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Discussion Starter · #1 · (Edited)
I've read a lot of posts from different guys that were having trouble with their fuel gauge without ever hearing what the fix was if any. So, despite checking everything very carefully during the build, when I put gas in the tank, I got nothing. Zero, barely to E. Last week I checked the gauge with an Ohmmeter and determined that the fault was at the tank. Today I lowered the tank and replaced the sender. I checked it for operation before putting it back up and adjusted the arm for approximately what I had in gas. I was happy. Bolted everything back up and hit the key. Zero!!! Let it back down, works great. :confused:
Okay, no suspense, here's what I found. The tank and anything attached to it, like fuel pickup and/or fittings, must be completely insulated so as not to touch the frame or any grounding sheet metal . The instant the tank is grounded you're dead. I went through and insulated every possible contact point and now have a functioning fuel gauge. :D
Hopefully this will be helpful to others with balky fuel gauges and save them from buying a new $80 sender for nothing.
Frank
PS: Keep reading there's more to this.

[ September 06, 2004, 01:08 PM: Message edited by: Frank Messina ]
 

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I think Chuck nailed it.....your leads are probably reversed. I think one lead on the sender is grounded to the tank. If you reversed the leads, the gauge would still work if the tank was completely isolated, but it would go straight to ground (zero ohms = empty) if not. Sounds like you have it working now by isolating the tank from any and all grounds. If one of your "insulators" wears through over time, you'll be back in the same boat. Might be worth it to swap the wires now? Test the theory by swapping wires, then intentionally grounding the tank.
 

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Discussion Starter · #4 ·
Yeah Wade, I think so too. So much for my big discovery.
In isolating the "no read", it just didn't occur to me that the polarity could be the culprit, but now in the daylight it seems logically obvious. At the time, I was so pleased that I had figured it out,I didn't think to chase it down any further. Another thing that clouded my reasoning was that I had previously soldered in the plug leads. I'm all put back together now so I'm just going to leave it for the time being. If I need to switch them later though, I can easily get to the leads through the rear wheel opening.For anybody under construction using a non donor harness, this is a good heads up.
Check gauge functionality before making your final connections and definitely before installing the lower trunk aluminum. You can do this while holding the sender in your hand with the gauge leads temporarily hooked to the sender with alligator clips and a third wire clipped to the sender flange and a frame ground. Turn the key on and watch the gauge. If it doesn't register, reverse the leads to the sender. Simple and no gas to worry about. I wish I had known about this 6 montths ago.
Frank
 

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Frank - the very fact that you took the time to put all this together and share it with your fellow builders to hopefully keep them from having to go through the same thing - speaks volumes about your Character!

You get a big

 

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Frank (& others):

Thanks for the heads-up. I'm almost that that point in my build. I have printed this topic and will add it to my "Build Book" for future reference!

Bob
 

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Frank, C-wood and others..THANK YOU... I was just getting ready to tackle this issue... think I'll try reversing my leads...
Ken
 

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I'm confused. :confused: Is this a stock Mustang sender? The stock Mustang sender is just a variable resistor. It shouldn't matter which way the wires are hook up to it; the resistance through the sender is the same. I believe that both terminals on the stock Mustang sender should be insulated from the sender housing. It sounds to me that your sender has a defect. One of the terminals on the sender is shorted to the sender housing, which would cause the wire on that terminal to short to ground when the fuel tank is grounded. This condition would explain why Chuck's gauge worked after he swapped the wires. If the shorted terminal is attached to the wire which goes to ground, the short won't cause a noticeable problem. But if the shorted terminal is attached to the sender wire coming from the gauge, the gauge will read zero.
 

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Discussion Starter · #9 ·
Yes it is a stock Mustang sender ( two of them) and I got the exact same results with both senders, one of which was brand new. The thing that I didn't grasp at first, is that this is a variable resistance GROUND circuit. This is not a problem you would likely encounter with a donor harness because the ground is automatically on the correct pin. All you do is plug it in and go. With a Painless or American Autowire harness, one leg from the sender goes to ground and the other leg goes to the gauge. Problem is, you don't know which pin is for the ground and which is for the gauge, so you've got a 50 50 chance of getting it right. When the pin that is supposed to go to ground is hooked to the brown gauge wire, and the pin that is supposed to go to the gauge goes to ground, as soon as the sender is grounded by tank or fastener to frame contact, resistance goes to zero and the gauge reads "E" or less. Unhook the tank and as soon as it comes away from the frame it reads fine. You can either insulate the tank like I did or be smart and switch the wires.
Frank
 

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Paul Thats why I said it doesn't make sense.
I was hopping mad...the thing would work out of the tank, I would put it back in and it would stop working...then I noticed when you touch it to the tank at all and the needle would go to E. I tested the voltage etc and could think of nothing other than swapping the wires...which didn't make sense but it worked ! When you look at the sender one of the contacts is continuous with the flange...so intended to contact the tank.
 

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Who cares if it makes sense... I just switched the two leads and my gas guage works now!! Thanks Frank!! Saved me lots of time and trouble!
Ken
 

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Now I'm really confused about the fuel gauge issue.

I installed the Autometer vintage fuel gauge via my '88 donor harness. Grounded the gauge to the chassis. It worked fine for a day, then a drive over a rough road and it stopped. I'm thinking loose wire. I checked the power and ground leads and they were fine. I then used my ohm meter attaching one lead to the sender wire and one to ground. I had continuity there. Using the voltmeter, the current flowed from the power through the ground and I had current coming from the sender wire.

I spoke to both FF and the autometer tech. support and they said it sounded like a bad gauge. Sent the gauge back and waiting for a replacement.

Bad gauge or something else?
 

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I was looking at this problem in my electrical fire post, and I still haven't understood this:

The negative (left hand, when looking from the rear) male terminal on the plug for the sender is grounded in two ways --directly through the gas tank to frame and also through the ground wire that attaches to it. So why does it matter which of these paths the electricity takes when it grounds? Or has my stock fuel sender/wiring already had its wires fixed? All I know is that in a straight-out-of-the-donor the electricity can't ground until it passes through the variable resistor, so why does it matter how it grounds after that?
 

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The easiest way to get it right is to use shielded bullet connectors to connect the fuel level sender. Make them a little longer so that the bullet connectors are by the front of the gas tank. You can zip tie them to one of the 3/4" tubes that support the tank. Then it's a simple swap-a-roo on the bullet connectors and the gauge works. Ding! Next!

I was really pleased with the painless harness. The color coded and labeled wires makes it super easy to get everything right.

Hope this helps someone.

B
 

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I have SW gauges/sender connected by the Painless cab wiring harness. Within 2 days (1999) of my first tank my gauge read 3/4 when full. I have swapped leads( went to E) , done resistance tests and swapped senders. Maybe it is my gauge. I will inspect for ground interrupts although that would just send me back to E. I learned to accept it as SW gauges are really quirky.
 
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