Toyota Trucks: Flaky charging system issue in 89 P/U

1989 P/U, 2WD, Carbureted, 4 speed manual. California Model.

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

Bruce L. Bergman wrote:
 

Theory and practice. Yes, in theory, there can be a sudden surge in
voltage with the loss of the load, which then might, possibly, cause
other damage. However, in practice, extremely unlikely and a
field-proven technique.

Only take risks with which you feel comfortable, of course.

GTr