Tacho problem on F80 BETL

[donbelcher]

The tacho on my F80BETL wavers around often and sometimes spends a while at full scale irrespective of engine revs,

The wiring, right from the engine loom to the meter is continuous and not shorting.

The D.C. voltage at the meter is about 13 volts whether working or at full scale so I would think the supply to the meter is pulsed.

Could it be the sender on the engine or would you guess it is the indicator at fault?

I have an oscilloscope available but what should the tacho output on the engine look like. The circuit diagrams seem hard or impossible to obtain on the net for this motor.

[boats.net]

It's probably a bad tach, you seldom see tach signal be constantly changing.

[donbelcher]

Thanks, I will get a probe for my 'scope today, confirm a steady signal from the motor, and then have to either try to sort the problem with the guage or buy a new one.

Does anybody know how the signal is produced in the motor? Is it an optical sensor?

Don

[donbelcher]

I know I am replying to my own post, but the problem for others, was the switch contacts for switching from 2-4-6 cylinders in the meter.

In a salt environment, bare brass contacts in an essentially unsealed meter is not good enough from a manufacturing point of view, and will cause failure in a relatively short time.

I opened the meter carefully and cleaned the contacts and he-presto problem solved. As an ex instrument engineer I can do this but most would have to just throw it away and buy a new one.

Hope this helps others and yes the waveform from the motor seemed O.K.

Don

[99yam40]

Yea, they should have been plated with **** or at least silver to control the oxidization, but they get by as cheap as they can.
Usually switching it back and froth a few times will help, but not a cure.

I am curious as to what the waveform looked like,could you describe it?

I have two old scopes that the battery packs went bad and started leaking and I was thinking of seeing if I could get them going again. They will not power up when plugged into 120V even with the batteries out, could be the power supplies are bad. I have not looked into very far yet.

[donbelcher]

The wave form is basically a square wave of amplitude 1.5 volts peak to peak the frequency is dependent on engine revs.

Superimposed on that waveform is a high frequency sine wave of about 0.3 volts.

The leading and trailing edges of the square wave have about a 2 millisecond slope with an overshoot peak (0.2 volts) on both the rising and falling edges.

I thought at first that the AC element might be the problem causing the unstable indication and tried to eliminate it by taking a ground lead for the meter from a separate source instead of through the loom. There was no improvement.

I decided that as the motor was charging the batteries quite hard as I had started the motor a number of times and it was the limitations of the rectifier system, rather than a fault, that was causing the superimposed sine wave.

After fixing the gauge contact which when open causes a full scale reading and was intermittent, I am hoping that as the battery charging settles down to a lower level the wavering about will reduce. If not I may have to live with it.

P.S. Your scope may not work with the batteries out as they could form part of the smoothing for the power supply which may not work without them.

[99yam40]

Thanks for info. I was thinking the same thing but was wondering whether or not to buy the battery packs just to see. They are Tecktronix 212 and 221.They have 2 - 6 volt packs in each one, do you know if they are set up in paralell or series? I looked for a circuit board schematic but did not find one. If they run off of 6 volts or even 12 volts I could rig up some emergence 6 v batteries just to check if the scopes function before I buy the packs needed


Did you try putting a charger on the battery over night to make sure it was good and charged up, so it does not have to depend so much on the motor to charge it.
What brand of tach is it?
Is it still wavering since you cleaned the contacts? If the input is not changing but the tach is then there must be something happening inside with the conditioning of the signal

I had always heard that they are pulse counters, but never knew what type of pulse it received. and that the voltage goes up with higher RPM when measured with a DMM but I do not think the voltage mattered any.
It would be nice if someone would show a proper signal wave form so at least we would know when a signal was going bad.

[donbelcher]

My Scope is a Tectronic 2215 60Mhz twin beam, and over here we are on 240volts 50Hz.

I'm sorry I can't help you with your scope problem but I from experience would think the batteries are wired in series as it would be quite unusual to put two rechargeable batteries in parallel and then charge them that way.

It is quite bad practise to parallel charge, because no matter how similar cells are made to each other they always 'peak out' at slightly different voltages. Having said that it is often done on boats but then one battery will usually only reach about 80% charge but then who is to know.

The batteries have been fully charged but by the time they are fitted and have started the engine a few times they are not at their peak of about 14.7volts and have dropped a bit

Due to where my boat is on dry land I cannot run it for long because of the cooling water getting all over the place on the lawn etc. and the wife complains.

The Tach is a Yamaha one bought second hand ex a Fletcher and I would think it is in all fairness, not one of their most expensive but if I can continue to use it at least i would not have to buy a new one.

I don't think the tacho depends on the DC voltage because measured with a DVM the change between idle and say 2500 RPM is only similar to the increase in battery voltage due to the increase in charging current.

It is the frequency of the wave form that the tacho is measuring not the voltage IMHO. The square wave is 'sitting' on a DC voltage but I think this is only being used to power the circuit components. Unfortunately I have not measured this factor as for my purposes I did not think it was important.

As to your final point.. I have dowloaded FREE an 800 page service document and a 300 page electrical service document for a Japanese 200SX. So why the secrecy on outboards? On another thread on this forum I have asked for an electrical diagram for my F70BETL but I don't hold out much hope and will probably have to pay for a full service document just to look briefly at one page.

Finally I am resisting making further negative comments on the lack of free information, waveforms etc and it will not stop me trying to solve my problem, but may influence the purchase of another outboard.

[99yam40]

Thanks for the info and your opinion.

Yea, from what I have read the manuals have copy right clauses in them and it is illegal to copy ( reproduce) without their permission. Most of the forums I have seen over hear follow them. But if you find a place just to view and not copy, let me know and I will do the same.

[donbelcher]

Ah. So in the USA it is more important to get your money than to make sure that you get the best information to repair a product that has failed.

I get the idea.

Thanks,

Don

[99yam40]

Are there copy right laws in the UK?
Or can you legally do what ever you like with anything you can get your hands on? A person could get rich doing that. If you did it right.
This is just the way the system works over here.

Check your PM