is lit.
Don't know why, but 220 ohms rings a bell - that would dissipate a much more likely 0.65W.

[Demlotcrew]

I must have my decimal point out on the meter as the bulb measured 12ohms, but that was measured when the bulb was not lit!

Ive used an LED with no resistor and on its own it excites the alternator perfectly.

Once the alternator is excited, doesnt it just send 12v back so there is no current flowing? And there seems to be a few diodes that are on the same circuit?

[Brianmoooore]

Plenty of diodes on the same circuit in the alternator - two complete three phase bridge rectifiers.
Yes, when the alternator is up and running, one of the bridge rectifiers in the alternator sends 12 volts back the blue wire to the bulb, which then has 12 volts on each side of it, so doesn't light.
You should never connect a LED directly to a voltage source - it's a current operated device, which requires a minimum threshold voltage. No current flows until that threshold is reached, then the current increases rapidly with rising voltage, until the max. permitted current is reached and the device is destroyed.

[Demlotcrew]

There is a diode on the instrument cluster PCB which is on the same circuit as the alternator bulb and resistor.

Absolutely that is true about LED's, but these are regulated 12v and will be absolutely fine.

[Demlotcrew]

Well technically they are 5v LED's with a regulated supply, compatible with 12v DC.

[Brianmoooore]

Voltage on the blue wire is also used to control the 'bulb test' function of several of the warning lights on the cluster, so there's quite likely a blocking diode associated with this.
Sounds like your LEDs already have built in resistors.

[Demlotcrew]

I would need to look in to the wiring, but I think the alternator has its own feed and the other tell tale lamps have a fused +ve.

Will dig out the wiring now.

[Demlotcrew]

Here we go, found it, also shown are the diodes.

[Brianmoooore]

The lamps have their own supply when working as warning lights, but there is a circuit that switches several of them on when you switch the ignition on, as a bulb test. This circuit switches off when the alternator produces a voltage on the blue wire, and the lamps, along with the battery lamp, go out.
If you nearly stall the car, you can sometimes see these lights flash back on for a short while.

[Demlotcrew]

Again, cant argue that, thats very true, I just don't think I have the block digram for that circuit. Would be good to find it!

[Demlotcrew]

This must be controlled by the SI board as there are no SS chips on the main PCB, so will try and focus on the SI.