1972 222 CC - Fuel Tank Gauge

[bluedolphin]

The gauge is not showing a reading. It's possible the culprit is the way it is wired. Can someone help me? I looked at the back of the fuel gauge, and the wire from the I (Ignition) is connected to a toggle switch, which is connected to the battery +. Ground is to ground, S or sender to the fuel tank sender. The ground on the tank is connected to the ground (-) bus bar. One thing i can see is that the I (Ignition) wire is connected to a toggle switch, then to the positive + on the battery, instead of to the purple wire coming from the ignition switch. Where and how do I start checking? When I turn the toggle switch on, the fuel gauge light turns on, but no reading.

[GoneFission]

The correct wiring to the + side of the gauge is from the hot side of the ignition switch - you don't want the gauge to work if the ignition is not on.  The sender wires go to the back of the gauge - there usually is no polarity to fuel sender wires (see below).  

Check the terminals and connections on the sender on top of the tank and continuity between the sender and the gauge.  If all that checks out, test the sender with an ohmeter to determine if it might be open or shorted.  No real power actually goes through the sender (safety measure), so essentially the gauge acts like an ohmeter and reads resisitance on the sender.  Check the specs on your sender or gauge to see what reading you should be getting.  

Hope this helps - Good luck!

[bluedolphin]

Thanks, Cap'n John. I need a little bit of help on this. I'm not very "electrical" savvy. 1.) Hot side of the ignition switch - is that the purple wire? 2.) Check continuity between sender and gauge - I have a voltmeter, where do I set the dial, and do I just touch the red probe to the sender (neg), and the black probe to the gauge (neg)? And then do the same for the sender (s) to the gauge (s)? 3.) Test the sender with ohmeter to determine if open or shorted - where do I set the dial, and which color probe and to where on the sender? Thanks again.

[GoneFission]

See the info below - I am assuming you have a Teleflex or other standard marine gas or fuel gauge, not a VDO gauge or some wacky digital level monitoring system.  

I loaned my book with wiring diagrams to someone yesterday, so I can't say for sure if it's the purple wire, but it IS the wire that has power when the switch is in the on position, and has no power when the switch is off.  If you got that right, it's close enough.  You can check this with your meter in the volts position or sometimes meters just have a "V" or "DCV" for Direct Current Volts.  You should read about 12 volts DC when you touch the black meter lead to ground and the red to the "+" or "ign" terminal on the back of the meter.  

OK, set the meter on the Ohms scale - it will either say OHM or use the Omega greek symbol.  First the continuity test:  Touch the red meter lead to the wire/terminal on the sender and then touch the black meter lead to the same color wire/terminal on the back of the gauge.  It should read 0 ohms. Do the same for the other wire.  Give all the leads a little tug to be sure they have not corroded inside the terminal.  Replace any bad wiring or terminals.  If you have continuity and wires and terminals are OK, then we can move on.  

To check the sender, remove the wires from the terminals, leave the meter on the Ohm scale, and put the red lead on one sender terminal with the black lead on the other.  The meter should read somewhere between 30 and 200 ohms, depending on fuel level.  If it reads 0 (shorted) or open (no reading), the sender is defective.  Usually that means replacing the sender, but sometimes the float just comes off the end of the float rod inside the tank.  If you get between 30 and 200 ohms, the sender is OK and you can replace the wires back on the sender terminal.  Be careful if you have to remove the sender - it only comes out and goes back in one way - also note the screw pattern orientation and be careful with the gasket and tightening the screws.  Tighten the screws down to make contact first, then a little more all the way around, and then a final snugging down.  Tight is tight - stripped or broken is too tight - and you can't just tighten it till it breaks and then back of half a turn...      Also try not to drop the screws down the sender hole into the tank - believe it or not, they really want to go there, and will take every chance to jump out of your hand and go right down that little hole into the gas tank!  

To check the gas/fuel gauge itself, remove the wires from the sender terminals on the back of the gauge and bridge the terminals with a 100 to 150 ohm resistor (if you don't have one laying around, you can buy them for a buck at Radio Shack - 1/4 watt 100 ohm or 125 ohm something near that is fine).  The gauge should read somewhere between 1/4 and 3/4 full when you turn the ignition switch to on.  If you have power to the gas gauge (see above) and you get no reading with the sender terminals bridged with the resistor, the gauge is bad - time to go shopping.  

Hope this helps - good luck!

[John Jones]

Here is a good reference for gauge application, installation, and troubleshooting.

http://www.teleflexmarine.com/cgi-bin/f ... link2=1047

[bluedolphin]

Thanks, JohnJones and GoneFission! I'll be doing those tests tonight and hopefully get the gauge working soon. I opened the sender yesterday, and used a clean stick to see how much gas was in there - 2". The sender mechanism looks ok with no blockages or kinks, and the piece that floats inside the tank and goes down when gas is used, is moving freely. After tonight, hopefully I will have the gauge working. Thanks again.

[John Jones]

Just remember that gas gauges are notoriously inaccurate anyway.  My little boat has all new stuff.  When it starts bouncing on E it still has 5 gallons in a 16 gallon tank.  Suits me as long as I know where it is.

[JimCt]

My 170 had a mahogany dipstick.  Pretty fancy  8) ... and accurate too.

Since were on the subject, is there any practical way of calibrating a sender to the actual fuel level in the tank?

[John Jones]

Yes, check the Teleflex link I posted above and look under Installation, then fuel sender.  All brands will be similar, by either adjusting the length of the arm or on some, the position of the float on the arm after measuring tank depth and comparing to a chart.

It's trial and error but you might get it somewhat closer.  It's still going to be ballpark.  Mine is too hard to get to.  MY AQ has the old mechanical gauge through a (used to be clear) deck plate.

If one really has to keep close tabs, I suggest a fuel management gauge.

[GoneFission]

The instructions on the Teleflex sender, if followed, will give you a pretty precise sender reading.  You measure the depth of the tank, then cut the lever arm to a specific length, and then mount the float at the specified distance from the sender.  I did this and have found it surprisingly accurate.  Half full on the gauge is about 40 gallons out of the 85 gallon tank, and full is full.  It's not as precise as the fuel flow meter (which reads to a tenth of a gallon), but gives you a quick "eyeball" of fuel level in the tank - which is what a gas gauge is supposed to do...  

Tank shape also has an effect - the gauge will be more precise as the shape of the tank becomes closer to a rectangle.  A V-shape tank will give readings that are slow at first and fast toward the bottom, so half full is really more like quarter full.

[bluedolphin]

Thanks to everyone. Fuel gauge works now. Culprit: 2 wires from sender to gauge were no good. When I replaced them with new wires, the gauge worked. Thanks again. One down, more to go. Next is the bilge pump. Then hook up the tach, oil pressure, water temp, etc.