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Old 06-12-2017, 08:32 AM
JOHNDRIVESF3 JOHNDRIVESF3 is offline
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Fuel Gauge not working

My fuel gauge is not working. When I put a test light on tank wire it is getting power. As I follow up to gauge and check, the test light blinks. While blinking the gauge reads (Works) Once I take the test light off gauge drops back to empty. Any thoughts?
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Old 06-12-2017, 11:12 AM
rusty48 rusty48 is offline
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Bad wire, bad ground, your test creating complete circuit and it works.
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Old 06-12-2017, 11:45 AM
JOHNDRIVESF3 JOHNDRIVESF3 is offline
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Thanks Kevin

I will check ground wire
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Old 06-12-2017, 07:19 PM
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The sender unit IS the ground. See schematic below.

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Old 06-13-2017, 08:21 AM
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The tank is usually insulated from ground if you have the rubber strips underneath. What I did was run a separate ground wire from under one of the sending unit fastening screws to a ground. In my case to the cab itself.
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Old 06-14-2017, 07:26 PM
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Kinda hard for the uninitiated to understand how the gauge and sender work. The gauge is attached to the ignition and grounds through the sender. The gauge's bi-metal arm distorts/deflects with current passing through and, in turn, moves the needle. The sender also has a bi-metal arm but in its case it serves as a voltage regulator. It doesn't quite act as the one in the gauge though. It actually opens and closes a set of points that regulate voltage and current to ground (or, for those that like to follow electron travel, from ground.) As generator voltage and current fluctuates and also as outdoor temperature varies the speed that the regulator pulses (now you know why your test light was flashing) also varies, supplying a reliable value. It doesn't actually provide a constant voltage/current, rather it supplies a on/off ground that, for ease of explanation, is averaged out by the heated strip in the gauge not being very quick to respond. It also provides dampening so that sloshing fuel in the tank doesn't have the needle going berserk while driving.

The missing regulator is why the aftermarket sending units have no chance of ever working correctly with an original gauge. I've got a CVR (Constant Voltage Regulator - basically the same exact thing but from a 12 volt car) from a '60s Ford that I've been meaning to install. Kinda thinking and hoping it will work, even though it's for a 12 volt system. My theory is its current that makes them work, so.... *fingers crossed*

Last edited by CBeav; 06-14-2017 at 07:33 PM. Reason: Add'l info
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Old 06-14-2017, 09:50 PM
Flat Ernie Flat Ernie is offline
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Quote:
Originally Posted by CBeav View Post
Kinda hard for the uninitiated to understand how the gauge and sender work. The gauge is attached to the ignition and grounds through the sender. The gauge's bi-metal arm distorts/deflects with current passing through and, in turn, moves the needle. The sender also has a bi-metal arm but in its case it serves as a voltage regulator. It doesn't quite act as the one in the gauge though. It actually opens and closes a set of points that regulate voltage and current to ground (or, for those that like to follow electron travel, from ground.) As generator voltage and current fluctuates and also as outdoor temperature varies the speed that the regulator pulses (now you know why your test light was flashing) also varies, supplying a reliable value. It doesn't actually provide a constant voltage/current, rather it supplies a on/off ground that, for ease of explanation, is averaged out by the heated strip in the gauge not being very quick to respond. It also provides dampening so that sloshing fuel in the tank doesn't have the needle going berserk while driving.

The missing regulator is why the aftermarket sending units have no chance of ever working correctly with an original gauge. I've got a CVR (Constant Voltage Regulator - basically the same exact thing but from a 12 volt car) from a '60s Ford that I've been meaning to install. Kinda thinking and hoping it will work, even though it's for a 12 volt system. My theory is its current that makes them work, so.... *fingers crossed*
You probably realize this, but that is not really a voltage regulator. The bi-metallic strip has pressure put on it by the float that makes it require more heat before it will break contact. It doesn't "regulate" anything in reality, it merely turns a full 6V on and off at intervals consistent with how much pressure the float is putting on it based on fuel level and how much heat is required to open the contacts. Full system voltage will flow through the gauge until the sender heats up enough to open the circuit and turn off voltage.

This heat is merely an expression of current flow through the bi-metallic strip at the sending unit. In turn, this current flows through the bimetallic strip in the gauge moving the needle through the deflection of the strip via a small gear train.

All you're really doing is measuring average current as heat.

Which means, you CAN do the same thing with an aftermarket sending unit just fine. It uses variable resistance to control the average current through the bi-metallic strip in the gauge. The trick is finding the correct resistance range.

