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Old 28-05-2010, 20:27   #1
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Depth / Speed Monitor Not Working Properly...Help !

I have an older Autohelm Depth/Speed gauge on my Newport 30.

Currently the depth is displayed properly, but the speed is not shown anymore.

It has only done this once before, on the day that I bought it. But within a few minutes it came back on. I hadnt had any trouble with it until the other day it just never showed a speed at all. I went out again and still nothing.

So, my question is, Where do I start in figuring out what casused it to stop working?


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Old 28-05-2010, 20:48   #2
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Do you mean that the display shows 0.0 knots, or that the display itself is gone?


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Old 28-05-2010, 21:42   #3
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Hey must have come from the same batch as mine. Guage reads zero, paddle-wheel spins freely so I am still chasing the problem. Do you have the dummy plug for the transducer hole when you pull the fitting or are you in the same boat as me- the PO lost the plug and the newer fitting is a slightly different size. Means that I cant even pull the sender and identify if that is the problem. Good luck and let us know how you go.
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Old 28-05-2010, 22:17   #4
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The gauge shows 0.0

Uncle Bob:
Im not sure. I am comletely ignorant of how it is set up, how it works, etc.
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Old 29-05-2010, 03:41   #5
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As previously posted at:
Testing Knot-Log Speed Instrument

Paddle Wheel Type Transducers:
A Paddle Wheel type speed sensor consists of a magnetized paddlewheel, wherein the paddlewheel is rotatably supported in a cavity adjacent to a (Hall Effect) magnetic sensing device. The sensor generates electrical pulses corresponding to (proportional to paddle wheel speed) paddlewheel speed, as the wheel rotates when the vessel moves through water.

Knot Log Tap-Test:

1) With 12VDC Power and Instrument “On”; remove the coax cable from the connector found on the rear of the speed instrument (display).

Bend a paper clip into a U shape and place one end of the paper clip into the center hole of the connector (signal) on the rear of the unit.
With the other end of the paper clip tap on the outside (ground shield) of the same connector (essentially shorting the inside pin to the outside collar) rapidly while watching the display.

If the speed increases as you tap, then the instrument is capable of producing a speed. You should be able to see about a 5 knot speed with rapid tapping.

If not; you will need to send the knot meter in for service.

If this is ok, proceed to the next step.

2) Re-connect the cable to the rear of the knot meter, and locate the Paddle Wheel Speed Sensor in the hull.
Follow the cable about 10 feet back, and disconnect the cable at the junction.

Perform the same tap test at the end of the cable leading to the knot meter, by shorting the center pin to the outside shell.
Have someone observe the reading on the knot meter.

If you can produce a good reading on the knot meter, then the inter-connect cable is ok, and you will need to check the Paddle Wheel.

3) Remove the speed sensor from the thru hull fitting, and insert the dummy plug (if afloat).
Spin the paddlewheel by hand. If the display shows a speed you will need to replace the Paddle Wheel Transducer assembly.

If it does not show a speed, you will need to replace the Instrument.

Knot-Log Voltage Test:

1. Connect a DC Voltmeter to the centre (Signal) and “bare” (ground) wires of the Transducer.
2. With the Instrument “on”, slowly rotate the paddle wheel one paddle at a time.

The Voltage should switch from about “8” volts to near “0”, volts as the magnets in the paddles pass by the body of the transducer.

3. If the Voltage does not change, as the paddle is slowly rotated, replace the transducer.
Gord May
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Old 29-05-2010, 13:27   #6
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just went out this morning and it worked...

I will get try the tests and see what happens.
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Old 29-05-2010, 13:31   #7
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I had a similar problem but mine turned out to be the electronics inside the unit.

Called Autohelm and they actually had a unit on the shelf so for a few bucks, I got a refurbished but far more reliable unit. You might give them a call. If they've got one on the shelf or can repair it, it might be cheaper than replacing, or having to fiddle with it to get it to work.

Capt. Douglas Abbott
USCG/MCA IV/M.I./C.I. 500-ton Oceans
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