fuel guage causing power drain

[mbfish]

I've traced a slow power drain down to my fuel guage (I think). When I have the key off and the guage connected, the wires that would be connected to the battery show a couple Ohms less resistance than when the guage is disconnected, then it shows the same resistance as an open circuit. So, is it a short in the guage? Does anyone have any insight or experience out there? The boat is a 2006 Lowe 175 FS.

 

[Bob_VT]

Re: fuel guage causing power drain

The power to the fuel gauge should be connected to a fuse block that is powered only when the key is on. Is it wired right??

 

[mbfish]

Re: fuel guage causing power drain

Yeah, it's all the factory wiring and seems to be correct.

To trace the power drain issue to the guage, I just put a multi-meter where the battery would go, took note of the resistance value and started taking plugs apart until something caused the resistance to jump up to the open-circuit value. That connector ended up being the fuel guage plug. I'm not really sure if there is a better way to find a current drain. If someone has any ideas or better methods for this kind of thing, I'm game.

 

[generator12]

Re: fuel guage causing power drain

Checking ohms is a good step, but if you feel you have it down to a circuit, use the multimeter's milliamp current measurement capability.

The meter needs to be placed in series with the circuit you're measuring. So you will remove one of the wires to the gauge, place the meter (observing polarity) into the circuit such that any current flow must go through the meter. Then methodically change the scale setting from maximum (say, 30 amps) through the more sensitive ranges until you either find a current draw you can measure, or reach the most sensitive range (say, 300 milliamps) and still read zero current draw. If the meter reads zero, you haven't found your offender as yet.

As mentioned above, the fuel gauge should not be powered up when the key is off. If it is, this should be investigated.

 

[tpenfield]

Re: fuel guage causing power drain

Amperage would be the correct way of testing the gauge for faults, as the gauge will show fairly low resistance on an ohms check.

Most fuel gauges have 2 coils in them and the sender feed connects at the junction between the 2 coils, thereby adjusting the current flow through each coil as the sender float level changes.

If you put an ohm meter across the gauge, you will be measuring the total resistance of the 2 coils.

 

[mbfish]

Re: fuel guage causing power drain

Thanks guys. I'll be checking amperage in the circuits today.

 

[Silvertip]

Re: fuel guage causing power drain

First, the fuel should not be active at all when the key is off so how can it have a power drain? Next, the power lead for any gauge does not go to the fuse panel. Gauges are powered by the "A" (accessory) terminal on the ignition switch for an outboard and the "I" (ignition) terminal for an I/O. The entire fuse panel is fed by a separate #8 or 10 gauge pair (black & red) from the battery via a 20 - 30 amp breaker at the battery. But then prior owners tend to do some "creative wiring" at times so it's hard to tell what the actual wiring currently looks like. Resistance measurements need to be made when the item you are testing is the ONLY item in the circuit otherwise you will likely be reading a combination of parallel and series resistances which is not very conclusive. If you have a battery that is losing power over time, it in itself may be the issue or there is some other drain such as the station memory and clock functions on your radio.

 

[mbfish]

Re: fuel guage causing power drain

Okay, I've done some current measuring...

There is small amounts of current flowing through the fuel guage when the key is off.

Wire colors are as follows:
Black - ground.
Blue - light.
Purple - power.
Pink - sending from tank.

Current measurements:
black-blue = 3 microAmps
purple-blue = 0 microAmps
purple-black = 0 microAmps
purple-pink = 16 microAmps
pink-black = 11 microAmps
pink-blue = 11 microAmps

...so... Is my logic on track here? The pink wire seems to be involved in the current draw. It is either grounding somewhere or the sending unit is shorting. Is that enough current to drain my battery over a few days or a week?

 

[Silvertip]

Re: fuel guage causing power drain

Stop and really think about this. With the key off, there should be zero "voltage" on the purple wire ("I" terminal) on the fuel gauge. If there is, then that circuit is being powered by something else. To make "current" you need voltage. If there is none, you can't have any current. Next -- 16 microamps is .000016 amps. .016 is 16 milliamps. So then you need to be sure you the numbers you post are really accurate. Your battery powered watch probably draws tht much so that current draw is not running down a battery.

Here is a diagram of a fuel gauge circuit. Sending unit resistance is 33 ohms full tanks, about 109 ohms at 1/2 tank and 240 ohms empty. Remove the pink wire from the sending unit and with the key off, check between it and ground for voltage (or current if that what you feel concerned about). There should be zero of either. Check for voltage between the "I" terminal on the gauge and ground with the key off. If present, that circuit is being cross-fed by some other circuit. You can also pull fuses, or disconnect components until the meter says that current flow is no longer detected. Now look for why that circuit is causing the issue. See where this is going. You are convinced the fuel gauge circuit is to blame when in reality it is probably not. By the way -- spit between a penny and a dime, place the two together and you can likely create 16 microamps of current.

