Calibrating tachometers

IMacC

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I have recently fitted two Prestolite alternators to my Volvo TAMD41B engines and also need a new tachometer for one engine to replace a faulty Volvo/VDO one. I thought it would be simpler and cheaper to buy two new matching tachos and opted for KUS units (which appear to be very similar to those from Wema).
There are two control positions on the boat and I am only going to change the tachos on the lower internal instrument panels.
The instructions for ‘calibrating’ these tachos are written in best Chinglish and despite following them as best I can, I cannot get sensible or stable readings. What puzzles me is that I would have thought that the pulley ratios would have had to be an initially set somehow before then setting the instrument to correctly display a known RPM, but there is no way of doing this. I am using the ‘W’ terminal of the alternator as was previously used with the Volvo tacho, and indeed this still works perfectly with the existing good Volvo unit.
Has anybody done this exercise before and succeeded?
 

IMacC

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There is nothing wrong with the pulley, which is the original one. The issue is trying to calibrate a non-Volvo tacho to work with the ‘W’ alternator output.
 

IMacC

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Yes they are, from the 'W' terminal. I am using the existing wiring which will work the Volvo tacho OK, but I can see no way of calibrating the new instruments despite what the specification and 'instructions' say. In addition to that, the new display flickers which there may be a cure for but the main problem at the moment is the calibration.
As I see it there are two issues to solve:
It must be possible to tell the tacho what the pulley ratio is.
Having done that, the tacho would then need to be set to correctly interpret the signal received from the alternator.
This would imply two steps but there is only one facility (rubber button) on the back of the gauge which has only one function. That function does not properly calibrate the instrument.
I am beginning to suspect that it is not suitable for an alternator source and is really intended for petrol engines where an direct engine driven electrical source is available, although the specification says otherwise.
 

IMacC

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Yes thanks, I have seen the same diagram.
You will note that the table of 'ratios' indicates a probable value of 1.3 to 8.0, however the available settings are from 0.5 to 250.0 in 0.5 steps. It is not made clear what you are actually setting, all you do is set a number which gives a correct idling speed and then the rest is supposed to work, but it doesn't. I don't see how it could possibly work without a second separate setting to tell the tacho how much faster it is than the actual engine speed. Suppose you got the tacho to correctly read an idling speed of 750 and then you increase the engine speed to 1500; the tacho would now probably be rotating twice or three times that speed depending on the pulley ratios.
I remain mystified!
 

IMacC

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I have a pair of wema tachos on tmd41a's and had to calibrate, I'm sure it was quite straightforward but it was a long time ago now.
Can you download wema instructions and try from there? Optical handheld tachos are cheap and accurate for checking against.
Yes I have the instructions from them and I have an optical tacho also. The tachos I have are KUS but appear the same as Wema and I spoke to Wema yesterday who seemed to think that their tachos should be OK so I may yet go down that route. Wema were very helpful despite my not having bought their instruments this time.
These are the Wema instructions:

SPEED RATIO ADJUSTMENT:

a. Hold down the rubber button on the back of the gauge for 3 seconds; it will then enter 'settings mode'.
If the button is held for more than 3 seconds, it will exit settings mode.
b. As the button is pressed, the value will increase.
To decrease the value; release the button and press again. The value will now decrease. The longer you press the button, the faster the value will change.
[The minimum step value is 0.5 and the adjustable range is 0.5 - 250]

c. When you get to the required speed ratio value release the button.
The tachometer will save the selection after 3 seconds.

To re-adjust the value, repeat steps a-c.


[If the deviation is big, the speed ratio can be a bit bigger than calculated value;
If the deviation is small, the speed ratio can be a bit smaller than calculated value.

The gauge needle points at the corresponding positions when adjusting the speed ratio.]


If above calculation description is not clear to you, you can adjust it according to the idle speed of the engine.

The engine remains at idle state after it is started; enter the settings mode and adjust the speed ratio until the gauge needle is close to the idle state.


Not really very clear at all, and you will see they end up saying just get the idle speed correct!
 

