Rev counter generator

[Andrew_Fanner]

The new to me boat has rev counters that are fed from some little generator type thingy driven off a flexible drive from, I suspect, the timing gears. Now one rev counter shws nowt, whatever you do, and the other one shows 500rpm, again pretty much whatever you do. Of course, this is with the engines turning

At present I've no idea if the gauges are knackered, the generator thingies are knackered or even if the wiring between them is defective in some wise. Alternators don't have W posts so the easy way out gets expensive at new alternators as well. ANyone any idea what the little generator thingies should produce across their terminals? If I can see that via the trusty meter then its gauges or wiring, if not then at least I know I need to get new bits from wherever to start with.

Ta muchly.

 

[fireball]

We fitted Rev counter to our little Yanmar ... the Yanmar part was expensive - so we got the output added to the alternator - cheap, quick and easy by our local alternator fixit guy ... I was lucky enough to be able to borrow a rev counter that uses light relfections so I could calibrate it from that.

 

[catmandoo]

I understand that the little thingies generate a voltage proportional to engine speed so you could check the voltage across the terminals without disconnecting any thing .

Next thing is to disconnect the cables one at a time and do a continuity check . I use the thing that burglar alarm specialists use . . A little box with wires that buzzes if you get a complete circuit . ( Good for latching as well as continuous circuits but not applicable here - info only very useful tool though )

I f you get no response then disconnect the other end and check the cable by joining the two wires at that end end together and testing again . That failing it looks as if the problem is in the instrument .

One thing I am not sure about though is whether you can hook up two instruments in parallel . I am thinking of doing this on mine as I have two steering stations one inside the cabin and the other in the cockpit . Rev indicator is inside but not outside . however outside is where the boat is normally run so I want to put an instrument there . However I am not sure if the generator has enough current to power two instruments at the same time

One thought though could also fit a change over switch .So then try each instrument on its own . That failing you need to look at the faulty instrument .

 

[Lakesailor]

Make it easier by swapping the instruments (or their feeds) if the odd effect follow the rev counters it's them. If the inert one stays inert but the other one changes it's display then ....oh well....you get the idea.

 

[catmandoo]

Not so sure about that one Doesn't check the generator out put . It may be just displaying a constant voltage no matter what engine revs are

Also easier to try one at a time only need to disconnect at generator end

 

[Lakesailor]

But it eliminates the instruments.

 

[Gordonmc]

Just opionion, but if you put a multi meter on auto range across the terminals it should show a pulse reading. If the thingies are knacked you could replicate the W post on the alternators by soldering a wire onto the field winding. It will mean getting tachos suited to the RPM range although I found a tacho with dip-switches which allow selection of the revs range. You might be lucky and find the thingies were set up to count revs in which case your tachos might be ok.
Thingies, BTW, is a technical term.

 

[rogerthebodger]

is this what the unit looks like

I have just purchased 2 units and I have some data somewhere ????

Will try to find and post details if unit is as above

here is another site I found

this is the VDO
web page and another showing the unit web page

 

[Andrew_Fanner]

Looks not dissimilar.

 

[fishermantwo]

On my fishing vessel I use a push bike speedo. Pick up and magnet mounted on the front pulley of the Gardner. There is a couple of metres of two wire flex added so it reaches the dash in the cabin. The "speedo" is set at 24 inch wheel and K/hour. This gives the rpm of the Gardner but short one digit so 1040 rpm is indicated as 104. Batteries need replacing earlier as the unit activates when the boat rocks. Instead of 4 years I get about 2 years. Has worked faultlessly for about 4 years so far.

 

[William_H]

Aircraft tachos of some time back used a tacho genrator which was a 3 phase alternator. That is a magnet rotating past 3 coils. This was carried up to the instrument which had a 3 phase motor driving a drag cup type speed indicator. Now it is doubtfull if this is what is used in your system but the number of wires connecting the generator will be indicative. These had 3 and no earth.

