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Rev counter wiring, BMC 1.5

Guest Cattleya

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My rev counter isn't working, I'm told it did work by the engineer who refitted my engine. It's on a JGM Madeira with a BMC captain 1.5 diesel engine. On inspection I found one of the wires is just dangling and isn't connected. It might be that this has always been the case, or maybe this is the problem. There are 4 wires connected to the blocks to the left, then a red wire comes off towards the centre (circled) and isn't connected to anything.

I have asked Steve (Ellaboat) as he has the same boat, he is going to check for me next time he is on Ella. In the meantime, does anyone know how these are wired up?

If it's not this wire that is an issue, would there be a fuse fitted to this circuit?

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The wire in the picture is for night illumination.

Most i see are not connected, connect to your nav light circuit

and you will see the rev counter in the dark .

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Oh! Ok, so that's not the problem then. I did wonder if it was for a light.

Thank you very much, that rules out that as the problem, it was never connected to anything I guess.

Is there a way I can test the rev counter pickup (on the front of the engine) with a multimeter? I guess I need to make sure that is working before looking at the wiring.

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I have not yet been to the boat but having seen your picture it has jogged my memory.

All the necessary 4 leads appear to be in place at the rev counter. (I have never bothered connecting the illumination either).

The feeds are:-

Battery positive

Battery negative

1st wire from sensor

2nd wire from sensor


One of the sensor leads sharing a battery terminal at the gauge and the other sensor lead being the output signal feeding the gauge (so there will be only 3 connection terminals).

As per my PM, I need to check which battery terminal a sensor lead shares (+ve or -ve). I have no info on the sensor to prove which battery terminal it shares so I need to confirm that.

Again looking at your picture it is certain that the colours will be:-

1 x red = positive (12 volts)

1 x black = negative (ground)

2 x red/white = sensor wires

Don't despair if it turns out that your rev counter display itself is u/s. As long as the internal movement is o.k., there is just one transistor inside which fails.

Ella's rev counter did not work when I bought the boat and changing the u/s transistor fixed it.

If you don't get to the bottom of this in the meantime, I can help when I am up there.

All the best,


p.s. observations re sensors:-

Logically, it would be a safer design if the sensor causes the sense line to be grounded as it pulses (since the entire surrounding area is at battery negative, so any damaged cabling would not be capable of shorting positive 12 volts to ground) so I will be happier if I find that the negative terminal is shared.

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You could check the basics first:-

1) Check that 12 volts DC is present across the red and black wires at the gauge

2) Check connections at the sensor - The sensor is in an area that gets dirty/oily which can result in poor connections. Loose spade connectors and dirty connection terminals at the sensor will both cause loss of signal.


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Thank you for the suggestions. I have fixed it now :dance

How the marina engineer missed the problem I have no idea, one of the connecters had fallen off the engine. It was a simple case of pushing it back on.

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