I drive a 86 2 door 4 wheel drive S10 Blazer with a 2.8 liter V6 and TBi. I keep receiving trouble codes 12, (Basic code for that year to say it is in diagnostic mode), and 34, (M.A.P. sensor), I have replaced both the coil and the M.A.P. sensor and still receive code 34. One terminal on the coil reads as both a positive and a ground at times. It normally only reads as a positive with the key on. and as a ground sometimes with the key off. Earlier the dash light were not working and one day when getting on the highway they just started to work. I am curious if the problem may be within the steering column near the ignition switch?

 

I would look for wiring damage on the wiring for the MAP sensor.