How to test a tachometer

[Mickeyrouse]

The Teleflex tachometer/hour meter on my boat (engine is Westerbeke 55B4) is not reading. Instrument lamp works, so Igues I have ground. Checked continuity from Balmar Max Charge 612- it’s good. Anyone have any ideas on how to check the tach itself to see if it isn’t toast?

[GordMay]

"Not reading", as in reads zero?

There are, typically, four types of problem that can occur with the tachometer:
- it’s totally inoperative, and always displays zero
- the needle is stuck, or permanently pegged
- the needle is erratic
- the RPMs are consistently off (either too low or too high)

[Mickeyrouse]

It is curtain 1, totally inoperative, always reading zero. Don’t think it’s stuck. It is erratic only in its error is sometimes only 900 RPM off, when it reads a non-vibrating zero at idle, to when it it is 3600 RPM off, when it reads a a non-vibrating zero at full speed. The RPM’s are consistently off, always reading zero. And then there’s the engine hour meter. Never advances. Tach pilot light burns. All other instruments in instrument cluster give readings, assumed to be more or less accurate, showing in expected ranges. Could be inoperative feed from voltage regulator, but can’t find anything in Balmar manual on that, or from Balmar support, who are non-responsive.

[tkeithlu]

This is an analog tach?

Does it respond to adjustment through the little hole in the back? It could be turned down.

I have two tachs, one digital based on a magnetic pickup on the fly wheel and one on the back of my Balmar alternator. I'm finding that the big analog tach off the Balmar has some of the same problems. It is not linear, and seems to forever need adjustment. The little $15 digital tach just sits there and reads accurately to 10 RPM. There is a reason - analog tachs and speedomenters rely on a spinning magnet attracting the spring-loaded meter lever, causing it to displace varying amounts. That's a recipe for calibration properties. I suspect that the solution on mine, and perhaps yours, is to buy a more expensive analog tach with the hope of better stability and action that is closer to linear. The adjustment paper work on mine says calibrate it to mid range, about 1500 RPM in my case, but then the ends of the scale are off.

I'm inclined the recommend a digital tach with a magnetic (mine) or optical pick up. They read the time internal between rotations of a flywheel, which digital circuits can do very well. Here's similar to mine. I added the analog tach because my digital tach has a very small screen. Currently, I use the analog tach to measure change in RPM, not absolute RPM.

https://www.amazon.com/DIGITEN-Digit...518106&sr=8-13

[tkeithlu]

The programming manual for the Balmar voltage regulator identifies the contacts for a tachometer.

Having a hand held optical tach helps. You put a white mark on the edge of the flywheel or front pulley and let it count rotations.

[GordMay]

Many inboard Diesel engines use the alternator output as the tach input, and the number of pulses per revolution is a function of the number of alternator poles, and the ratio of the alternator pulley size to the engine pulley size.

Other engines use a magnetic pickup mounted next to the flywheel and generate one pulse for each gear tooth - in which case, there are two or three hundred pulses for each engine revolution.

1. Check the obvious - when the ignition is switched on, is there 12vdc (or 24vdc) between the positive terminal and negative terminals when measured with a voltmeter? If not, use your voltmeter to determine whether the problem is in the wiring on the positive side or the ground side.

2. If the voltage is correct, the next step is to check the input signal. Remove the signal wire. Set your voltmeter to AC volts and connect it between the signal wire and ground.
Start the engine and set the speed to idle.
You should get a reading on the multimeter that varies with engine speed. If the tachometer is connected to the alternator, this reading should be a minimum of about 5VAC, otherwise it will be more like 0.3VAC.
If you get any indication of a pulse stream, reconnect the signal wire, and if the problem is still evident, the culprit is most likely the tachometer itself.

3. If you get no indication of a pulse stream, disconnect the tachometer signal wire on the engine, and repeat the test there. If you detect a pulse stream at that end, the problem must be in the wiring. Otherwise, the problem is in the signal generator.

Hope this helps.

[Mickeyrouse]

Originally Posted by tkeithlu

This is an analog tach?

Does it respond to adjustment through the little hole in the back? It could be turned down.



https://www.amazon.com/DIGITEN-Digit...518106&sr=8-13

Nah, it just sits there, unresponsive to calibration efforts, in a state of total repose. I’m trying to determine as a first measure if the tachometer is functional. It reads analog.

