Can you see the wire colors? Less than battery voltage is normal on the dark green control wire but it must not be 0 volts. If that 9.5 / 12.5 volts is on that wire, that part of the system is working and you didn't make good contact with the probe on the other wire. It had to have full battery voltage for there to be anything on the one you found voltage.
If that 9.5 / 12.5 volts was on the dark green / orange wire, that is the 12 volt feed and what you found is too low. Since that circuit runs injectors and ignition coils and is obviously working, that leaves a corroded splice as the likely suspect.
I think the first scenario is the one that pertains to your van because to actually have 0 volts on the dark green control wire, that wire would have to be grounded or the brushes inside the alternator would have to be worn and open. Worn brushes are common but not on a replacement alternator unless you installed a used one. If that wire was grounded, the system would be charging wide open and you'd be having an overcharge condition. Either way, this would imply there are two different problems.
Assuming the first story is right and you found that voltage on the dark green wire, measure the voltage on the fat bolted-on output wire. If it's real high with the engine running, as in 16 or more volts, there's a break in that wire going to the battery. Look for a blown large bolted-in fuse under the hood. You will also find 0 volts there with the engine off.
If you find full battery voltage at the alternator output terminal all the time but it's not making it up to 13.75 to 14.75 volts when the engine is running, suspect a failed diode inside the alternator. That can be identified with a professional load test. Most of these were 90 amp units. With a failed diode you will lose two thirds of its capacity and only be able to get around 30 amps. That would explain why the system voltage was dropping to 12.9 volts with many things turned on. The alternator can't keep up. That low voltage will turn on the "battery" warning light. Most load testers also display a bar graph to show relative "ripple". Ripple will be very high with a bad diode.
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Saturday, June 16th, 2012 AT 11:02 PM