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

 

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

Which one is more affected? The oil pressure reading or the temperature one?

 

And another question, how can you be so sure it's resistance related?
Have you tried using shielded cable?

 

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.

 

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

 

Tempreature reading makes sense if you're not using the proper thermocouple wire as an extension.
But your oil pressure doesn't make sense.

 

Nope, I haven't tried anything but the end run with a dedicated ground.

 

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

You beat me to the adequate grounding question...

 

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

Thanks! I'll give that a try.

 

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)

 

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)

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 ?

 

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 ?

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.

 

Wise words from Siemens.
http://www.automation.siemens.com/doconweb/pdf/840C_1101_E/emv_r.pdf?p=1
Max.

 

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.

 

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.

Boat, saltwater, corrosion, crusted insulation... it all adds up

 

Here's a drawing from some other person's problem.

 

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.

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.....

 

Here's a drawing from some other person's problem.

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?

 

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

 

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.