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Category: Sail Boats
Asked By:
Richard C. Tanner
Date:
15/03/2010
Boat:
Footprints
Hello,
I have a Perkins diesel (4-108) and the engine runs fine but when I energize the guages the Tachometer pegs the needle to max rpm. This is even before I start the engine! I have replaced the voltage regulator and installed a rebuilt alternator and both test fine. I know the power to the Tachometer is working fine since the hour meter which is part of the gague works fine. I replace the old wire with new wire to the power, ground and tach sender from the new Baldor voltage regulator and still have the problem. I'm really stumped now.
Side note, I have passed the USPS Marine Electronics class and have always been pretty handy with a VOM meter. All my other circuits are fine. Oil pressure, voltage and water temp gauges work flawlessly.
I'm stumped, so hopefully you can help.
Thanks,
Rick
S/V Footprints
Irwin 37 CC (1977)
Long Beach , CA
I have a Perkins diesel (4-108) and the engine runs fine but when I energize the guages the Tachometer pegs the needle to max rpm. This is even before I start the engine! I have replaced the voltage regulator and installed a rebuilt alternator and both test fine. I know the power to the Tachometer is working fine since the hour meter which is part of the gague works fine. I replace the old wire with new wire to the power, ground and tach sender from the new Baldor voltage regulator and still have the problem. I'm really stumped now.
Side note, I have passed the USPS Marine Electronics class and have always been pretty handy with a VOM meter. All my other circuits are fine. Oil pressure, voltage and water temp gauges work flawlessly.
I'm stumped, so hopefully you can help.
Thanks,
Rick
S/V Footprints
Irwin 37 CC (1977)
Long Beach , CA
Answer:
"
Hi Rick,
So far it looks like you've been doing all the right things and it sounds like you know what you are doing with the volt meter. I always start with the potentially simplest cause first, so in this case I would check the Tachometer unit itself and perhaps you have already. There is usually a fuse in the back of the unit, so check that first. Then remove the Tachometer unit from the circuit and use the volt meter to test the continuity to make sure there isn't a short in the unit itself. Commonly, there can be corrosion in the ground wire, or in the ground circuit that could cause the Tachometer to peg when powered. Although the clock works, it is independent of the Tachometer and does not guarantee that a short isn't present in the Tachometer circuitry. The fact that it pegs before even starting the engine tells me there is something wrong with the ground circuit.
If it is not the unit itself, then you will need to systematically check the continuity of the wires you replaced, right down to the terminals they are attached to, and perhaps you have already done that.
Assuming the continuity checks are OK, then it is possible that you have a ground fault somewhere in your engine circuit. I once had a battery cable that passed very close to the engine mount. Eventually, the plastic coating wore off of the cable and the wire came in contact with the engine block and all my electronics went crazy. Try testing the voltage between the battery -ve and the engine block, of course it should read close to 0.7 volts, but if you have more than that, then there could be some +ve volts getting in somewhere. The oil pressure sender is often a culprit, but you indicated that it is working properly, so it is unlikely to be this, but I would check just to be sure.
Lastly, I would try to get hold of a spare Tachometer, wire it into the circuit and see if it behaves the same. You're doing all the right things, but I would concentrate on the ground circuit, or the unit itself.
Once you get to the bottom of this, drop me a line with what you find, I'm curious. Hope this puts you on the right trail.
Good luck,
Captain Ian Fagg
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