Need some help: Tachometer displays four times 3.200k @ 800RPM

[lordoffrisco]

Hello FABO Members!

I need your help. I wanted a tachometer for my 1965 Barracuda Commando V8 and was searching for different types. When I found these pictures, the decision was made...

The Golden Commando Goldfish ran with the Sun Pro Tach in 1965. So I bought the Repro Sun Pro Tach. Everything was fine (set the switch to 8Cylinder), but when i tested the Tach it displayd way too much. Fourtimes too much. 3.200-3.600 (800-900RPM) when cold... So i switched the hole Cylinder Set Up at the back of the Tach (1-2-4-6-8-10) but it got only worse.

Last year I changed the original dual point distributor to a matching pertronix electronic ignition - thanks to the FABO
Working on electronic ignition conversion for HP273 dual point distributors

Maybe this causes the problem? I got a tip that there is an adapter (by SunPro), which could help and solve the problem...

But this Adapter is sold out everywhere

I was at a local Tachometer specialist (VDO Instruments and Gauges), but they couldn't help me either with my problem...

So at this point I do not know what I can do to fix the problem...

My Car
1965 Plymouth Barracuda Commando V8
273 4bbl
dual point distributor -> now -> Pertronix electronic ignition.

Maybe someone has an idea.


Thank you in advance!!!
Lord of Frisco

 

[Streetsboro Rick]

Did you talk to the people at Pertronix i have run that tach and no issues with points or factory electronic ignition

 

[halifaxhops]

Sounds like a bad tach that is out of calibration etc. You don't need that adapter.

 

[67Dart273]

Should work fine with pertronix. No idea..........

 

[KitCarlson]

I assume you have a good ground on the tach, and are using the coil (-) as the tach trigger. Problems of over reading, may be that the tach is seeing multiple (4) triggers for each ignition event. Tachs have an input circuit to attenuate and clip, followed by frequency to voltage conversion, to drive the gauge movement.

To separate a possible ignition problem from tach, you could possibly test tach with a 24 ac transformer. They are fairly common, a doorbell or hvac transformer comes to mind. At 60Hz, there are 3600 pulses / min, but divide that by 4 since a V8 puts out 4 pulses/revolution, so a V8 tach should read 900 RPM. Test that, if tach reads 900, good.

The next step for me would be use a high voltage probe 1-2kV and scope to view the coil (-). The scope view will provide the ability to identify multiple triggers. There may be issues in wiring, including ground that is retriggering, such a problem may lead to early failure of ignition, and poor high RPM performance.