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How to test fuel gage?

Posted by jeffnoll 
jeffnoll Avatar
Jeff Noll
Western MA, USA   usa

Not having any idea how much gas I have is kind of driving me nuts, so tonight I pulled the gage. But I have no idea how this thing is supposed to work. With ignition on, both wires green/black and green light up my tester bulb. So it's getting power, but I'm not sure how it's supposed to work. Does the fuel gage controller in the tank somehow fluxuate the voltage on one of them? Any way to test further to see if its the gage or something else? Is there a way to go about and fake it into thinking there's a full tank just to see if the needle moves?
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tim65mgb Avatar
Tim Guntly
Cherry Valley, Illinois, USA   usa

If you connect the gauge and go under by the fuel tank there should be a wire that goes to the sending unit. Disconnect it at the connector and ground it to the body. Then turn the ignition switch on and see if the gauge goes to full. If it does it is likley a bad sending unit in the tank or a poor ground. If not could be bad gauge, voltage stabilizer or wire problem.

i just went through my nonworking fuel gauge problem and it turned out to be a bad wire.
jeffnoll Avatar
Jeff Noll
Western MA, USA   usa

Ok, I'm starting to get how this thing works. Grounding the two connectors on the back sends it up to F, so gage looks functional. Guess I'll crawl under and take a look at the ground tomorrow. Hopefully that is it, I really don't feel like pulling the tank to investigage more.
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MG Cruiser Avatar
Keith Delta4
Rockingham, WA, Australia   aus
1967 MG MGB "'Blue'"
1970 MG MGB "Moneypit"
1978 MG MGB GT "The Other Woman!"

Don't pull the tank! This will have nothing to do with the gauge reading.
jeffnoll Avatar
Jeff Noll
Western MA, USA   usa

In reply to a post by MG Cruiser Don't pull the tank! This will have nothing to do with the gauge reading.
But the sending unit is in the tank. If I go right to ground the gauge goes to full, so the gauge seems ok. The green wire that goes to the sending unit goes into the tank, so there isn't much else I can do without pulling it, is there?

Presumably this is all just a bad ground on the sending unit, but I can't get to that either without pulling the tank, can I? At this point there's nothing I can get to with the tank on. Though I guess I can cut the wire, add a few resistors and go to a ground and see what happens. But again, that just verifies I have to pull the tank and check out the sending unit, no?

Update. So I did a continuity check from the outside of the tank to a ground and it came ot fine. I then unplugged from the guage and did an ohm reading to a ground an it showed 250ohm.



Edited 1 time(s). Last edit at 07/09/2010 07:50PM by jeffnoll.
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7mg2 Avatar
Andrew Hardie
Calgary, Alberta, Canada   can
1969 MG MGC GT "Mr "C""
1972 MG Midget

Dropping the tank is child's play compared to almost ANY other car once it is empty. If you have to drop it just make sure you soak all the securing nuts with penetrating oil long before you try to twist a nut. The studs are a b%&*h to replace if you break one.



Andy

NAMGBR# 20-7738
AMGCRA# 1678
DaveK Avatar
Dave Kulka
Dickson, TN, USA   usa

I have a used sending unit sitting on my desk. I replaced my before doing enough investigating. My reads 250 ohms in the empty position and about 20 ohm in the full position. Can you have someone bounce the car to slosh the gas around while to check the ohms.

The gauge is nothing more than a bimetal coil with a needle attached to it. When the current heats up the coil the coil expandes and moves the needle. So if you put 12 volts to it it should read full. Don't hold the current to it too long. I assume you could burn out the coil. The voltage stabilizer is a little bimetal unit as well. As current heats this unit up it opens a contact in affecting turning off the current. As soon as the bimetal cools off it closes the contacts again. This happens about every 0.7 seconds. This blinking of current makes it difficult to read on a digital voltmeter. The volt meter will act like it's going crazy. An analogue meter will show you a better view of the on/off pulse.

My problem was the wire came off the voltage stabilizer.
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jeffnoll Avatar
Jeff Noll
Western MA, USA   usa

Ok, so I dropped the tank and I'm pretty sure its a ground problem. Sender directly to a chassis ground shows 212 ohms. Sender to the tank alligatored to a ground shows 290 ohms.

Thing is I have no idea how the sender/tank is grounded. Is it something in the sender that grounds to the tank that could be undone? I can't get the sender out because I lack the special tool and Hayne's tells me to use a series of complex levers or something along those lines, like that's a lot of help smiling smiley Anyone have any idea how to get this thing off without the special tool other than figure out how to use a bunch of makeshift levers?
Kerr Avatar
Norm Kerr
Ann Arbor, Mi, USA   usa

it is a ring that needs to be rotated a partial turn to remove, you can use a large screwdriver, or punch to tap it around with a small hammer

there is a rubber ring that is between the sender and the tank hole that you will want to replace (and maybe that metal ring as well, if it gets pretty banged up by the removal.

early senders earthed through a dedicated wire, but the later models used that ring against the body of the tank for this so shifting that ring a bit might scrape that and renew your sender's function.


Norm
jeffnoll Avatar
Jeff Noll
Western MA, USA   usa

In reply to a post by Kerr it is a ring that needs to be rotated a partial turn to remove, you can use a large screwdriver, or punch to tap it around with a small hammer

there is a rubber ring that is between the sender and the tank hole that you will want to replace (and maybe that metal ring as well, if it gets pretty banged up by the removal.

early senders earthed through a dedicated wire, but the later models used that ring against the body of the tank for this so shifting that ring a bit might scrape that and renew your sender's function.


Norm
Sounds good. I think I have two issues going on actually. