tach reading

[Don Lucas]

My tach doesn't read rpm all the time on my 410. The problem started last year and when onto the "when I get around to it list". I'm had the same type of problem in the past and it was just some corrosion on the wire terminals for the sensor so the other day I cleaned those.

But this problem is different and I don't really understand why it would do what it it doing. The tach reads fine on start-up when the engine is cold. But after the engine warms up it just reads zero. So it seems that the issue is a temperature related one. I can not believe that the wires themselves could possibly heat up enough to result in enough resistance to stop the current for the sensor.

Since it works on cold start-up I feel I can disregard a lot of the troubleshooting items such as voltage to the unit, wiring connecions etc. And the only other thing to check would be using a syroscope (which I don't have anyway) when the engine is running and warms up to check the output of the snesor.

Is it possibe that the flywheel sensor itself could heat up a point that it stops working?

Anyone else ever seen this as I hate replacing sensors based on the "if" principle of troubleshooting?

PS - the circuit has nothing to do with the alternator and has nothng to do with charging.

 

[Bill Roosa]

The sensor just outputs an AC voltage which has a frequency that varies with the engine speed. You can use a simple multimeter set to AC volts to see if there is a signal. Should be 1-2 volts.
With that said it does not take much corrosion to cause a voltage drop high enough to make it unusable to the control head.
I agree with your assessment. Since it does work and it seems to be temp related that kinda limits the problem to somewhere around the sensor.
It the wires and connectors check out then it could be the sensor is "going open" or "shorting to ground" after it gets hot. You can check both with a multi meter. the going open by checking for a resistance between the terminals of the sensor. Should be a low resistance. The shorting to ground by testing between each sensor terminal and the sensor case. Should be a high (> thousands of ohms) resistance.
I'm betting that it is the sensor terminals that are corroded though as
99.44% of all electrical problems on boats is corroded terminals or wires.

 

[Sailgunner]

Mine was pegging sometimes so I cleaned some terminals and it was till not working correctly. So I went throught he process (was not easy) of trying to find a replacement tack and was able to do that. I checked terminals again and after cleaning the ones on the sensor it is working fine. So I have a spare tach. If you decide you need a new tach send me a private message and I can tell you what to buy.

 

[faisond]

I contacted the manufacturer about erratic tach readings and a possible replacement. He responded that they'd be happy to sell me the tach but before I ordered one, he suggested that I mainpulate a switch on the back of the tach a few times in case my symptoms were due to a bad contact. I was unaware of the switch on the back side of the tach as I've never pulled it. It's on my "to do" list.