Croftie

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I have fitted similar with no probs,

You said you have 2 helms and are going to fit new to lower, set engine speed to say 1000rpm on the upper helm then adjust the new tach's to as near as you can to 1000rpm. If you increase rpm to say 1500 on the upper helm the new ones should show the same 1500. The setting you do is to set the ratio of crank rpm to alternator rpm, that ratio will never change. eg if for every rotation of crank the alt turns .75 then that is fixed whatever speed the engine is turning assuming no belt slip
 
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IMacC

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I have again today recalibrated the tachos according to the given instructions and get the following result:
The only setting which gives an accurate idle speed (700rpm) is 17.5, but on increasing the revs to 1200 the gauge then reads 2000, at 1300 actual it reads 2200 and at 1400 it reads 1700. Changing the setting to 22.0 makes the idle speed indicate 600, 40.0 reads 300 and 100.0 reads 100.
I do not think it can be possible to properly calibrate these instruments where the sender is an alternator which is not driven at engine speed and that, despite what the literature says, they must be either intended for petrol engines of for diesels where an engine driven sender is available.
 

vas

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I have again today recalibrated the tachos according to the given instructions and get the following result:
The only setting which gives an accurate idle speed (700rpm) is 17.5, but on increasing the revs to 1200 the gauge then reads 2000, at 1300 actual it reads 2200 and at 1400 it reads 1700. Changing the setting to 22.0 makes the idle speed indicate 600, 40.0 reads 300 and 100.0 reads 100.
I do not think it can be possible to properly calibrate these instruments where the sender is an alternator which is not driven at engine speed and that, despite what the literature says, they must be either intended for petrol engines of for diesels where an engine driven sender is available.
can you please explain how an alternator pulley off the crankshaft is NOT in a clear analog relation to the engine speed? Cannot see how this is happening, because it cannot!
Mind you need TWO adjustments, one for the relation between the two values and one for the rate of change. If yours has only one, doubt it will work
I've setup a KUS rpm counter for a friend years ago but that was on a 2T outboard.
Try a mobile app which is "listening" to the engine noise, you setup cylinders, 2/4stroke and it accurately measures rpm. On my android mobile I've used successfully RPM Gauge
 

Croftie

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I have again today recalibrated the tachos according to the given instructions and get the following result:
The only setting which gives an accurate idle speed (700rpm) is 17.5, but on increasing the revs to 1200 the gauge then reads 2000, at 1300 actual it reads 2200 and at 1400 it reads 1700.
That does not make sense, reading high at 1200 and 1300 but LOW at 1400. Is this happening for both tac's on both engines. AFAIK the W terminal gives out an AC voltage but the frequency increases with RPM. The tacho "reads" the frequency and gives the RPM reading so cannot understand the sudden drop at 1400 >> 1700. Have you a link to what you bought, alt and tacho.
 

IMacC

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can you please explain how an alternator pulley off the crankshaft is NOT in a clear analog relation to the engine speed? Cannot see how this is happening, because it cannot!
Mind you need TWO adjustments, one for the relation between the two values and one for the rate of change. If yours has only one, doubt it will work
I've setup a KUS rpm counter for a friend years ago but that was on a 2T outboard.
Try a mobile app which is "listening" to the engine noise, you setup cylinders, 2/4stroke and it accurately measures rpm. On my android mobile I've used successfully RPM Gauge
I agree, there needs to be two calibration stages, but unfortunately these gauges only have one. I think I will have to bite the bullet and buy another exorbitant Volvo/VDO one!
 

IMacC

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That does not make sense, reading high at 1200 and 1300 but LOW at 1400. Is this happening for both tac's on both engines. AFAIK the W terminal gives out an AC voltage but the frequency increases with RPM. The tacho "reads" the frequency and gives the RPM reading so cannot understand the sudden drop at 1400 >> 1700. Have you a link to what you bought, alt and tacho.
It makes no sense to me either! The alternator is a Prestolite 70A unit and the tachos in question are KUS Q series 4000 rpm. I bought the alternator from Yacht Boat parts and the tachos from Furneaux Riddall. There are instructions on the Furneaux website and in various other places, e.g. Wema. Nobody can tell me what it is that you are adjusting when you do the 'calibration', simply that you need to set it to give the correct idle speed and the rest should follow, but it doesn't.
 
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