Very strange charging problems... Please help!!!

[91DSMdave]

Ok, I have searched and searched and tested and tested, still finding no solution to my problems.

I recently purchased a 90 GSX for a daily, thinking yeah got a good deal on this car! Get it back and start working out its bugs. Battery light is on, so I check voltage at the battery. 11.8 while running, so I say no good and quickly swap in a known good alternator. Same thing, at both the alternator charging post and the battery.

Checked all the fuses, tried known good relays. Nothing.

Then I tried jumping the alt relay... and the car idled down. Alternator kicked on but the voltage barely changed. It read around 12.5 at 3000rpm.

Checked resistance on the charge wire and it was fine. But even with the relay jumped and the alt "half ass" charging, the car will not remain running when the terminal is pulled off the battery. Here are some readings from idle:

Relay not jumped, everything how it "should be" voltage at alt- 11.8v

Relay not jumped, charging wire not attached measured at alt- 3-4v

Relay jumped, charging wire not attached measured at alt- 1-0

Relay jumped, charging wire attached measured at alt- 12v

I don't get it. I know even if the battery is junk it should run with the terminal disconnected. And the voltage isn't getting near where it should be even tho the alt charges with the relay jumped. So why wouldn't the relay be kicking on to begin with? I'm so lost right now.

Really. No one.

 

[wrencher]

Disconnecting the battery for a testing procedure is useless. Not advised either.
Modern vehicles are wired in such a manner to use the battery as the voltage stabilizer in the charging system for the solid state componets in the car. (I.E. ECU etc)

The results from disconnecting the battery running are of no consequence. Should never be done....ever.


As for your charging issues, the relay is really a resistor w/a diode. It does no switching, more of a back up/by pass circuit for the battery lamp circuit.
Make sure your battery is good/fully charged & the terminals are clean & tight.
At the alternator test the 2 wire plug to the regulator.
Disconnect the connector. Turn the key on, you should have very near currently available battery voltage @ both yellow & black/red-black white wires.

If that's ok reconnect connector do some volatage drop testing. (google it)
Run engine.
Check the positive side. From the output lug of alternator to the positve battery terminal measure voltage.
Should be very low voltage .1-.3 getting a little high.

Do the same for the negative side. You should see the the same (hopefully less) voltage from the alternator case to battery negative.

If anything is abnormal here you need to find the cause & rectify.
Test these things & post back.

But get the battery charged & confirmed OK first.