Fuel Gauge Troubleshooting

1941 - 1945, MB, GPW Technical questions and discussions, regarding anything related to the WWII jeep.
Wolfman
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Re: Fuel Gauge Troubleshooting

Post by Wolfman » Fri Jun 14, 2013 11:44 am

Ok. This is getting confusing. I went back and read all the posts.
Sean, I agree. On a properly wired gauge, either style gauge, single or balanced coil, should read full with the sending unit terminal on the gauge grounded.
If a balanced coil gauge, which must have a good ground to work, were wired backward. Power to sending unit post. Sending unit post to ignition. Looks to me like, with the ign. post open. No ground. The current would go through both coils to ground through the gauge housing. Basicly( disregarding coil resistance) equal magnetic pull to both coils and the gauge would read somewhere around half full.
Ground the ignition post. Electricity takes the easiest path to ground. The current would go from the sending unit post. "Remember this is if wired backwards." Through the full coil but go to ground through the grounded ignition post. Bypassing the empty coil and the needle would read full.
On a balanced coil gauge. The empty coil is a fixed resistance. never changes it's magnetic pull on the needle to empty. The full side coil is attached to a variable resistance. The sending unit. So, the magnetic pull the full side coil applies to the needle changes as the resistance in the sending unit changes. When the sending unit is in the full position, resistance through the sending unit is low. Current through the full coil is high and the full coil generates a high magnetism, pulling the needle to full, against the pull of the empty side, fixed resistance coil. As the sending unit float drops, the resistance through sending unit increases, decreasing the magnetic pull of the full coil so the fixed, empty side coil begins to take over and the needle moves toward empty.
The only difference in a single coil gauge, the empty coil is replaced by a spring that pulls the needle toward empty. otherwise it works the same.
On either type gauge, properly connected, a grounded gauge terminal, at the gauge, will make the gauge read full. If not, there is a problem with the gauge.
If the last test result proves positive but the gauge does not work when the sending unit wire is connected, ground the sending unit wire at the sending unit. Gauge reads full. Wire is OK. Gauge reads empty , there is a problem with the wire.
If grounding the sending unit gives a full reading but still the gauge won't work with the sending unit only, either the sending unit is bad or the sending unit has a bad ground.
With either type gauge the sending unit and gauge must be matched.
On a single coil unit, the gauge does not have to be grounded to work.
Mike Wolford
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Sean Collins
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Re: Fuel Gauge Troubleshooting

Post by Sean Collins » Fri Jun 14, 2013 4:21 pm

Mike:
If a balanced coil gauge, which must have a good ground to work, were wired backward. Power to sending unit post. Sending unit post to ignition. Looks to me like, with the ign. post open. No ground. The current would go through both coils to ground through the gauge housing. Basicly( disregarding coil resistance) equal magnetic pull to both coils and the gauge would read somewhere around half full.
I hadn't considered that scenario. My first thought was to agree with you, but an other characteristic I had discovered about these gauges made me think again, so I ran some tests:

With 2 original gauges and 1 NOS gauge (all Autolite), connected as in your scenario (+6v to sender terminal, gauge case grounded, v-in terminal disconnected): all 3 gauges remain on "empty"

What I believe is happening in this scenario: the combined series resistance of the 2 coils cuts the current flow (and magnetic field strength) to less than half of what the "full" coil would normally have, and slightly more than half of what the "empty" coil would normally have.

So the "full" field strength is no longer sufficient to overcome the combination of "empty" field strength + gravity pulled "return-to-empty" counterweight, and the needle stays at empty.
Ground the ignition post. Electricity takes the easiest path to ground. The current would go from the sending unit post. "Remember this is if wired backwards." Through the full coil but go to ground through the grounded ignition post. Bypassing the empty coil and the needle would read full.
Agree completely with this, and the remainder of your descriptions.

