erratic tach Stewart Warner

[healer1340]

My cobra has been on the road since1992. I had to replace the tach about 5yrs ago because it would be erratic, jumpy, and would often indicate twice the actual RPM's.
Now I'm having the same problem. I wonder if the MSD ^AL is causing a problem with the tach circuitry oor if I just got two bad tach's.The is a company , APT that supposedly repairs them and makes them more "friendly" with MSD units.
Any information would be appreciated.
Steve

[Dangerous Doug]

First, check that it is properly grounded. The "-" should be connected to a good, solid ground, as well as the case.

I had one SW tach fail, and replaced it. I recently had a problem with the tach (again) but it seemed to be related to having managed to drain my battery---once I recharged the battery with my garage charger the tach worked fine.

DD

[ItBites]

My SW tach only works when it wants to, otherwise, it reads more than actual...

I have the MSD 6AL and I have the MSD tach adapter box. 14 years ago I had the SW tach worked on by SW and also had the 6AL worked on by MSD and changed the tach adapter a couple of times. Nothing really made a difference and so I gave up eventually.

Maybe the tachs or the MSD stuff play better together today, 14 yrs later, but I just listern for when to shift. With the quick revs from my 540, I can't look at the tach anyway

[CSXDreamer]

I've only had two S&W tachs, and they both had crazy-wavy needles. I had the S&W in my LAX "repaired" by an auto intrsument repair shop, and it worked fine, for the first 50 or so hours. Then it started acting erratic, although sometimes it works just fine. The repairman mentioned something about the front bezel being rolled over the back shell of the tach, chipping chrome, and maybe not going back together correctly.

[jagrolet]

does anyone have diagnostic information on S_W tachs,voltages etc.
Or where i can get it. This is my second tach to just quit. I am using the tach signal connection on a GM type HEI


thanks
Wally