 

[mbfish]

Re: fuel guage causing power drain

Okay. Thanks for the reply. Makes sense. I'm starting to think it's not the fuel gauge. However, it seemed proper at the time when there was a change in resistance when it's connected and not connected.
If a battery has a 50 amp-hour capacity then it would require 100 hours to drain it on a 1/2amp current, right. So, I'm looking for a current in the order of 0.1 amp or more? Since the battery was killed in a week and a half... 10 days x 24 hrs = 240 hrs. 50 amp hrs / 240 hrs = 0.2 amps required to drain the battery to dead over a 10 day period?

 

[Silvertip]

Re: fuel guage causing power drain

Have you thought about the radio station memory and clock? Did this problem just start or is the boat new to you? Stop looking for current. Think about what is being left turned on. Or, put your meter in the positive lead of the battery and see what the current draw is. Pull fuses until it drops to zero. That's the killer. I'm guessing the alternator or regulator on the engine may have some issues causing the drain. And as you can see from the diagram, measuring resistance at any point in the system will change when you disconnect something. That's the way resistance is. Two 50 ohm resistors in series means you measure 100 ohms when measured across both of them. If you measure across just one you still get 50 ohms. Two 50 ohm resistors in parallel will measure 25 ohms across them whereas you still measure 50 ohms if you disconnect one of them and measure across either of them. You need to understand the circuitry when making resistance measurements or you end up chasing your tail.

 

[mbfish]

Re: fuel guage causing power drain

Thanks for the direction. You are 100% right. I was chasing my tail using a poor indicator.
I put the multimeter inline with a battery lead and started measuring current. Right away there was 0.28A of current running through the meter. I popped off the face-plate of the stereo head unit that I put in late last year and the draw dropped to 0.12A. A few seconds later the draw dropped to 0.00A (on the 10A setting). On settings that allow detection of less current, I still see 20mA. Is this at all typical? Anything to be concerned about?

 

[bruceb58]

Re: fuel guage causing power drain

I would love to know how you measure micro amps!!!

 

[tpenfield]

Re: fuel guage causing power drain

Does the boat have an I/O engine with a corrosion protection system (like Mercathode)? Those will draw around 20ma in the water.

 

[mbfish]

Re: fuel guage causing power drain

I would love to know how you measure micro amps!!!

With a meter....

 

[mbfish]

Re: fuel guage causing power drain

Does the boat have an I/O engine with a corrosion protection system (like Mercathode)? Those will draw around 20ma in the water.

It's a honda 90 outboard. At this point I don't think I'm going to worry about it. I may mess around with it in a few days. That current will take close to half a year to kill a battery.

 

[bruceb58]

Re: fuel guage causing power drain

With a meter....

What kind of meter? I have a fairly decent Fluke at home and the best resolution I have on mine is 10 mA...that is milli, not micro.

 

[generator12]

Re: fuel guage causing power drain

Thanks for the direction. You are 100% right. I was chasing my tail using a poor indicator.
I put the multimeter inline with a battery lead and started measuring current. Right away there was 0.28A of current running through the meter. I popped off the face-plate of the stereo head unit that I put in late last year and the draw dropped to 0.12A. A few seconds later the draw dropped to 0.00A (on the 10A setting). On settings that allow detection of less current, I still see 20mA. Is this at all typical? Anything to be concerned about?

Now you're cooking...! That's the most direct way to look for a current draw. Resistance checks can help to break down a circuit once that circuit has been determined to be faulty, but it's seldom the best initial move in checking a boat (or automotive circuit).

If you're sure of the .002 amp draw with everything shut off, that is something you should pursue. That represents a 600 ohm load, and will reduce your starting capacity over a period of weeks. 600 ohms could be something like an accumulation of grease or moisture across a pair of terminals (+ to -) somewhere under the motor cover. It could decrease on its own over time, but could also increase with time. No emergency, but if it were mine I would go over everything that could be so exposed with a good light, a non-conductive grease/moisture remover and a small brush.

 

[mbfish]

Re: fuel guage causing power drain

What kind of meter? I have a fairly decent Fluke at home and the best resolution I have on mine is 10 mA...that is milli, not micro.

Well, mine's probably false resolution anyway if a good meter doesn't measure that small. It's silly to be measuring on that kind of scale for this type of application anyway - as Silvertip illustrated with the 'spit between a penny and a dime' statement. I'm just using a $20 meter from the hardware store. It has a setting for between 0.1 and 10A, two settings in the milli amp range and two settings in the micro amp range.

 

[mbfish]

Re: fuel guage causing power drain

Now you're cooking...! That's the most direct way to look for a current draw. Resistance checks can help to break down a circuit once that circuit has been determined to be faulty, but it's seldom the best initial move in checking a boat (or automotive circuit).

If you're sure of the .002 amp draw with everything shut off, that is something you should pursue. That represents a 600 ohm load, and will reduce your starting capacity over a period of weeks. 600 ohms could be something like an accumulation of grease or moisture across a pair of terminals (+ to -) somewhere under the motor cover. It could decrease on its own over time, but could also increase with time. No emergency, but if it were mine I would go over everything that could be so exposed with a good light, a non-conductive grease/moisture remover and a small brush.

Thanks for the tip. I'll try to free up some time in the near future to inspect things and maybe clean up some contacts and such. Thanks for all your help, guys!!