There is another variable too - voltage. At idle with a fully charged battery, there's probably only 6.0-6.3 volts (no load), but with the engine running and the RPM above the generator cut-in, you may read 7.5V which will cause your gauge to read 'fuller' than at idle. This is why when you get off the exit after running it all the way down to E on the highway and you're waiting for the off-ramp light to turn green, you might notice you're even lower than E on the gauge - you didn't just magically burn more gas between the off-ramp and the light, you just changed the voltage. That's why I really like true CVRs when doing a 12V conversion - stable 6V to the gauges no matter RPM or battery voltage. This variable voltage affects temp and oil pressure too (and BATT meter on '40 Fords).
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Old 06-15-2017, 03:57 PM
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I thought I covered that by stating it doesn't actually provide a constant voltage/current but rather an on/off ground. Yes, the cam on the float arm causes the dwell time to change and cause the gauge to reflect the varied voltage/current (could call it wattage but didn't want to introduce another term. And the term wattage is correct as the nichrome wire wrapped around the bi-metal strips react to both volts and current, the same as a heating coil on an electric stove.) It's the dwell time of the points being open/closed that 'averages' the wattage, buffered by the response of the heated strip in the gauge. Other than the introduction of the float arm's influence, the sender's operation is exactly the same as a mechanical CVR such as that used by Ford for years following. Electronic CVRs don't work as well since they do not provide any compensation for temperature.

The available aftermarket sending units aren't exactly calibrated for us (at least none match the requirements I need for my truck. I assume most will follow suit but I have been wrong a few times in my life ;-) ), they're just a kind of a generic make-do/it must be you/it must be your car/etc. product. The resistance coil in those sending units are wound to provide a range of resistance that is close, but not quite close enough, to result in an accurate readings. Using a Kent-Moore instrument gauge tester (J-24538-B) I found the following readings applied to my '53 F-100 (resistance readings verified with a Fluke VOM to all be within 0.1Ω):

Full (needle centered on mark) 18Ω
21Ω
3/4 23Ω
25Ω
1/2 28Ω
32Ω
1/4 35Ω
39Ω
Empty (centered on mark) 43Ω

These readings were taken with the engine idling so as to provide a stable 7.15 volts. I moved to the settings both up and down and from 1/8 spans and 1/2 spans to reach the readings. I've measured both "Must be used with original gauges ONLY" style sending units (standard nichrome wrapped and 'electronic'.) Both measured identically (I wish I had written the measurements down but they were in the 22-98Ω range as my memory recalls.) The spec sheet included with the tester shows Ford fuel units to spec at 10-76Ω but make no mention of years or K-S or CVR type units. My truck has had the dash harness replaced by PO and I've installed a new ground to frame wire. I have the stock (replacement) running board tank ('53) and it measures 11" deep. I made the specific arm length adjustments as directed (DC 'electronic' version sending unit) and ended up exactly in the same place I was with the 'standard' replacement unit. I sped up the engine RPM to allow for 7.56 volts and measured about an 1/8 difference in gauge range as a result. I realize that the missing regulator in the original sending unit is the reason for the gauge inaccuracy. Hoping that even though the CVR I have is 12 volt it should still work, just slower as the wattage will be lower (volts X amps = watts. My '6' volts will come with the same amps, dictated by the ability of the heating coil, so figure half the wattage. Plus, most Ford CVRs are adjustable so another hope of mine is I'll be able to tweak the voltage to make up for the lower wattage.
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Old 06-15-2017, 09:31 PM
Flat Ernie Flat Ernie is offline
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Quote:
Originally Posted by CBeav View Post
Electronic CVRs don't work as well since they do not provide any compensation for temperature.
You do not need temperature compensation with a CVR...that's sort of the point.

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Originally Posted by CBeav View Post
The available aftermarket sending units aren't exactly calibrated for us (at least none match the requirements I need for my truck.
They're not exactly calibrated for your brand new 2017 Ford either - ever notice that it takes a long time to go from Full to 3/4, it goes quicker from 3/4 to 1/4 and that last 1/4 tank seems to go in no time flat? That's because it's not linear...but it works just fine.

Quote:
Originally Posted by CBeav View Post
Using a Kent-Moore instrument gauge tester (J-24538-B) I found the following readings applied to my '53 F-100 (resistance readings verified with a Fluke VOM to all be within 0.1Ω):

Full (needle centered on mark) 18Ω
21Ω
3/4 23Ω
25Ω
1/2 28Ω
32Ω
1/4 35Ω
39Ω
Empty (centered on mark) 43Ω

These readings were taken with the engine idling so as to provide a stable 7.15 volts. I moved to the settings both up and down and from 1/8 spans and 1/2 spans to reach the readings. I've measured both "Must be used with original gauges ONLY" style sending units (standard nichrome wrapped and 'electronic'.) Both measured identically (I wish I had written the measurements down but they were in the 22-98Ω range as my memory recalls.)

