Altenator and Tachometer problem

[Phil23]

My woes continue!!! I was running a Teleflex Tachometer in my down stairs station. The unit just refused to operate so presuming it was faulty and replaced it with a new WEMA brand unit.
Once installed and on powering up the needle goes from 0 to 4000 then back to zero and will not show rpm under load. The wiring is correctly installed with POS to ignition, GROUND to ships earth and SEND to alternator exactly as per specification and previous instrument.
I have continuity on alt/sender wire which is running of a Lucas alternator with dedicated spade fitting.
While not mentioned in the installation instructions I have been told I should have a minimum of 4VAC between GRND and SND ??
The alternator is charging well with 14.7v on charge to batteries and is not running hot.
Before I pull alternator out (dog of a job) and suggestions

 

[catalinajack]

My woes continue!!! I was running a Teleflex Tachometer in my down stairs station. The unit just refused to operate so presuming it was faulty and replaced it with a new WEMA brand unit.
Once installed and on powering up the needle goes from 0 to 4000 then back to zero and will not show rpm under load. The wiring is correctly installed with POS to ignition, GROUND to ships earth and SEND to alternator exactly as per specification and previous instrument.
I have continuity on alt/sender wire which is running of a Lucas alternator with dedicated spade fitting.
While not mentioned in the installation instructions I have been told I should have a minimum of 4VAC between GRND and SND ??
The alternator is charging well with 14.7v on charge to batteries and is not running hot.
Before I pull alternator out (dog of a job) and suggestions

If the alternator is charging, why would you remove it? If the AC tap for the tach is putting out a signal, then your problem is not the alternator. I would run a test wire directly from the alternator's AC tap to you tach and see what results. You need to remove the existing signal supply wire from the AC tap to the tach as a source of the problem. Or, use a tone generator to test that wire's continuity.

 

[CharlieJ]

The tachometer converts the frequency of the AC signal from the alternator. To test the alternator, disconnect the tach signal sending wire from the alternator and install a short test lead to the quick disconnect tab. With the engine running, measure the AC frequency at the test lead while increasing the engine speed. The frequency should increase with increasing engine speed. If it doesn't, the alternator needs to go to a good alternator shop.