by CowsPatoot » Mon Apr 01, 2013 8:41 pm
With the 4.9, the tach gets it's signal the same way it does from a stock Fiero....white wire (this may be different color in your custom harness) goes from the coil to the tach filter to pin C3 on the C500, then from there it goes straight to the instrument cluster.
So...one of the following things is bad.
1. Coil quit sending a signal (unlikely...never heard of this happening from a running car)
2. Tach filter has gone bad
3. Tach has gone bad
4. wire has lost connection somewhere along the line
I would start by doing a continuity test between the tach filter and the pin on the back of the tach. That will eliminate the wiring from the equation (assuming it shows proper continuity). From there....well, I have spare parts laying around, so my next step would be to swap the tach filter, then the tach. Keep in mind that your tach has been modified to read properly with the v8...a stock v6 tach will read 33% high, a stock 4 cyl tack will read double.