The spec sheet included with the tester shows Ford fuel units to spec at 10-76Ω but make no mention of years or K-S or CVR type units. My truck has had the dash harness replaced by PO and I've installed a new ground to frame wire. I have the stock (replacement) running board tank ('53) and it measures 11" deep. I made the specific arm length adjustments as directed (DC 'electronic' version sending unit) and ended up exactly in the same place I was with the 'standard' replacement unit. I sped up the engine RPM to allow for 7.56 volts and measured about an 1/8 difference in gauge range as a result. I realize that the missing regulator in the original sending unit is the reason for the gauge inaccuracy. Hoping that even though the CVR I have is 12 volt it should still work, just slower as the wattage will be lower (volts X amps = watts. My '6' volts will come with the same amps, dictated by the ability of the heating coil, so figure half the wattage. Plus, most Ford CVRs are adjustable so another hope of mine is I'll be able to tweak the voltage to make up for the lower wattage.
You're making it incredibly more complicated than it needs to be...you could just fill up at 1/4 tank and not worry about being accurate down to the last 1/2 gallon.

A true CVR will always provide 6V to the gauge. The Ford "chopper" works just fine, but is also subject to voltage fluctuations due to generator output.
There are also plenty of aftermarket senders with 0-90 ohm range, which is close enough for a gas gauge. Now, just adjust the arm so you can tweak the end you find more valuable (full or empty) to be more accurate. Done.

This topic seems to be another one where folks with a little knowledge one way or the other have hard-over opinions - not unlike which oil to use, 6V vs 12V, protestant vs catholic, tastes-great or less-filling.

Enjoy your truck the way you want to enjoy it.
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Old 06-16-2017, 01:37 PM
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Thanks for guage lesson

I must say i learned something from this conversation. My sender was completely rotted away so i had little choice but to replace it with aftermarket. Sounds like mine will work at least enough to give me an idea once i ever get it on the road. For my 52 using a 12v battery i cannot get it to work at all, not sure if the PO ruined the guage or not but nothing seems to make it move so for now i have a heavy string with a large nut tied to the end that i use to measure the level of gas in the tank Hey it works in a pinch
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Old 06-17-2017, 10:20 PM
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Not to hijack the thread, but seems there are some people who are knowledgeable on the subject.

I used the MidFifty 12V replacement stock-looking gas gauge. I tested the new sending unit (ohms) to make sure it was working before putting it in the tank. When I turn the key to "on" the gauge went to half so I assumed it was working. However, I put another 6 gallons in the tank and it is still at half. So now I realize that it is always reading half. Any thoughts?

I thought maybe the float was stuck so I jumped up and down to see if it would help. No gauge movement. Anything else to try? I'd really like to avoid taking it all back apart. It was installed with the bed off.

Thanks!
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Old 06-17-2017, 10:35 PM
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...
I used the MidFifty 12V replacement stock-looking gas gauge. I tested the new sending unit (ohms) to make sure it was working before putting it in the tank. When I turn the key to "on" the gauge went to half so I assumed it was working. However, I put another 6 gallons in the tank and it is still at half. So now I realize that it is always reading half. Any thoughts?

I thought maybe the float was stuck so I jumped up and down to see if it would help. No gauge movement. Anything else to try? I'd really like to avoid taking it all back apart. It was installed with the bed off.

Thanks!
What tank are you using? Was the MidFifty gauge supposed to work with whatever sender and tank you have? Was the MF gauge/sender a package deal?
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Old 06-17-2017, 10:40 PM
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What tank are you using? Was the MidFifty gauge supposed to work with whatever sender and tank you have? Was the MF gauge/sender a package deal?
It is a Ranger tank. Yes, it was a generic sending unit. You had to cut the float arm to length. I cut it like the one that was replaced.

To clarify - the needle doesn't seem to go up or down. It's a constant half full (or half empty ).
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Old 06-18-2017, 04:36 PM
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It is a Ranger tank. Yes, it was a generic sending unit. You had to cut the float arm to length. I cut it like the one that was replaced.

To clarify - the needle doesn't seem to go up or down. It's a constant half full (or half empty ).
Did you check to be sure that the float arm could move through its range of motion when you put it in? It's possible he arm could be hitting an internal baffle or something else inside the tank...
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Old 06-18-2017, 10:08 PM
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Did you check to be sure that the float arm could move through its range of motion when you put it in? It's possible he arm could be hitting an internal baffle or something else inside the tank...
I'm pretty sure I checked because the first time I installed, the arm was going the opposite way and it was hitting a baffle. There was still a half tank of gas so I was only able to check upper range of motion. I tried to put a tube down in the tank to see if I could hit the float arm (thinking it may be stuck), but couldn't get anything down there.
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