I had a similar issue with my A3 several years ago. Headlights were bright, but the truck wouldn't start. I put a battery charger on each battery separately and the charger said each was fully charged. It turned out that one of the batteries was going open internally when under heavy load. I discovered which battery was faulty by using jumper cables to my F250 on each battery in turn. The rear battery was at fault. With the jumper cables hooked up to my F250 on the rear battery, the A3 fired right up. Do you have another MV you can use a slave cable with? If not, try using the jumper cables to another good battery like I did. New batteries can be faulty.
Thanks for the suggestion. I put jumpers on each battery 1 at a time, but no luck. Tried the same using 2 jump boxes on both batteries at the same time and still no luck. I think batteries are good, bc I'm getting 25v to the Aux switch #11 wires.
Here's what I'm seeing...
-When I check power to the Aux switch, I get 25v to each of the power wires (#11 wires), so that appears to be working correctly.
-When I turn on Aux switch (without aux wires connected - #27 and #27-400) I get 25v to the ports where those 2 would plug into, so that appears to be working correctly
-Where it gets weird... When I plug wire #27-400 into its port in the aux switch, the port where #27 would plug into than only gets 6V. Once I plug that one in, it somehow seems to rob the other port of also getting the full 25v to go to #27-400. I've tried this on 2 different Aux switches, both with same result. Either both of those switches are somehow bad, or something coming from #27-400 is messing with it.
