Yes you will eventually burn it up without the harness connected. It's probably not slipping, it's just when you are in 4th gear without the convertor locked it just seems like they are. If it has the correct lockup solenoid for the model it is, I can help you get it wired correctly. Look on the right hand side and towards the rear just above the pan you will see the year and the model and the date of assembly. You should see a 6 folowed by a 2 letter code such as MK, TK, YK and so on. Let me know what you have and we'll go from there.