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Re: Fuel Gauge Troubleshooting

Post by Farrell Fox » Fri Jun 14, 2013 6:53 pm

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Last edited by Farrell Fox on Wed Oct 16, 2019 12:24 pm, edited 1 time in total.
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Re: Fuel Gauge Troubleshooting

Post by Wolfman » Sat Jun 15, 2013 5:32 am

You hit on an interesting point Sean.
Connecting the gauge backward has the two coils in series and the polarity of the coils are different.
Normally, the gauge power would be applied to the ign. terminal. This would have both coils connected in parallel to ground.
With the gauge connected backward. Power to the sending unit terminal and the ign. terminal open, the two coils are in series.
Sending unit post being positive instead of grounded. This reverses the polarity of the full side coil.
The empty side coil would still have the same polarity so, looks to me like in this scenerio, one coil would cancel out the other coil. The needle is weighted to empty so the gauge reads empty.
What do you think ??
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Re: Fuel Gauge Troubleshooting

Post by Sean Collins » Sat Jun 15, 2013 8:01 am

Mike:
Wolfman wrote:You hit on an interesting point Sean.
Connecting the gauge backward has the two coils in series and the polarity of the coils are different.
Bingo, you're right! I forgot all about coil polarity: full side is reversed, so they do effectively cancel.

Bottom line, you can't use polarity to determine which post is which, since the gauge will exhibit the same behavior either way.

I still think Rodney has it backwards.

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Re: Fuel Gauge Troubleshooting

Post by Chuck Lutz » Sat Jun 15, 2013 8:36 am

Which Post Is Which...???

When MOUNTED in the dash:
Autolite = the post on the pass side is where the feed from the switched side of the Ignition Switch (via the circuit breaker) attaches
Autolite = the post on the drivers side is where the sender wire attaches

Stewart Warner = the CENTER post is where the feed from the switched side of the Ignition Switch (via the circuit breaker attaches
Stewart Warner = the TOP post is where the sender wire attaches
Paint Can FUEL Rear.jpg
Paint Can FUEL Rear.jpg (107.97 KiB) Viewed 2656 times
The Autolite gauge has a SPRING on the bracket that is used to provide a GROUND to the gauge case, but the bracket arms must not be rusted/corroded/dirty and the place on the back of the dash must also be clean enough to provide a good GROUND. Internally the guts have another spring that conducts a GROUND from the inside rear of the case to the mechanism. All these can be a cause for insuffecient GROUND.

The Stewart Warner gets a good GROUND from the case-to-dash-opening and the Autolite can also do this if it is not painted!

As noted, if the GROUND strap on the fuel line behind the air cleaner is missing and you have a RUBBER hose between it and the fuel line on the motor, you may not have a duffecient GROUND to the tank and the SENDER.

Guys have gone so far as to run a separate wire to the SENDER mounting screws in some instances in an effort to get a sufficient GROUND for the tank.

There are a few problems to eliminate before purchasing a new Gauge or Sender or matched set...

1) Improperly wired gauge
2) Insufficient GROUND
3) Miss-matched Gauge & Sender
4) Bad Sender
5) Bad Gauge
6) Wiring shorted/broken/loose
7) Bad Circuit Breaker
8) Bracket shorted to post
Chuck Lutz

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Re: Fuel Gauge Troubleshooting

Post by thidisbogus » Sun Jun 16, 2013 9:31 pm

Hi guys,

The site has been up and down with maintenance so this is the first chance I have had to update. I bought the gauge as a repro from Ron. Not sure what that means in relation to Autolite and Stewart Warner. I also bought the sending unit from Ron as a repro.

"Guys have gone so far as to run a separate wire to the SENDER mounting screws in some instances in an effort to get a sufficient GROUND for the tank."---I am one of those guys.

