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I have a C3 car starts normal after turning off it doesn't start completely dead no battery life does this random checked battery its full charged ,alternator good .
after boosting runs fine mechanic says could be ground problem how can I fix this where are grounds
You have two major ground wires. One short cable runs from the battery underneath the battery box to the frame. The other runs from the starter brace to the passenger side motor mount.
Battery cables may look okay, but may be corroded inside. Replacing the positive cable with the transmission in the car is quite a job (the positive cable runs up the trans tunnel).
I hope you can read this and see where I am going with this the battery cables at the battery can be loose or dirty and need to be cleaned and installed tight the end at the starter solenoid is also a big issue and can be loose even if the wire is tight the stud is loose on the cap of the solenoid if you have power but can not get it to crank and when you try it clicks I would look at your starter and not knowing WHAT YEAR you C3 is it makes a DIFFERENCE I do not know if it is an automatic or manual.
Google "drop voltage test". You will be able trace any drop in voltage along connections from starter to battery, and isolate where the resistance is occurring when you turn the key to start.