Pilot resistor on Alternator signal

I know there's no auto-leccys on here but maybe some of the sparkies can confirm this one for me.

It took quite a bit for me to nut this out so the info might be handy to somebody one day.

None of this crap is present in the D22 wiring harness as the dash cluster is computermerised.

In the D21 TD27 diesel there's a "pilot resistor" teeing a <12V supply into the signal wire from the alternator to the dash. Downstream of the resistor the signal wire from the alternator is patched in.

What I really needed to know was if the resistor is likely alternator-specific, or combination-meter specific, as I'm changing alternator but not combo-meter.

Here is what I think so far, some confirmation would be great:

IGN OFF, ENGINE OFF: Fused 12V field excitation feed to alternator, no feed to "charge" lamp or resistor.

IGN1 RELAY CLOSED, ENGINE OFF: Fused 12V field excitation feed to alternator, 10A fused 12V feed to resistor, signal wire from resistor to "charge" lamp.

IGN1 RELAY CLOSED, IGN2 RELAY CLOSED, ENGINE OFF, KEY TURNED TO "START": Fused 12V field excitation feed to alternator, 10A fused 12V feed to resistor, signal wire from resistor to "charge" lamp, second 10A fused 12V feed direct to "charge" lamp.

IGN1 RELAY CLOSED, IGN2 RELAY CLOSED, ENGINE ON: Fused 12V field excitation feed to alternator, 10A fused 12V feed to resistor, unfused 12V signal feed from alternator to low side of resistor (making both sides of resistor 12V), signal wire from resistor to "charge" lamp.

So the "charge" light comparator in the combination meter sees <12V vs. 12V when the key is "ON" but the engine "OFF" and lights up, another 12V feed when the key is held at "START" so the lamp goes out while cranking (running through that ever-present hardworking "METER BACK UP LAMP" fuse) and so sees 12V vs 12V, and a 12V feed from the working alternator once the engine is on, again 12V vs 12V and the lamp is off. I think?

Yes I'm aware it's closer to something like 13.8V. 12V "nominal".

Cheers everyone.

It sounds like you've nutted it out right, and the resistor would be meter-specific.

The alternator is capable of producing 18V or something like that (maybe more). Attached to the alternator circuit is a regulator providing an output of 14.4 to 14.7V.

I'd go right ahead and change that alternator, it shouldn't affect the meter one bit.

Just on the <12V input ... as soon as the alternator fires, that will see a >12V on the other side of the resistor so power should flow the other way. The only thing you need to light a lamp is potential difference and when the engine's not running, the pilot resistor provides that until the alternator feeds into that line.

12V-----------meter lamp-----------alternator feed--------------resistor------------12V

When the alternator feed is off, there's a potential difference between the resistor and the 12V on the left, so the lamp should light. The resistor determines how much potential difference and therefore how many volts the lamp will get => brightness. Switch on the alternator and since the left-hand 12V is coming from the same source as the alternator feed, the meter lamp sees no potential difference and therefore is unable to light.

Thanks for that Tony. It will be interesting to try and relocate the resistor outside the cab, as the branch it used to live on can no longer exist with the new harness.

Electronics = not my strong suit.