[davepl]

I converted my idiot-light dash to a rally (gauge) dash in my 2+2. While that was going from a Canadian idiot light cluster to a US rally cluster, wiring changes were required. I would imagine that if they could be made to use the same setup (PCB and harness and connector) they would have. But I think the idiot light took a different path on the PCB.

I can't tell you why, just what my experience was.

I also believe the ammeter and the idiot light provide some kind of resistance load to help excite the alternator. If you go to a one-wire alternator you lose that and they have to self-excite or be taken over a certain RPM to even start working. So the light does something useful.

Actually, that just reminded me! When I was a kid, circa 1972, the car's ALT idiot light would dimly glow now and then. Drove my dad crazy. He carried an alternator with him because he just didn't trust it. No one could ever find the problem, but it never manifested into any failure, just spooked us that the idiot light would light sometimes.

In the 90s I noticed somehow that when it did it, if you reached under the dash and grabbed the bulk of the harness, you could move it around and change the degree to which the ALT light was lit.

In the 2000s (and this is all the same car) I started a tear-down for restoration. When I pulled the fuseblock, one of the connectors between the engine side and interior side of the fuseblock bulkhead was just laying there and HAD NEVER BEEN CONNECTED from the factory. So if things moved or jostled one way or the other, they might touch, but never reliably.

Sadly my Dad had been dead for 14 years or so at that point. I really wanted to be able to tell him the solution to the decades-old mystery!