Tachometer

[duke3]

Hello.<br />Last week I have installed a tach (0-6000rpm) from an 6 years old 4 cyl. Mercury on my 60HP Evinrude VRO from 1990.<br />The tach can be adjusted from 2 to 8 pol.<br />So since my Evinrude is 3 cyl. 2-stroke, I did adjust the tach to 6 pol.<br />So when I start up the motor, the tach was working fine, and was operation from 0-5500 rpm.<br />But this week the tach was working fine for about 30 min. From idle to apx. 3000 rpm, it works okay, but if the motor is running more than 3000 rpm, the tach is going down to apx. 1100 rpm.<br />And if I slow down again to apx. 3000 rpm, then the tach is jumping from 1100 to 3000 rpm. ????<br />Whats going on ??????????????????

 

[duke3]

Re: Tachometer

Thanks.<br />Ill try to connect a 0,5watt, 2K resistor on the back of my tach.

 

[Seasport]

Re: Tachometer

Yes but why did it work and then stop again? Something has changed. Ideally you want to get a scope and have a look at the tach signal. Make sure you are getting decent signal levels (close to 12V).<br /><br />I had a situation where I was running an OMC tach from a Force motor. Worked fine until I replaced the rectifier/regulator. Then it didn't work anymore. Basically, the tach was loading the rectifier output too much and there wasn't enough voltage to drive the tach. Got similar symptoms to yours. I fixed the problem by building a simple little transistor buffer to give the tach signal more current capacity.

 

[duke3]

Re: Tachometer

Thanks Seasport for your answer.<br />Its like my tach is working ok the first 3 to 5 min. Then when the motor is warm, the tach starts to operate funny (goes bach to zero, and jumping up and down from 0 - 3000 rpm)<br />First Ill try the resistance this week, and if it does not work, then I realy could use more info, about how you did build up a "simple transistor buffer".

 

[Seasport]

Re: Tachometer

Maybe the rectifier/regulator is operating right at its current limit in driving the tach and then as it heats up it doesn't have enough power. Maybe, it's just speculation. You really need to have a look at the signal with an oscilloscope to figure out what is happening. You should have a reasonably clean square wave of close to 12v when the sender wire is connected to the tach. If the signal is good when the sender wire is disconnected and then drops down to a low voltage when connected to the tach, you know you are loading the rectifier signal too much with the tach.<br /><br />I don't know how the Merc tachs differ from the OMC ones. There may be another difference but I was driving an OMC tach from a Force (Mercury) rectifier/regulator.<br /><br />The transistor buffer was a simple emitter follower using a transistor and resistor. The idea is to create a buffer between the rectifier and the tach to prevent the signal from the rectifier being loaded too much.<br /><br />Maybe the resistor will do the trick. This might be connected from the signal wire to earth or +12V depending on the output of the rectifier.

 

[duke3]

Re: Tachometer

Well Seasport.<br />I believe you know more about it, than I do.<br />But I cantt right follow your theori.<br />You are writing, that it is the tach, there is loading the rectifier, but as I see it, the load from the tach, must be taking from the stator.<br />There is four wires from/to the rectifier (A:ground, B:+12V battery, C and D from stator.)And the gray wirer to the tach, are connected together with wirer D.<br />Also when I connect the 2.2K resistor, the load must then be bigger.<br />But lets wait and see, what the resistor will do.

 

[Seasport]

Re: Tachometer

I think some tachs are fed directly from the stator and some from the regulator. Looks like your tach wire is fed directly from the stator (I think you just have a rectifer & no regulator). I'd try the resistor between the send terminal and ground. Should fix the problem. This resistor is in parallel with the tach and will decrease the apparent impedance (resistance) of the tach as seen by the stator and reduce the voltage levels at the tach. Maybe you're getting too much voltage at your tach rather than too little. Teleflex reckon you should have 5-7 VAC at 2000rpm.

 

[duke3]

Re: Tachometer

Have just installed the 2,2K resistor. And at 2000 rpm I have 16 VAC between "send" and "ground". And also 16 VAC at 1000 rpm.<br />After apx. 5 min. with the motor running, the tach was still working 100% from 0 to 5500 rpm.<br />But I dont know, why the VAC is so big at my tach. But never mind, it is working.<br />Thanks out there for all the response.<br />Best regards from litle Denmark.

 

[duke3]

Re: Tachometer

Well well well.<br />Back again.<br />Yesterday I went out for a short trip, but after apx. 10 min. the tach starts to back to zero again at full throtle ??? And after few sec. the tach works ok again. That happend 4 times yesterday. So after I have installed the 2,2K resistor, it hase been better, but still not good.<br />I have 12,7 Vdc at my battery, 13,2 Vdc at my battery when the motor is running at idle, and 16 Vac between ground and "send" at my tach.<br />Could it be a bad rectifirer ? or a bad stator ?

