OK I have a 94 with the check engine light on that I can't get the codes from. I tried a code scanner with no luck (a snapon mt2500). I tried the test light method. I also checked the data link according to the snapon guide and the positive and negative links are reading backwards to what the book says. The book says that I should get 5 volts at the negative and 0 volts at the positive and it is the other way around. Maybe that is a miss print in there book. Does anyone have the way to check the data link also the diagram to show where the wires from the pcm goes to the data link. Maybe somebody hooked them up backwards. I need to get the codes so I can repair what is making the light come on without replacing everything at the cost of a new vehicle. lol. Thank you for any help in advance. Matt 
