ok, so this all started with the tach inop on my 73 formula, points ingnition. I removed the instrument cluster from the dash, and the tach from instrument cluster. I tested the tach directly off the car-I have power at the pink and white wire in the dash and also made a jumper wire for the ground post and grounded that, and even sanded a spot on the housing and grounded that as well. Started the car and still no tach movement, needle sits on zero. Circuit board for tach shows no sign of a burnt resistor or anything obvious. The is also a QC sticker on the back of the tach dated in 2007 so I assume it has already been refurbished once. While going through this whole process, I noticed that the circuit board for the instrument cluster was new and the part number corresponds with 74 and later, not 73. The only difference that I could tell is that the 74 has an extra circuit for the brake light. There is no wire in the harness plug to correspond to the circuit board, which I assume is ok? My cluster does have a brake light, so did someone swap out the cluster at some point or did they all have it pre 74 and just not use it or is it one of those late 73 model year GM things where they just put in whats on the shelf that day? Should I change the circuit board back to the 73 part number? All the gauges and lights work correctly, just not the tach, and given that I have tested it outside the instrument cluster, should I just replace the tach and call it a day? Just worried I'll drop $300 and it still wont work.
