Erratic tachometer

Discussion in 'OnBoard Electronics & Controls' started by pennym, May 7, 2014.

  1. pennym
    Joined: May 2014
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    pennym New Member

    My boat was purchased from a deceased estate and the tacho has never worked for me. It is a diesel motor and has a 1970/80's 4.0 inch VDO with no labels on it and has 4 terminals (12V, Sense and 2 x earths).

    I did the normal thing and connected to the 'w' on the alternator. The tacho does not read under normal running but I noticed when a serious load is applied (ie the autopilot motor operates) then the tacho will read while the load exists.

    There is an earth shunt installed with my xantrex battery monitor. The bolt holes where a magnetic pickup would be installed on the ford lehman still have original paint in them so have I assumed the tacho was never connected like that.

    Anyone with thoughts as to why the tacho might behave this way.
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The problem may be the alternator. The Tach is reading the frequency
     
  3. pennym
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    pennym New Member

    Thanks for your thought but I have 2 alternators and they both do the same. The tach is reading something but only under a big alternator load.
    I think I need another tacho to try out - that way I will know if it is the tacho or a poor earth or something else.
     
  4. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Check the AC voltage on 'w' without load and do the same at the tacho input.
    Repeat the measurements when the alternator sees a serious load.
     
  5. pennym
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    pennym New Member

    I checked the AC volts at both the alternator and tacho. It measured 16.0 volts ac on both. When I applied the load it varied by about 0.4 volts ac (I did note that the w terminal was also reading 7.0v dc on the DC scale).
    However I got completely side tracked when I happened to place the voltmeter on the boats 12v supply and noticed it was reading 28 volts AC.

    I am so confused as to why this reading exists. I presumed I would read zero AC volts or maybe a small ripple amount but not 28 volts AC.
    Is this just some quirk or am I looking at a more serious issue.
     
  6. CDK
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    CDK retired engineer

    So the AC signal is superimposed on the supply or the ground lead is open.
    DC 14.4 V with a possible ripple of 0.5 V max is what you should find.
     
  7. pennym
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    pennym New Member

    CDK - In respect to the 'W' terminal only from the alternator what would be the AC and the DC voltage reading you would expect to see under normal operation.

    With regard to the other crazy AC reading on the 12V system I think I need to put a scope on to see what is actually going on because I just checked and still got that weird reading when the engine was not running, and I had both the shore power and solar panel disconnected. I have only been using a cheap digital voltmeter.
     
  8. CDK
    Joined: Aug 2007
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    CDK retired engineer

    The 'w' terminal is connected to the stator windings by 3 small diodes, so the output is a half sine signal, frequency =rpm times 3 divided by 60. Because with 3 phase signals the flanks overlap, a DMM sees a DC level of approx 50% of the supply voltage.

    If the erratic supply voltage is also there when the engine doesn't run, there must be something else oscillating, like a voltage converter or regulator. A large electrolytic capacitor between DC+ and ground should be able to silence it.
    A scope will show what's happening.
     

  9. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    big ac on the dc side means faulty diode in the alternator surely?
     
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