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PeterI...
PeterI...

Posted: Tue May 02, 2017 6:51pm

Post Subject: Tachometer wiring

Hi, Tony/Experts, Please could you help me with wiring my tachometer. Currently it has no wires at all anywhere near it. It's the original tacho, and clearly used to be connected (marks on the terminals). Made in 1979, it has 3 terminals marked "Pos", "Neg + Gen Z" and "Gen X." There is a label saying 022 adjacent. Engine is a BMC 1500 diesel with alternator. Any ideas please? Thanks Peter

Tony-B...
Tony-B...

Posted: Tue May 02, 2017 7:27pm

Post Subject: Tachometer wiring

As a matter of interest have you asked about this elsewhere recently because I think I have answered something very similar with a Smiths gauge recently. I have a horrible feeling that this revcounter is designed to be paired with an inductive sensor, often fitted close to the front pulley or the flywheel with the sensor being the "Gen". Such sensors would typically produce one pulse per engine revolution so there is no need for adjustment on/within the gauge itself. Nowadays typical electric revcounters are driven from the W terminal or phase tap on the alternator and as the alternator pulley ration is unknown to the revcounter maker and the alternator produces many pulses per revolution the gauges have an adjuster to set it to show the correct speed. Pos would be fed form your ignition on connection to the other gauges. Neg is any battery negative connection such as another gauge. Now things get difficult. If I am right about an inductive sensor the Gen X & Z would be fed form two sensor connection. As an inductor output is sort of AC it would not matter which wire goes to where but if it did not work one way round try it the other. I suspect that apart from adjusting for the number of pulses you could feed one (say Gen X) from the W terminal or a phase tap on the alternator but I would expect the needle to try to go way off the scale (too many pulses). I have also seen revcounters with their own little generators fitted to the engine in which case the two Gen terminals are fed from the generator. From memory on a BMC 1.5 such a generator would be located close the the back corner of the engine block under the manifolds. If this boat or engine came from the Norfolk Broads I know they fitted a lot of inductive revcounters a number of years ago. To be honest if the engine does not have a inductor close to the front pulley or flywheel I think it would save a lot of time and possibly cost if you just bought a new modern one and fed it from the W terminal on your alternator and if it doesn't have one get a phase tap fly-lead soldered into the alternator.

PeterI...
PeterI...

Posted: Sun May 07, 2017 6:26am

Post Subject: Tachometer wiring

Tony, Thank you for your informative reply. There's no sign of a sensor or generator for the tachometer. I'll go for a new one as you suggest. Thanks again, Pete

PeterI...
PeterI...

Posted: Sun May 07, 2017 6:29am

Post Subject: Tachometer wiring

Oh, and no, mine isn't a Smiths. Pete

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