Tach reads 1000 RPM low

[kstens]

I just returned from Rennsport III and had a strange thing happen. I was driving along and suddenly noticed that my tach had started reading about 1000 RPM low at all speeds. The next morning when I started out again it read normal, after about 20 minutes it dropped about 1000 RPM's again and remained reading low the rest of the day. My car has an MSD ignition with the MSD tach adapter. Up until this trip it has always worked fine. Please help! Thanks, Kevin

1965 911

[304065]

1000 RPM low at what speed? It wasn't reading zero at idle, do you mean more like 3000 at 4000?

The '65 tach has a one-shot (monostable multivibrator) that looks for a 24 volt square wave. Since we know that a 911 uses 38 degrees of ignition dwell, that's a clue to the duty cycle the tach expects: 38 degrees of dwell times six ignition events per distributor revolution is a 63% duty cycle.

The MSD output on the side of the box is a 12 volt square wave with a 20% duty cycle. So if your tach adapter is wired off the output, there's a difference right there, the MSD is only "high" for 20% of the time vs. 63% of the time, so it's not going to trigger the input transistor on the tach's one-shot often enough.

Now, if you are using the tach adapter wired to the points, then you get the full duty cycle. What is the tach adapter? It's basically a big coil that charges up with voltage when the points are closed, and then when they open, the magnetic field in the coil breaks down, sending a big voltage pulse forward to the tach (and also through the points, BTW). Sometimes there's a diode in-line to damp out the huge ringing and noise the coil generates. The coil gets hot in use, and that's what I think your problem is. . . as the coil heats up its resistance drops and that is changing the appearance of the pulse that comes out of it, causing the tach to read differently.

Bottom line: if you plan to stick with MSD, have the tach board replaced with an MSD board by North Hollywood or Palo Alto and use the output on the side of the MSD box, this is steady as a rock.

[ttweed]

Check your battery condition and alternator output. My tach was fluctuating the same way when the alternator was failing and voltage got flaky.

TT