[Mickeyrouse]

GordMay, I presume you mean positive and negative terminals on the back of the tach. Nothing is identified as that, but I suppose that would be “S” for signal and therefore positive, and the negative would be the post that has multiple wires leading to other posts in the instrument cluster, I assume would be a ground terminal.
I should point out that the signal(pulse?) as wired does not come from the alternator but from the Balmar regulator/ controller. This has been made confusing for me in that the engine as shipped does not conform to the published schematic from Westerbeke. That diagram references a Balmar ARS5, whereas I have the Max Charge 612, with an entirely different terminal array.
I realize I’m interpreting this incorrectly, but the terminals I would test for 12 VDC with ignition on is also the terminal that is connected to the pulse from the regulator, where I understand you to say I’m looking for up to 5 VAC.

[FPNC]

The tach signal comes from the Balmar alternator stator. (white wire or stator terminal) Since you do not tell us what alternator you have it is hard to know. Tach signal can run through the Balmar regulator. It does not have to run through the Balmar regulator. However it works better that way for various other reasons.

The tach signal voltage can be measured at the white wire or stator terminal. It should be 6-9 volts AC. If voltage is not present or varies a lot, you have your problem there. The tach reads changes in the tach signal frequency. It is hard to measure that without an oscilloscope.

On the bench, I test tachometers using a simple 6 volt AC power supply. This provides a consistent frequency of 60 hz or 3600 cycles per minute. Enough of a signal to make the tach read predictably on various settings. You can also connect the tach up to another alternator. Its only 3 wires.

[dw1979]

On my boat the tachometer signal is fed thru the temp and oil pressure senders. You might want to follow from the alternator output and see if it does in fact go there. Is so it is a matter of seeing if you have a broken wire or one if the senders is not working properly.

[Mickeyrouse]

Originally Posted by FPNC

The tach signal comes from the Balmar alternator stator. (white wire or stator terminal) Since you do not tell us what alternator you have it is hard to know.

Sorry- the alternator is a Balmar 621 100 amp 12 volt unit. (I was trying to focus on testing the tach- I see that knowing the alternator is pertinate.)

[GordMay]

Originally Posted by dw1979

On my boat the tachometer signal is fed thru the temp and oil pressure senders...

I cannot imagine how that would work. Could you explain?

[StoneCrab]

Originally Posted by Mickeyrouse

The Teleflex tachometer/hour meter on my boat (engine is Westerbeke 55B4) is not reading. Instrument lamp works, so Igues I have ground. Checked continuity from Balmar Max Charge 612- it’s good. Anyone have any ideas on how to check the tach itself to see if it isn’t toast?

Does your tach have a wire coming from the alternator? There is usually a tach post on the alternator. It puts out a pulse at the engine rotates. Of course you then need to know the belt pully ratio and how many poles your alternator has to accurately set up your tack.

My tach's usually have some set up dip switches for telling the tach the pulses per revolution. If those were changed, or if the tach becomes ungrounded, that can effect the reading.

Another way for a tach to be fed a signal is if you have a proximity switch or a capacitive switch mounted on the engine. This can be set to count the teeth on the flywheel, or count bolt heads as they pass by.

The signal can be a voltage pulse, or a grounding of the signal line with each count. Any break in these wires, the power wire to the tach or the grounding wire can cause this failure.

To test, first determine the type of signal... whether from the alternator or from a sensor and then put a handheld scope on the signal wire to see if your are receiving a signal from the sensor or alternator. If you are, and the tach has its dip switches set properly, your tach is faulty.

On the alternator, it is probably easiest to take it to a rebuild shop and ask them to test it. I've had these types of problems myself and it is usually a diode inside the cover of the alternator.

[Mickeyrouse]

There is no identified tach output on the alternator, though surely there is something. Since I assume the Balmar Alt is expected to be used with a Balmar regulator/controller and wired accordingly I’m guessing the Balmar folks expect that the designated tach signal output is how the tach does its thing. As indicated previously, the alternator is making electricity and keeping the batteries charged. Thus, I’m trying to find a way to test the tach unit. BTW, the alternator had been recently rebuilt by an alt. shop with Balmar parts.

[Mike Banks]

Get it timed and counted. There is a device that counts the number of revolutions in a set time frame--then you just convert that figure to RPM. We used them in sawmills all the time to ensure the correct spindle speeds were attained on all kinds of motors and machines. You center a rubber buffer on the shaft to be tested, start the motor, then when it reaches operating speed, start the counter/timer.

I am pretty sure you could hire one.