Gas gauge woes

1941 - 1945, MB, GPW Technical questions and discussions, regarding anything related to the WWII jeep.
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JAB
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Re: Gas gauge woes

Post by JAB » Fri Jun 24, 2022 6:33 pm

Grounding the sending unit terminal will make it read full. That's what it does. The reostat (or what ever the proper name is) inside the sending unit grounds that terminal through a regulated means (winding of high resistance wire around a non-conductive card) based on where the float arm puts it. Full gauge is fully grounded, or nearly so. What you've done will make it read full all the time, regardless of the fuel level, because your new wire bypasses the reostat. Something else is going on.
-Jeff

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Raflad
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Re: Gas gauge woes

Post by Raflad » Fri Jun 24, 2022 8:48 pm

Just curious as to what fuel gauge you have, Stewart Warner or Autolite? If it's an AC gauge as used on CCKW's it might not be compatible with the jeep sender.
Adam Smith
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Re: Gas gauge woes

Post by raymakr » Fri Jun 24, 2022 9:11 pm

I have two new 12 volt fuel gauges for my GPW Jeep. First one is a USA made unit that immediately pegged full and fogged up. Probably my fault for sanding the back edge. Its needle moved from empty to full doing a sweep hooked up to sending unit outside the tank so I thought the two were compatible.
The second non USA made gauge also seemed to read find doing a sweep outside of the tank. Reads half full with sending unit installed in tank.
These gauges are reproduction gauges and non original. So I have no idea who makes them. Ron apparently no longer stocks them so thinking about ordering a new sending unit and start there.
I might need to switch the existing wiring out for new and maybe even the circuit breaker that feeds power. Kind of at a loss right now. So open to suggestions. Thanks.

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Re: Gas gauge woes

Post by Raflad » Fri Jun 24, 2022 9:30 pm

Sorry, missed the point that your jeep is 12 volt.
Adam Smith
GPW 80535, 11-19-42
MBT 2470, 8-42, USA #0160905

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Re: Gas gauge woes

Post by borgy76 » Fri Jan 06, 2023 3:42 pm

After a lot of fault finding I have finally managed to get the charging circuit to work correctly. So I moved on to the next problem. The fuel gauge and sending unit. I am running a 12V system in my MB. I did not convert the jeep to 12V, the previous owner has done this. Attached to the fuel gauge was ( I removed it ) is a reducing unit that converts 12V to 6V. I rewired the fuel gauge without the unit, and it read E. I took the jeep for a drive and the fuel gauge came to life. The thing is as dove the gave moved with the amp metre. The faster I went the closer to F it got.

So my question is. Will I need to attach the reducer to make the gauge function correctly? Or is it something else?

I have checked the resistance on the gauge and sender, both are correct. I have run the extra earth wire from the sender to the footman loop.
Joe Borg
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Re: Gas gauge woes

Post by JAB » Fri Jan 06, 2023 5:34 pm

I can't answer all of your questions, but if the jeep was converted to 12 volts and the gauge is the stock 6 volt gauge I would think you should reinstall the resistor so that you don't damage the gauge.
-Jeff

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Re: Gas gauge woes

Post by borgy76 » Sat Jan 07, 2023 11:13 am

Thanks, I now known what Im doing on my days off work this week.
Joe Borg
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Re: Gas gauge woes

Post by Wolfman » Sun Jan 08, 2023 5:55 am

Without going into a lot of detail, an original balanced fuel gauge will not be much effected by voltage increase.
The OEM fuel gauge has two coils working against each other to move the needle and an increase in voltage is applied to both coils in the gauge so the pull on the needle changes equally. Not much change in reading with voltage change.
On the other hand, an aftermarket (cheap) single coil gauge has one coil pulling the needle one direction against a spring that pulls on the needle in the other direction.
An increase in operating voltage on this type of gauge will give you a big difference in gauge reading.
Been running 12 volts on my 6 volt CJ fuel gauge for a long time with no ill effects.
One more thought. Anytime I find something weird with an automotive electrical circuit, it usually turns out to be a ground problem.
Mike Wolford
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Re: Gas gauge woes

Post by borgy76 » Sun Jan 08, 2023 6:44 pm

I have reconnected the resistor and all is working well. Still having trouble though. So the situation is, I have the wire running from the gauge unit to the sending unit. Turn the key and I have 6.01V at the end of the wire that goes to the sending unit. Connect the wire to the sending unit and I get 3.66V. I have removed the sending unit from the tank, wired it up and it works correctly. I move the float arm and it corresponds with the needle movement on the gauge. When the sending unit is on the tank I get the voltage drop. I have an earth from the footman to the unit as stated previously. I run a 10amp wire from the negative battery terminal to the sending unit, still the same problem. I removed the gasket from under the sending unit, still the same problem. Remove the sending unit from the tank and it works perfectly.

