How to avoid resistance?

Discussion in 'General Electronics Chat' started by duncan mccallie, Apr 2, 2015.

  1. duncan mccallie

    Thread Starter New Member

    Apr 2, 2015
    7
    0
    Hi ! I've recently replaced old analog oil pressure and water temperature gauges in a boat with digital units.
    They have sending units that utilize resistance to register. My issue is apparently the distance from the pilothouse to the engines. My ground wire and sender leads pass through two or three terminal strips before reaching the engines. Apparently the minute additional resistance in each joint is driving the digital gauges crazy. Water temps are way too high and oil pressures way to low. I can alleviate this by running one unbroken #12 ground wire from instruments to engine....but I trip over it :)
    If I were to eliminate the terminal strips and solderless connections with soldered joints, would that possibly reduce the resistance?
    Any suggestions appreciated.
    Thanks in advance!

    duncan mccallie
     
  2. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,552
    2,482
    Which one is more affected? The oil pressure reading or the temperature one?
     
  3. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,552
    2,482
    And another question, how can you be so sure it's resistance related?
    Have you tried using shielded cable?
     
  4. duncan mccallie

    Thread Starter New Member

    Apr 2, 2015
    7
    0
    Oil pressure should read 50-60 lbs and they both read 7-8 lbs. Temps should be around 175 ish and they read 200 and 215 respectively.
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    Your analysis seems highly dubious to me. As you said "the minute additional resistance...". I think there is something else afoot here. Like a "supposed" ground wire that isn't really grounded perhaps. You can do a test with a multimeter and a short loop of wire in place of the sensor. You should be able to estimate the observed resistance from the distance and the wire guage. See the following table:
    http://en.wikipedia.org/wiki/American_wire_gauge
     
    cmartinez likes this.
  6. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,552
    2,482
    Tempreature reading makes sense if you're not using the proper thermocouple wire as an extension.
    But your oil pressure doesn't make sense.
     
  7. duncan mccallie

    Thread Starter New Member

    Apr 2, 2015
    7
    0
    Nope, I haven't tried anything but the end run with a dedicated ground.
     
  8. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,552
    2,482
    You beat me to the adequate grounding question... :confused:
     
  9. duncan mccallie

    Thread Starter New Member

    Apr 2, 2015
    7
    0
    Thanks! I'll give that a try.
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    You have a ground loop! Other, non-related equipment is sending large current(s) down the return wire going to the sending units on the engine(s)
     
    #12 and cmartinez like this.
  11. duncan mccallie

    Thread Starter New Member

    Apr 2, 2015
    7
    0
    This sounds quite possible. I've built (and wired) this vessel over a period of several years. On a boat, the DC ground and the AC ground are bonded together.
    Seems like that is ripe for a ground loop? Can you point me in the direction of how to diagnose the source ?
     
  12. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,552
    2,482
    The way to avoid a ground loop is to make sure that all grouds meet at a single point... think of a star and NOT a tree.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,493
    2,363
  14. #12

    Expert

    Nov 30, 2010
    16,246
    6,736
    Your diagnostic method is valid, but it only locates the cure, not the cause.
    The obvious answer is to search for crusty connections, especially in crimp connectors. The less obvious is about other loads, like lamps, sharing the ground wire. If the ground wire was perfect, as in, "no resistance at all" this would not be a problem. Unfortunately, wire has resistance and there is no reasonable way to make one wire so large that its resistance is insignificant. Most people that can wire a boat can operate an ohm meter. This suggests that your problem is the less obvious one...too many amps already using that section of ground wire.
     
    cmartinez likes this.
  15. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,552
    2,482
    Boat, saltwater, corrosion, crusted insulation... it all adds up
     
  16. #12

    Expert

    Nov 30, 2010
    16,246
    6,736
    Here's a drawing from some other person's problem.
     
    cmartinez likes this.
  17. duncan mccallie

    Thread Starter New Member

    Apr 2, 2015
    7
    0
    Thanks! This has given me lots to think about. I recently installed a high output alternator with a regulator I need to revisit in the morning.....
     
  18. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,552
    2,482
    I read a while ago tha there are pros and cons to using shielded cable for an AC motor's power lines. I remember that the recommended practice is to use metallic conduit to wire that separately from signal cables. Maybe @MaxHeadRoom could explain this further?
     
  19. duncan mccallie

    Thread Starter New Member

    Apr 2, 2015
    7
    0
    Thank you everyone for the help.

    My wife and I live aboard. We're currently in San Francisco but headed up the West coast for British Columbia in the next few weeks. It would be good to have engine vital signs for that trip.
    I'll head to the engine room with my multimeter first thing in the morning.

    Thanks again
     
  20. #12

    Expert

    Nov 30, 2010
    16,246
    6,736
    Instrument wires are often in danger of picking up interference, but these sensors are usually in the range of 100 ohms and not very sensitive to local magnetic fields.

    Oh no! You installed, "new digital units". That means you might NOT be using old fashioned low impedance engine sensors.

    Well, if running a dedicated wire fixes the problem it is not the impedance of the sensors that went wrong.
     
    Last edited: Apr 3, 2015
Loading...