Battery light is on?

How many miles on the van? If it's high, suspect worn brushes in the alternator. To determine that you'll need an inexpensive digital voltmeter. Measure the voltages on the two small wires on the back of the alternator. This must be done with the engine running, and the numbers we need are when the problem is occurring.

This guide will help us fix it

https://www.2carpros.com/articles/how-to-check-a-car-alternator

Please run down this guide and report back. caradiodoc

IF the voltage is low with the engine running, have the alternator checked. If it checkes good, have the fuseable link between the alt and battery checked.

Check and or replace belt and tensioner

Recharge the battery on a slow rate for an hour or two, then use an inexpensive digital voltmeter to measure its voltage. If it's near 12.6 volts, it is ok. Measure again with the engine running. It must be between 13.75 and 14.75 volts. If it is low, dig down to the back of the alternator and measure the two voltages on the two small wires that are plugged in or bolted to the back. The engine must be running for these tests.

One terminal will have full battery voltage. The second one should be less, but if you find 0 volts, the brushes are worn. They can often be replaced without removing the alternator from the engine.

Caradiodoc

I'd really want to know what those two voltages are on the back of the alternator before I headed for the least common cause, (and most expensive). If you can get to the connectors on the Engine Computer, (Electronic Control Module - ECM / Powertrain Control Module - PCM), you can take the important reading on the brown wire / gray stripe in pin 8 of connector 1 instead of standing on your head to reach behind the alternator. You will measure full battery voltage, 0 volts, or something between 4 and 11 volts. 4 - 11 volts means that part of the circuit is working properly.

Caradiodoc

Did you verify powers and grounds for the alternator>> basics first.

13.6 under a load is not bad at idle. Should jump to 14 with 1500 rpm. Check the fuses for the light.

What was the issue with the old alternator? The pcm does control alt output.

Roy

Hi guys. You need to check the voltage on the two small wires on the back of the alternator but that has to be done with the engine running. One must have full battery voltage. The other one must be less but not 0 volts. Holler back with those numbers.

So my wife was going on a trip and half way there the battery light lights up. She gets there after four days heads back stops at auto store they test battery say its 12.1 volts okay to drive! She starts van light is out she drives five minutes light comes on she almost makes it back it stalls at stoplight battery to low to start/run. Gets a jump light is out she drives a mile to house lets it idle for twenty minutes shuts it off. I get there start it light is out I take it for drive stop a few places light never comes back on. She gets in it next day starts it and light is on. She drives a few errands with it on gets back home shuts it off. Restarts it light is off. Shuts it off restarts it light is on. I bought van eight months ago have had no problems. Alternator does not look like its been replaced in a while looks like 135 amp model. I know the regulator is in the PTCM. I noticed when I was checking it out the battery light does not come on with 'key on' like all the other warning lights. Darn warning lights are hard to see tucked under the small flap on top of dash. Did not think to do a code read? Probably will not get back to it until weekend sometime.

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.

All indication are you have a charging system problem. You need to have it tested while the light is on.

Did test while light was out and it checked out fine 14.4 volts I may just replace alternator this weekend since I have extra one. Kind of weird it works and does not work depending on what? I do not know. The regulator being external adds the possible problem of wires/connections issues.

It really does not do any good to test it when the light is not on. Intermittent problems are not unusual. Although the alternator is regulated by the PCM, it will normally set a code if there is an issue there.