One of my pals that does auto electrical work told me what he wants on his tombstone.
"The man knew his grounds".
All circuits in a car must return to the negative side of the battery. When I had my original Mylar board out for a tach repair, I replaced all the grounds and added a few extra. When grounding dash devices to the sheet metal, there is no guarantee that the dash circuits have a good ground. Rivets and or spot welds can get loose, copper in the wires can fracture, and so forth.
I made a SAE J rated 8 gauge wire 17 feet long with 2 monster flat nose copper clamps. When I want to investigate a electrical gremlin, I attach one end of the cable to the negative side of the battery and the other end to a distribution block. From there smaller jumpers can be used to drop in grounds to qualify circuits. You would be surprised at how many electrical failures are caused by bad grounds. Most guys replace the device which failed due to a bad ground, and when they disturbed the ground during the device replacement the ground started to work. The problem is that American cars are minimally grounded to start with. American cars have 3 or 4. Japanese cars have 12 or 13. That is one reason how the Japanese earned a reputation for reliable cars.
The new Mylar board can be qualified to some extent with a multi meter, a variable resistor, and a 12 volt power source before it's installed.
How is it grounded? Where does that ground go? Is the negative battery to frame, to engine block, to firewall, to dash good? If a guy is cursing the repop Mylar board for not working, and it could very well be crap, has he replaced the grounds on his 50 year old car?
Go to a Marine supply house, usually they have the good electrical stuff.
