Fuel Guage. Getting close, still not working - Page 2 - Chevelle Tech
Electrical & Wiring Troubleshooting electrical problems.

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post #16 of 16 (permalink) Old Oct 20th, 04, 4:22 PM
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Join Date: May 2001
Location: Northern VA
Posts: 2,173

Maybe I can help you with your trouble shooting efforts. This may seem long but it may be worth reading and is backed by over 40 years of experience, so bear with me. In about 2 hours you should have the problem isolated and cure defined.

You will only need 2 small rolls of #12 or #14 insulated stranded primary wire, one RED and one BLACK (available at any auto parts store); and a few "clip leads" (you can make your own or pick them up cheap at radio shack). You should also use a digital volt/ohm meter but it's not absolutely necessary. But it will give you the most accurate readings and draw less current from the test point. Also, make sure the gas in your tank is half or over but preferably not full.

Sometimes electrical can be a bear. If you make assumptions rather than prove the circumstance, it will drive you to distraction. If the gauge and sender are indeed the correct units, you only need to verify about 4 things to isolate your problem.

One of the hardest things for people to troubleshoot is a bad ground. Mainly because there's no "absence of voltage or signal" to indicate where or if it exists. So, when I try to trouble shoot something like you are doing, I ALWAYS run a long black piece of number 12 insulated primary wire to the negative side of the battery. Make absolutely certain that this connection is clean and solid. A large alligator clip will do fine. All electrical ground paths lead to there. THAT wire is also what I hook my meter to when I take readings and when grounding other leads like at the back of the guage or the brown wire. I tie every ground of the circuit I'm testing (in this case, the tank sending unit ground and the instrument cluster ground) to that lead also. It only takes a few cheap clip leads from Radio Shack to do this. If things start working correctly when you are doing this, you know the answer, BAD GROUND(s). I never trust that because a ground wire is bolted to the frame or block that it is making a good connection. I've had too many that looked perfect but weren't because of engine paint or frame rust. I always tie my meter to that black lead connected directly to the negative battery post. You'd be surprised how many "wierd problems" I've easily solved this way.

First, forget the sending unit for now, diconnect the brown wire from it and tape up the end from the harness so it's not touching ground. Let's try to verify/isolate one element of the gas gauge circuit at a time instead of dealing with the whole thing and multiple variables.

- Disconnect the brown wire from the back of the gauge. Based on the info from others here, it should peg the full mark since it is now seeing 90+ ohms. If it doesn't, you should stop and find out why not.

- Next, verify there is a good 12 volts supplying the gauge (and the rest of the dash). With the black lead of your meter connected to that "home run" ground lead and read the voltage on the other connection of the gauge, that's the supply side. If 12 volts, proceed. If not, find out why.

- Next ground the Volt meter to the dash frame andd measure the same 12 volts at the same point. If it reads the same as when the meter was grounded to the home run lead, chances are the dash ground is good. If it read differently, you have a bad dash ground. (You can easily clip the home run ground lead to the dash and see if the reading changes. If so, that is a dash ground problem.)

- Now ground (to the home run ground) the side of the gauge that the brown wire goes to. Again, based on the info provided here, the gauge should read empty. If not, you have a bad gauge.

- Now we'll jump to the sending unit because the it's easy to eliminate the wiring in between. Use a piece of RED #12 insulalted primary wire and tie one end to the gauge terminal where the brown wire would normally go. Connect the other end to the the lead from the sending unit that the brown wire should normally connect to. Remember to connect the sending unit ground to the "home run ground lead" even if it's bolted to the frame. If necessary, unbolt it for this test and clip the black wire from the battery to it. Turn the key on and look at the gas gauge. Does it accurately reflect the amount of gas in the tank? If yes, congratulations, you have a bad ground or wiring from the gauge to the sending unit. If not, you have a bad (or wrong) sending unit. If everythnig reads correctly, clean the ring terminal and frame area to bare metal and re-connect the sending unit ground to the frame. Does the gauge still read ok? If not, fix the bad ground connection there, or elsewhere.

Note: at this point you have verified that the gauge works properly and you have eliminated ALL the wiring between the gauge and the sending unit to determine if the sending unit is functioning properly (or repaired it). The only part left in the equation is the wiring between the gauge and the sending unit.

- With the brown wire disconnected from the sending unit, connect the other end back up to the gauge. If the gauge doesn't continue to read FULL, you have a short or partial short to ground in the harnesses between the gauge and the other end of the wire. Stop, find it and fix it. Start by disconnecting the first connector from the tank end and then the next towards the gauge unitl the gauge reads FULL. Fix the short.

- Now ground the other end of the brown wire to the "home run" ground lead. The gauge should read empty. If not, you have a bad or dirty/corroded connection in the brown wire circuit between the gauge and the tank end. At this point you can find and fix it or just bypass the whole circuit with the red wire. Your choice.

Hope this helps.

/herb sr.

Herb Sr
'67 Coupe (frame-off)
489BB Full Roller Stroker
M20/3.73 Eaton posi

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