97 & 98 Ford Mustang 3.8 need help. On the data link number 4 pin it show no chassis ground . I found 1 at the battery 1 at the starter . And one from block to frame . And 1 in the cab by the computer . I clean them all . But it still say no chassis gronnd . How do check a ground to see if it good or bad . And are there more ground that I do not knowabout ? Help if you can thank you