Re: Battery voltage decreases to 10volts
Good point, Dave. If you think the ign switch might be bad, it will only show up when its has been used, say for 10-12 minutes of time while current flows through it . This is case when there is a high resistance on the contact points of the switch.
First thing I'd do is to external charge the battery, have dinner, then come back and measure the charging voltage across the battery posts while being charged. if it measures above 16 volts, then battery is bad.
If battery charges up, install it and run the engine. As the battery warms up from charging, less voltage is needed for charging. After 10-12 minutes, I would suspect to see min 14 volts across battery posts. Anything less measured here, as Dave suggests, is a severe short, disconnect the battery post to witness change. Voltage pops up, bad battery - measure the battery cables. Voltage low, either a short in the car or alternator has a low output.
Feel the alt alum case. If hot to the touch, electrical system short, alternator is working its buns off creating current for the demand.
If everything is okay in the engine compartment, meaning you measure the red power supply wire to ground and voltage is 14 volts, good, horn relay buss bar, alt BAT terminal, etc.
Now move into the interior to the fusebox. You will see three spade terminals labelled BAT , IGN, ACC. BAT is supply from the red wire. IGN is from the ign switch, so is ACC, but another ign switch terminal. If BAT & IGN read different, then either a faulty connection to/from fusebox to ign sw or bad ign switch.
After the red supply wire comes through the firewall connector, the red wire continues to the headlamp switch - there is a factory crimp inside the electrical tape for a red wire branched to the ign switch.
So hopefully this helps. With headlights on, and you attempt to start the engine, the headlamps should dim when the starter drive engages and there should be enough power from the battery to start the engine with the lights on. Alternator belt may squeal until battery charge lessens up.
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