Stack Tach jumping

[nickal]

Good day! I am having bad needle jump with my Clubman tach. Conventional points setup. Above 4000 the needle jumps all over. Anyone out there have any ideas on this??

Thanks,

Nick

[Buc01]

Nick:

I have had similar issues in the past. First thing, check all your wiring connections (especially grounds).
If the problem persists, it can be bad tach. I had that happen to me once as well.

Aaron

[Phil Creighton]

Used to be Stack importer - bad ground usually. The tach needs something like 9 volts difference between tach signal and ground to work. Any resistance on either side narrows the gap from 12v and the tach thinks its not working and drops out. Swift DB4s were really bad for this with Electromotive ign. Make sure you have a ground from tach to battery is first check - not to a pop rivet somewhere on the chassis!
I still have a bench tester if you need to check the tach operation out of the car
Phil

[BorkRacing]

Used to be Stack importer - bad ground usually. The tach needs something like 9 volts difference between tach signal and ground to work. Any resistance on either side narrows the gap from 12v and the tach thinks its not working and drops out. Swift DB4s were really bad for this with Electromotive ign. Make sure you have a ground from tach to battery is first check - not to a pop rivet somewhere on the chassis!
I still have a bench tester if you need to check the tach operation out of the car
Phil

Absolutely follow Phil's advice on the ground wire. Also, never weld on the chassis with the tach connected to ground. It will make it jump forever.

Robby

[Rick Brannon]

Absolutely follow Phil's advice on the ground wire. Also, never weld on the chassis with the tach connected to ground. It will make it jump forever.

Robby

Excellent advice and Phil is 100% correct it's the lack of a good ground reference.

Welding on a chassis while any electronics is terminated is a no no and will affect the device remember silicon devices have a .7volt valence barrier easy to tattoo this junction. Somewhere when I was at Boeing we produced a video depicting welding on a piece of material (arc, tig, mig) with a bearing installed and you would be surprised where conductivity showed up on the bearing no matter where the ground from the welder was placed.

[David Ferguson]

If you are using conventional points, I would consider checking your condenser (the little round device mounted to the side of the distributor). If the wiring to that is broken, or it's not grounded properly, you will have voltage fluctuations which would cause a noisy tach signal.

[GregSmith]

Both Nick and I had the same problem, both DB1’s, Kent engines, both with Pertronix ignitions. Grounds were established, all connections freshened, still the same problem. Turned out that the rivets that mount the Pertronix module to it’s base plate had loosened, allowing the module to flop around a little bit. No wonder the tach was confused. Tightened the rivets, no more problem. Mine is a relatively old Igniter II and I will be replacing it sooner than later because the little “window” on the module has been scuffed up a bit from occasional contact with the rotor hub.