Tach Problem

[captain465]

Perkins 4-108........on start up the tach does not show any rpm. If I race the engine, it starts to show rpm, and continues to show proper rpm.
Any ideas as to what might be the cause? Rpm indication is correct when checked against a hand tach.

[Pblais]

Perkins 4-108........on start up the tach does not show any rpm. If I race the engine, it starts to show rpm, and continues to show proper rpm.

It's common to see this. I think it's just that you start the engine and draw off some power but the engine is still in idle and not really making much power off the alternator. Once you start in gear it is working more and the power is coming out of the alternator high enough to enable the tach. My Westerbeke does the same thing and my previous boats Perkins did the same thing too.

[nylajo]

If you have an external 3 stage regulator such as Xantrex this is not unusual as they ramp up slowly to save shock loading.

[MarkSF]

Originally Posted by captain465

Perkins 4-108........on start up the tach does not show any rpm. If I race the engine, it starts to show rpm, and continues to show proper rpm.
Any ideas as to what might be the cause? Rpm indication is correct when checked against a hand tach.

You should also notice that the alternator charging voltage also needs that quick burst of rpm to get going. The tachometer output comes from the alternator.

It's normal with some older one-wire / self-exciting alternators.

[Cheechako]

If you have a high output regulator, some of them dont kick in until you initially reach above idle rpm. they drive the tach.

[MarkSF]

Originally Posted by Cheechako

If you have a high output regulator, some of them dont kick in until you initially reach above idle rpm. they drive the tach.

I've heard that one-wire / self-exciting is also called high output. They are the same thing. Rather than excite coming from the ignition switch, it's done via the regulator which may have a highish rpm threshold before the alternator activates. The advantage of one-wire is you can get higher charging current. The disadvantage, you have to give the throttle a blip to get it started.

[GordMay]

Alternator tachometer signals may be derived from the (“W”) stator windings. This signal is, thus, dependent on the amount of energy being produced by the alternator. When batteries are being charged, signals are strong enough to produce good tach stability. When multi-step regulators are used to charge batteries, the tach signal strength can vary according to the charge state. When the regulator switches from the absorption state to the float state, there may be complete loss of tachometer signal during the time that battery voltage decays. During this period some regulators shutdown completely, and the tach signal does likewise.

When the regulator finds that the battery is full, alternator field current is reduced, as necessary, to avoid overcharging the batteries. This often results in a tach signal which is too weak to trigger some tachometers. Erratic tachometer readings result; particularly at low RPM. One possible solution is to turn on some electrical loads; forcing the alternator to produce more output, and hence, more tachometer signal strength.

Virtually all diesel engines have a port which will accept an RPM sensor*. This mechanism produces a reliable tach signal without regard to battery state of charge. The RPM sensor does not suffer from belt slippage, and continues to operate even if the alternator has failed.

* See ➥ Magnetic Proximity Tachometer - Tachometer Wiring Diagram Cruisers & Sailing Photo Gallery

[AnchorageGuy]

OR, like our engine the tach does not start to respond until the oil pressure starts to build, or all of the above. Chuck

[nigelmercier]

Originally Posted by Waterwayguy

our engine the tach does not start to respond until the oil pressure starts to build...

I have the same "problem", I never thought of that.

[curtis742]

Also check alternator belt tension. Tighten until you can no longer rotate the cooling fins by hand AT ALL. Look for belt dust from the alternator belt wearing away. A full output demand may cause belt slippage initially until the pulley heats up and causes the belt to become sticky enough to drive the alternator. There also could be a bad\loose connection inside the alternator on the AC line feeding the tach.