 

[Seasport]

Re: Tachometer

Oh dear. I think you have a basic incompatibility between that tach and your send signal. As I said above, I think your motor doesn't have a regulator and your tach signal is taken directly from the stator winding. That tach is probably designed to be fed from a regulator circuit (which will be a lower voltage square(ish) wave. Probably nothing wrong with your stator but it is putting out too much voltage for your tach and somehow "cooking" it.<br /><br />Check out this document. http://www.tflx.com/PDF/99320.pdf <br /><br />They mention your symptom and say that if a resistor doesn't fix the problem, you need to fit a regulator.<br /><br />You could try posting in the electrics forum to see if anyone else has a workaround for this.

 

[duke3]

Re: Tachometer

Well well well…………………Seasport.<br />Yesterday I did make some more readings at my boat.<br />When the motor is stopped I can read 12,7V on my battery, but just when I start the motor I can read 16,8V at 1000rpm, and 17,6V at 2000rpm on my battery. I did try to let the motor run for apx. 5 min. but the voltage would not go under 16,8V at 1000 rpm.<br />I think 16,8-17,6V is fare to high. (last month my fishfinder stopped working, maybe because of the high voltage ????) And I believe the voltage is getting more than 17,6V when the motor is working at 5000rpm.<br /><br />so I have just controlled my 3-wire rectifier.<br /><br />Same as on this link, where it is called OMC 3-wirer rectifier:<br /> http://ishopmarine.com/ishop/jsp/As...~A0000021~M0009890~M0009973~M0009975~C0200076 <br /><br />And same rectifier can be seen at this link. But here they call it OMC Voltage regulator.<br /> http://www.outboardparts.com/omc/mall/dynamic-results2.asp <br /><br /><br />Yellow/gray to ground I can read infinity in one direction, and 1000Kohm in the other.<br />Yellow to ground I can read infinity in one direction, and 1300Kohm in the other.<br />Red to ground I can read read infinity in one direction, and 450Kohm in the other.<br /><br />The two wires to the stator, I can read 2 ohm.<br /><br />So are there voltage regulator on my rectifier ???<br />Does the reading at my rectifier show that anything is wrong ???<br />Does the high voltage on my battery show that there is anything wrong ???

 

[Seasport]

Re: Tachometer

Kim, you don't have a regulator - just a rectifier. You should be able to fit a regulator to that motor (or a combined rectifier regulator). I see there is a regulator kit available from Bombadier.

 

[duke3]

Re: Tachometer

Okay Seasport.<br />Ill try to move my qustion under "el-troubles"<br />But thanks a lot.

 

[Seasport]

Re: Tachometer

Kim. I think your stator and rectifier are fine. The problem is you don't have a regulator to keep the DC voltage at a constant level. This is why you are getting such high DC voltage when your motor revs up. This is probably also the reason why your fishfinder failed.<br /><br />The rectifiers (like yours) are cylindrical and the rectifier/regulators are square with fins. You should be able to fit a rectifier/regulator to your motor. I'd bet that your tach problems disappear when you do this and you won't fry your electronics.

 

[duke3]

Re: Tachometer

I think you are right Seasport.<br />I can also see, that Im not the only one, with high voltage. I can see, that some are always controlling the battery via a voltmeter. And if the valtage are getting too high, the just turn navigation-ligt etz on, to reduce the voltage.<br />But since its just a normal rectifier on my motor without voltage regulator, should I then not read 0 ohm and infinity on this one ??<br />And not the reading as below ???<br />Yellow/gray to ground I can read infinity in one direction, and 1000Kohm in the other.<br />Yellow to ground I can read infinity in one direction, and 1300Kohm in the other.<br />Red to ground I can read read infinity in one direction, and 450Kohm in the other.<br /><br />Why is this resistance in my rectifier so big ????

 

[Seasport]

Re: Tachometer

Your test results look OK. Your resistance readings do sound high. However, this is probably due to the meter you are using (or you are using the wrong meter scale). What you are doing with the rectifier test is measuring the forward current flow through a diode. You need a certain voltage (>0.6V) across the diode to get current flow. When you reverse the leads you get a -ve voltage across the diode and should get no current flow. I'd say if you tried a different meter (or even changed the ohm scale of your meter), you'll get a better reading.

 

[duke3]

Re: Tachometer

Super Seasport.<br />You get the point.<br />My readings must have been wrong.<br />I dont think I did have >0,6V, so the diode did not open. Ill make a new reading, with another meter. But I think you are right, my rictifier seems OK.<br />Tanks a lot