Can someone please explain how I am getting the voltage drop when hooked up to the fuel tank? What steps can I take to rectify this issue other than getting a new sending unit. As explained the unit works when its not inserted into the fuel tank. Any advice would be great before I go nuts.

Cheers
Joe Borg
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Re: Gas gauge woes

Post by YLG80 » Sun Jan 08, 2023 10:50 pm

borgy76 wrote:
Sun Jan 08, 2023 6:44 pm
I have reconnected the resistor and all is working well. Still having trouble though. So the situation is, I have the wire running from the gauge unit to the sending unit. Turn the key and I have 6.01V at the end of the wire that goes to the sending unit. Connect the wire to the sending unit and I get 3.66V.
Hello,
With the resistor added by the previous owner after the 12V conversion, you are just inserting an additional resistor in series with the sending unit which is basically a rheostat.
Not sure that the previous owner has inserted a simple resistor?
Isn’t it a voltage step down circuit or regulator like the LM7806?
Does it have 3 contacts? One input, one output and the ground.
If it was a simple resistor, you should measure 12V in open circuit, i.e with the sending unit wire disconnected.
Here below you will find the wiring diagram. I have drawn a resistor in red but it could be a voltage regulator?

The point marked A is the point where you measure 6V in open circuit and 3.66V with the sending unit connected.
If the voltage reduction is made by an electronic circuit, your measurement shows that this circuit is faulty. It becomes likely very hot.
A voltage regulator with the appropriate power should provide the sender with 6V, not 3V as you have measured.

Yves

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JAB
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Re: Gas gauge woes

Post by JAB » Mon Jan 09, 2023 4:34 am

I had a similar issue with an original sending unit. I finally gave up and replaced it. I was guessing that when I moved it by hand the parts were moving smoothly but when it floated on the gasoline the arm was torqued by the fact that the cork bobber is offset causing it to twist enough that the 80 year old parts just didn’t connect as well as they should. I’m not one to give up on original parts easily either but it was so frustrating….
-Jeff

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Re: Gas gauge woes

Post by Wolfman » Mon Jan 09, 2023 4:51 am

The sending unit is an aftermarket unit installed by the previous owner ???
It has been a problem for a long time that the resistance value range of aftermarket sending units was different from the original sending unit. The new replacement sending unit makes the gauge read wrong.
The added resistor in Yves schematic was one way to correct the problem.
This correction would work, no matter if the unit was mounted or free.
Still not sure where the voltage regulator or a resistor is installed on your installation. ???
A voltage regulator would be between the power source and the "power in" terminal on the gauge.
The resistor Yves shows is between the gauge and the sending unit.
The fact that the gauge reads correctly when the sending unit is connected in free space but does not work when clamped into position on the tank, with or without a ground jumper, has me thinking something internally in the sending unit is screwing up from the clamping pressure on the unit from the mounting screw pressure.
Try using screws with nuts on them, in the sending unit mounting holes, to simulate the unit being mounted while the sending unit is not mounted. See what happens ??
???
Posible the float his out of alignment. Hitting the side of the tank or center baffle when the sending unit is mounted ???
I think was mentioned earlier.
:D Hi Jeff.
Mike Wolford
CJ-2A
VEP GPW
Comm./Inst. SEL
AOPA ( 50 yrs)
EAA ( 49 yrs)
4th Inf. Div. - 5th Inf. Div. - 2nd Armor Div. - CIB

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Re: Gas gauge woes

Post by dinof » Mon Jan 09, 2023 5:40 am

My gauge issues went on for 20 years of living with bad readings. Ron was working on a source
for a kit that was to be a gauge and the sensor. They were going to be a matched set. It
appears they let him down and never came through.

In trying 2 sensors, I replaced the gauge. It was from Brent MUllins. It all works now.
It was the gauge all along that was inaccurate.
Dino Falabrino
On the "G" since 1998.
1943 GPW 102310 DOD 3-3-43
1928 Model A Roadster Pickup
1930 Model A Tudor
1968 Taco Minibike

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Re: Gas gauge woes

Post by borgy76 » Thu Feb 09, 2023 1:04 pm

Thanks for the invite people. The missus came through with the goods and bought me a new sending unit as a surprise. It seems whinging and repeating "it must be the sending unit" worked. I now have a working gauge of sorts. It works but is not accurate, I can live with that :D
Joe Borg
Mackay
Queensland

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