Tach needle bouncing

[51504bat]

A while ago I installed a 4,000 RPM VDO tach in my '39 p/u with an 8ba converted to 12 volts and running a Bubba Chevy dizzie. When first installed the needle had a bit of a bounce to it but the readings seemed pretty accurate so I was just living with it. Then I installed a new set of solid core wires and the tach needle was all over the place and was reading over twice what it should at higher revs. A check with Mr. Google said that the tach trigger wire was picking up interference from the plug wires and to re-route the trigger wire away from the plugs and if that didn't work (which it didn't) to install a 10K ohm 1/2 watt resistor in the trigger wire which only made things worse. The company that I got the tach from suggested a tach noise filter for about $30.00 shipped. I then contacted VDO tech support which I should have done in the first place.They suggested a 1N4005 Diode in the trigger wire as close to the tach as possible. After a quick trip to Radio Shack for their part number 2761104 and less than 2 bucks for 2 diodes no more tach needle bouncing and the tach was reading correctly. If anyone is having issues with a tach needle bouncing or reading incorrectly you might try the 2 dollar Radio Shack fix before shelling out 30 bucks for a tach noise filter. JMO