Alternator/Tacho Query

[darrenyorston]

I think either my tacho has given up the ghost or the alternator is not outputting correctly. Is there a way I can test the output from the tacho post on the alternator to see where the problem is coming from. There seems to be no pattern to the gauge. When djsting the throttle or turning on some electrical component the gauge seems to move randomly to a new number.

[FAST FRED]

Dirty ground.

But remember a tach driven by an alt has almost no repeatable accuracy.
The belt temp, glazing , alt load and engine room temp all will change the reading.

FAST FRED

[darrenyorston]

What other options are there for a reliable tacho?

[Frosty]

Alt tachs are not accurate at all. There is an adjustable screw on the back that allows you to adjust the tach to the nearest settings. Obviously it is alt speed. They are just good enought for a repeater situation where calibration of say 2 engines was more important than actuall RPM.

You can then make a mental calibration from your known good ones.

As for your problem,-- if the adjustable screw is not making a good contact this would give the simptoms you describe

[FAST FRED]

For a gas engine some car makers offere tachs driven from the distributor.

For a diesel a tach pulse sender can be installed to a cable tach drive.

Ot the pulse sender can go on an aux power pad.

FAST FRED

[kapnD]

Have a look at tinytach.com. They make an accurate, economical diesel tach that senses the pulsation in a high pressure fuel line. No magnets, no moving parts, simple and effective. You can calibrate your tach with a phototach, but as mentioned above, an alternator tach can be fickle for a number of reasons. Fred's mention of dirty ground is where I'd start.

[Carioca]

kapnD,

Looked at Tiny Tach, yes. Gret t face value.But...

What is the accuracy ?

Attach it over any one of the injection-lines, between injection pump and cylinder nozzle ?

How do you adjust for differing number of cylinders, 2-stroke/4-stroke etc. ?

[kapnD]

The sensor is attached to any convenient high pressure fuel line between the pump and the injector. Since each cylinder fires one time per revolution of the motor whether 2 or 4 stroke, the sensor reads one fuel pulse per revolution. Much simpler and straightforward than the belt driven alternator sender, or the other options. I installed one in my engine room along with a set of mechanical engine guages and a start/kill swith. The first time I ran it, I compared it to the tach at the controls and found quite a lot of discrepancy in the mid to high rpm range. This worried me, so I borrowed a photo tach to compare to, and found the tiny tach to be spot on, while the instrument in the bridge panel was incorrect. The only drawback I see is that the tiny tach has a non replacable battery that lasts only about 5 yrs.

[Carioca]

Spot on kapnD !

At 65 bucks a piece and a battery lifetime of 5 years, the alternator sender option with its attendant drawbacks is definitely worse

[Capt. Mike]

According to the manufacturer's description the diesel version of Tiny Tach uses either a replaceable 9v battery or the ship's power.

[Carioca]

Capt Mike,
You are right. A 9 V external battery (or boat battery) is required to supply the power when the diesel engine is turned on.

An internal - and unreplaceable - lithium battery supplies the power to the instrument´s microprocessor and digital display at all times.

The former power source would seem to address the needs of the (proprietary) transducer and associated signal processing electronics.

The internal lithium power source probably has a low-power consumption mode when the diesel engine is turned off, which keeps the digital display operational.