1976, sbc350, th350

Stay with me, this is gonna be a long explanation.

Tachometer was stuck at 3k rpm. Went ahead and pulled it apart and notice that the board got hot and burnt one of circuit strips. Got a new tach board and went to 0 it and had nothing, needle would not move.

So, checked power and signal and both had power. Checked ground on tach connecter and it had ground. Also checked the ground tab that gives ground to all the lights and it was good.

Next I checked the tach motor, had 216 ohms on power and ground, and about 220ish on power and signal. I thought it was alot so the next plan of action was to make power probe. Took two wires and connected one to the stud for and ground and signal for power and touch one on ground battery post and one on the positive battery post and it shot to 7k rpms, so figured the tach motor was still good. On a side note when I went to check for ground on the ground stud it would not pick up anything. I don’t know if this will tell you anything but just trying to give all the info I can.

Now onto the board, on first visual inspection after I got nothing when going to 0 it, I could not see any imperfections. Everything was good. So went an ahead and checked resistance on the power, ground and signal prongs. Power to ground is 2.9M ohms, ground to signal about 2.2M ohms. This is my first time with a tach board so I don’t know the specs for how much resistance it should have. Last thing about the tach board, when I checked for ground on the ground prong is was not picking up anything.

Lastly, a few extra things, the tach got stuck at 3k when my fusible link blew (one on passenger side next to the starter). I also have replaced the alternator before all this and the first startup is when I was doing my testing. I got this tach board from top flight automotive.

To me everything is leading to me just getting a bad tach board off the rip, but let me know what you think. Is there anything else I need to test? Should I just buy another board?

Thanks, any info or tips would be greatly appreciated! Nate

 

Not certain how you checked your ground. Ground on tach to neg. Post on battery should be near zero ohms. Like .1- .2 ohms.
If indeed you do have a decent ground. I would suspect you got a bad board.

 

Sure you have already seen this but just to be sure....


I thought the signal from distributor is a pulsed (+) frequency, not sure if putting 12 volts to that terminal is a good idea. Remember, when you see the smoke its the magic leaving the circuit. Good luck.