Here's one problem fixed: I was using a remote starter to turn over engine (no start problem) and saw large sparks coming from the battery ground strap connection at the engine compartment wall. I had checked that connection before and it seemed tight. I checked again with a 10mm socket/rachet and found I could tighten it a couple of turns more.

After that most weird voltage readings changed. Now when I turn on interior halogen lamp voltage drops from 12.7 to 12.6v instead of 12.7 to 10.9v. Same with parking lights, same with pressing on brake pedal. What a difference a ground makes!

One last weird problem. Temp gauge reads real low with old temp sender, with new temp sender it reads very high. Measuring resistance at temp sender (which depends on coolant temperature) the reading varies greatly depending on which ground point I use. I'm hoping a good engine ground will fix this problem.

What's the best size wire to use for chassis and engine grounds. If I don't have welding cable, will a 12 gauge wire be good enough, or is 10 guage necessary (or larger?).