I've got an intermittent charging system problem. It usually happens after/during a good rain. It's definitely the alternator not switching on. I pulled the main lead going to the battery, put a volt meter between it and the positive side of the battery to bypass any short. I grounded the F terminal to switch on the alternator and bypass the regulator. No go. No charge. The alternator isn't kicking in. The engine is spinning faster. When it does kick in, the engine RPM goes down a little under the load. So, alternator suspect. Here's the thing: I have had the alternator checked twice and it's within spec.
So, I did some studying and took some readings. Here's what I got:
At the alternator 3 wire plug, there is 12.5V going to the S (voltage sensor on regulator) and L (load, iirc) terminals. There's about 3.8V going to the IG terminal. Across the 7.5 amp fuse labeled CHARGE, it also reads 3.8V. The electric choke heating coil (at least that's what the wiring diagram is calling it and it is going to the carb/aircleaner) is also on that circuit, from what I've read on the wiring diagram. I went through the wiring and traced a ~1V drop back to the engine side of the connector that goes to the choke heater (~11.8V). It should read about 12.6V as it goes straight back to the battery. So, I'd imagine there's a short in the circuit. Of course, after I wiggled wires while I was connecting and disconnecting stuff, the voltage jumped up to 12.6V at the terminal and IG jumped up to 4.5V and the alternator now kicks in. If I disconnect the choke heater coil, the voltage at IG kicks up to ~11.8V because the choke heater coil is now out of the circuit. Still, the alternator doesn't kick in. So, I assume that you're supposed to be reading about 4.5V at IG. Argh. Assumptions.
Hypothesis 1: Intermittent short in circuit containing the alternator IG terminal and the choke heating coil, causing drop in voltage in circuit. The IC in the regulator probably senses a voltage range and 3.8V is below its range to tell it that the ignition is on. So, the IC doesn't trip the proper transistor that sends current to the alternator rotor coil, even through it's getting a good 12.6V at the S/sensor terminal because it doesn't "think" the ignition is on at all.
Hypothesis 2: Although the voltage is a little low due to an intermittent short, the regulator is slightly defective or just going bad and should be sensing 3.8V and sending current to the ignition coil or something down those lines.
So, on the wiring side, trace back wires to fuse box, strip off wiring loom tape if needed, find short or excess resistance in a connection and fix. On the regulator side...um, give it a stiff kick in the teeth?
Am I barking up the right tree?
TIA.
-Beth