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Re: Fuel Gauge Troubleshooting

Post by Wolfman » Mon Jun 17, 2013 6:13 am

I have no idea what type gauge Ron sells or who made them but if both the sending unit and gauge came from him, I would think they would be matched.
Might be time for a bench test. Pull the sending unit and gauge out and wire them to a battery on the work bench.
Wired the same as in the jeep. Battery + to gauge ign. post and a wire from tank post to the sending unit only add a ground wire from the sending unit body to the gauge body and on to battery - post.
Work the float up and down and watch for gauge movement. If it works, the problem is in the jeep. Bad ground, sending unit wire or battery power to the gauge.
If the gauge does not work, the problem is with the gauge or sending unit. Short the sending unit post on the gauge to ground. The gauge needle should jump to full. yes. The gauge is ok, the sending unit has a problem. no. the gauge has a problem.
You can check the sending unit with an Ohm meter set to R. One probe to the unit body and one probe to the wire stud. The ohm meter needle should sweep back and forth as the float is moved up and down. If it does not, the sending unit is bad.
Also, while the gauge is out of the dash, determine which wire is which in the dash harness and which post is which on the gauge and that you don't have the wires from the harness connected backward on the gauge, as Sean suggests. 8)
Mike Wolford
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Comm./Inst. SEL
AOPA ( 46 yrs)
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Re: Fuel Gauge Troubleshooting

Post by Its me » Tue May 13, 2014 2:35 am

Hi All,

Hope you dont mind me bringing up a old thread, when i was out in the jeep over the weekend i filled the jeep up with fuel, and while driving around i thought she was doing to good on fuel, so needless to say i checked and there was about half a tank left, but gauge reading full. when i turn the ignition of the fuel gauge goes back to empty. I havnt had any time to check as of yet but has anyone had the same problem and can point me in the rite direction?

many thanks
Kyle

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Re: Fuel Gauge Troubleshooting

Post by Joe Gopan » Tue May 13, 2014 4:58 am

Many GI's have found that it is more reliable just to take a look into the Jeep fuel tank.
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Re: Fuel Gauge Troubleshooting

Post by Its me » Tue May 13, 2014 5:09 am

This is very true.....couldnt agree more lol.

kyle

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Re: Fuel Gauge Troubleshooting

Post by Sean Collins » Tue May 13, 2014 6:26 am

Its Me wrote:... there was about half a tank left, but gauge reading full. when i turn the ignition of the fuel gauge goes back to empty ...
Those are symptoms of:
  1. poor/missing gauge case ground, or ...
  2. sender stuck on full
Sean

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Re: Fuel Gauge Troubleshooting

Post by lucakiki » Wed May 14, 2014 2:45 am

Its Me wrote:Hi All,

Hope you dont mind me bringing up a old thread, when i was out in the jeep over the weekend i filled the jeep up with fuel, and while driving around i thought she was doing to good on fuel, so needless to say i checked and there was about half a tank left, but gauge reading full. when i turn the ignition of the fuel gauge goes back to empty. I haven't had any time to check as of yet but has anyone had the same problem and can point me in the rite direction?

many thanks
Kyle
Hope you can manage to troubleshoot your fuel gauge. They are made to work properly, and they should work. Even on an old Army Motors ( MVPA kind) there was a nice article on how to troubleshoot or repair a gauge and the sending unit. Obviously, just saying that they are unreliable is not going to help anybody.

viewtopic.php?f=4&t=118652 : even if you only need to check grounding and connection,and not to completely overhaul your gauge, this sticky post shows you how a gauge works, which is good to know.
Luca

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Re: Fuel Gauge Troubleshooting

Post by Division Leclerc » Fri Feb 14, 2020 12:17 am

hello gentlemen

this post - for sure a bit distant i n time ! - is however so valuable !

It will be very helpful to wire my SW fuel gauge (reduced from 12V to 6V withe a reducer)
Bertrand
GPW 01/19/1945
Restoration Thread : viewtopic.php?f=4&t=284126

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Re: Fuel Gauge Troubleshooting

Post by thidisbogus » Fri Feb 14, 2020 7:29 am

Very thankful it is a help. One of the most gratifying things about documenting a restoration is returning the help to others that they provided to you.


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