Bad Tachometer And Condenser/capacitor Fix

[Zed Head]

I had a collection of odd problems all happen in one day, hot start (unclear why), stuck fuel pump relay (had to use my pump switch to get home), and brake pads grinding on the disc (caught just in time), and one of them, just to confuse the overall situation, was that my tachometer stopped working correctly.  It would just bounce around between zero and 500 or not move at all.  

 

I pondered living with it like that for a while after I fixed all of the other problems, because changing tachometers is a pain, but remembered that I had had to install a condenser on the negative terminal of the coil years ago, when using the GM HEI module with a 300ZX ignition coil.  Since them I've installed another engine, a different module (same brand after I damaged the old one), and replaced the 300ZX coil with the proper GM HEI external coil.  But never removed the condenser, even though I thought it might be unnecessary now. So I got out the meter to start checking circuits and instead found that the wire to the condenser had broken.  It was just hanging there.  Spliced it back together and the tach is back.

 

Lots of words just to say that sometimes these old tachs just need some signal filtering.  Maybe it will show up in someone's future search.  I don't know why the condenser works, maybe one of the gurus can say, but it definitely does.  I still have a good condenser, the stock configuration, on the coil positive.  But without that extra one on the negative side, the tach refuses to behave.