I mean, how do I check which ground is bad? besides the battery grounds, will other grounds for other devices in the car affect the drop in voltage?

 

A very common problem with our cars is the grounding point from the negative battery cable to the frame, next to the battery. I used a multimeter and detected a voltage drop between the negative battery post and the body when running a lot of accessories. I simply picked up a short battery cable with lugs on each end from Advance Auto Parts and connected one end to the bolt on the negative battery post and the other end to the bolt going through the negative battery cable into the body next to the battery. I never had another problem --- and its gotta be at least 5 years now. Hope this helps ...

 

Most people look at the ground clamp on the battery and pay no attention to the cable going IN TO the ground clamp. A lot of issues are within that area.

 

EXACTLY O. L. T. I don't think people realize that this is the main body ground for our cars --- not just a clamp.