Alternator driven tachometer crapping out

[erasedhammer]

So I installed the alternator driven tachometer on my 60 amp a couple of months ago and then a couple weeks ago I upgrade to the 200 amp. Tach has been working great before the 200 amp.

As far as I can see the tachometer works fine and reads correctly between 0 and 2900 rpm. But as soon as I go above 3000 rpm, like on the freeway 60+, the rpms start to read radically lower. Like at 62 I would assume the engine is running around 32-3400 rpm but it reads 2400. Also above 3000 it bounces around a little, it's like the needle doesn't want to go above 3000.

Is this caused by the alternator upgrade, or could it be something else. Belts definitely aren't slipping, I have the correct ones installed and they are very tight.

Is there some sort of caveat with driving a tach off the 200 amp?

 

[mechanicjim]

for the Tach signal from the alternator what is coming out of the alternator is 1/2 system voltage AC, basically its unrectified voltage. now if you have a DVOM you can put it to Hrz setting and look at the RPM of the alt itself(it may show like 100.3 this is 1,003 RPM). for engine RPM you have to do some math as normally the alternator speed is 3:1 of engine speed.(on most commercial diesels) so 700 engine speed is 2100 alternator speed. so a engine RPM of 3000 is like 9000 at alternator give or take which is the high end of there working range.

 

[juanprado]

The easiest way is to just tap off the 2 wires for the tach drive for the ste in back of the engine. 2 butterfly in line taps and done.

To use the 200 amp niehoff alt to drive the tach you would need the corresponding harness to drive the tach for ambulances with the tach that uses that cannon plug harness. Relatively expensive and scarce.

 

[pjwest03]

Also, different alternators can have a different number of poles. This results in a different frequency on the output. Most/many of the after-market tachs have calibration adjustment to accommodate that.