A simpler system will have a DC generator feeding a voltmeter.
But for a more modern style it probably has a pulse or Ac generator going to a chip in th indicator converting pulses to DC as in many car tachos. All of the latter will have just one wire and an earth. or 2 wires)

After doing the swap around trick it is time to pull the indicator apart to see if you can discern the way it works. From there you may be able to decide if you can measure the output in some form.

So sorry Andrew not much help good luck olewill

 

[Andrew_Fanner]

Spiining the thingummy (more jargon) shows no especial voltage, just a few mV "noise" on the digital multimeter. Continuity across the terminals regardless of any rotation, or apparently so, spinning it at c1000rpm in the lathe. Needle more on the guage dial but no continuity across termnals.

Thingummy is marked Z on one terminal, D on the other and VDO on the back. Its now on my workbench where I can play around with it.

 

[Guest]

For a lot less hassle you can buy a couple of optical or magnetic rev-counters ........ which read from white patches / magnets stuck onto flywheels ........ you can get them with extension leads etc. to site displays where you want.

They are cheap, accurate and easily replaced if fail later ...

Just a thought ... saves all that crawling around fiddling with wires etc.

 

[rogerthebodger]

At 1000 RPM should be giving you 10 V AC with a frequency of 4000 cycles per min

meter must be set to AC range as if on DC will only show very small voltage (mV) if any at all

 

[Andrew_Fanner]

Aha. AC.

Back to the garage...

 

[comoxlarry]

There are a couple of ways it could work, you could have a small DC generator puting out a variable voltage to a meter type movment. If so you should be able to measure a DC voltage at the output that varies with engine speed. It is likely that you have a magnetic pickup that puts out a pulse as the gear teeth pass near it. This is fed to the tachometer which integrates the pulses producing a voltage proportional to the time between them. The voltage then drives a meter calibrated in RPM. Generally speaking , within limits , it doesn't matter how high the pulses are. I just finished replacing my tach with another old one. The out put on the sender was aprox. 1.5 volts P.P. at 500 revs rising to 8 or 9 volts at 2000 RPM. The replacment required a higher voltage to operate properly so I had to lower the input dropping resistor a bit. You should be able to measure the o/p of the sender with an AC meter and there isn't much that can go wrong with them, they are a magnet with a coil of wire around it that produces a small pulse of current when the magnetic field is disturbed by the iron teeth of a gear.
Good luck

 

[Andrew_Fanner]

It appears that the genrator is OK, happily producing AC when run off the lathe, ergo the guage is the problem. Curiosity compels me to take it apart as I can't make it worse. Naturally Mr VDO presses his gauges together with the bezel so dismantling is sort of a one way process, given that the pressing wasn't proof against the elements I don't think I'm losing out much. I don't even want to think about the cost of a new one, worse yet, a pair.

 

[Gumpy]

I was faced with a simailar problem and went here
http://www.tinytach.com/tinytach/index.php
and got the diesel one

Julian

 

[rogerthebodger]

Well if it the gauge you could go for the type that is driven ftom the alternator which tend to be cheeper than the generator driven and tend to be more on Ebay or boat jumbles.

I did get a generator one from a boat jumble 2 years ago when I was in the UK so they can be found cheeply.

If you went for the alternator type you cound then sell the generators in ebay like this one to pay for the new tachos

I paid 100 A$ for two a week ago.

I have both types but I think the generator type is better as I found the alternator type only starts reading once the alternator starts charging. This may be just my setup.

 

[Andrew_Fanner]

[ QUOTE ]
I was faced with a simailar problem and went here
http://www.tinytach.com/tinytach/index.php
and got the diesel one

Julian

[/ QUOTE ]

All looks very interesting, until you get to words like "battery not replaceable", however I have asked for a quote with 30' leads. Does allow me to flog the genrators as well. As the dash instruments all match nicely and all say Perkins I'm not giving up on trying to fix them.