Floating Tach

[72targa]

Took my S out yesterday (thanks for the prompt, Mr. Tilton!). Noted that at mid-RPM range (3500-4500) that the tach was "floating" vs holding steady on a specific RPM (at a particular level of throttle). What gives?

[Chuck Miller]

Peter,

First thing to do with the 'floating tach' deal is simple. Just disconnect the purple tach wire (black w/purple stringer) to the distributor and clean the spade connections thoroughly....

Then reconnect and see if that does it...

Hope this helps,
Cheers

[72targa]

Thanks Chuck. That will be my project for this weekend. I will give it a go and report back!

[Mr9146]

I'd also take some fine grit sandy paper to the points...can't hurt.

[Chuck Miller]

A little caution:

If you sand the points make sure you use a 'natural' sand paper (sand, garnet, etc (usually tan, brown, reddish) and NOT a metal based cloth or emery paper (usually dark grey, black, maroon) The fine metal dust you create using the latter will short the points.....

Hope this helps,
Cheers

[jameshtaylor]

If none of these suggestions does the trick, ship the tach to NH Speedometer and have them tear into it. I don't understand the concept but somewhere inside there is a silicon damper which often eventually fails to do its job. That was my problem when I had your symptoms. Under $100 to fix.

[304065]

Peter,

"Floaty" or "bouncy?"

Early tachs are a monostable multivibrator or "one-shot" that are based on a pair of transistors-- one receives a signal from the points circuit (420mA current with the Bosch '001 box) and triggers the other to send a voltage pulse to the tach needle. The more pulses, the higher the needle bounces, and the pulses are "integrated" by the meter movement to appear smooth.

Bouncing around is a clear sign of a defective voltage regulator, so check the voltage measured at the battery terminals first with the engine off, then with it running (and above about 2000 rpm preferably). If you get less than 12.5v in the first test and about 13.5 in the second I would investigate the charging system further.

[72targa]

Wow! I appreciate all of the insights!! I'll take them all into consideration and report back.

As for "bouncy" or "floaty", it's more "floaty".

[66S]

I think it is usually alternator or regulator.

[72targa]

Thoughts on this alternator replacement on Pelican?

High Output Replacement Alternator, New, 95 Amp, 911 (1966-77)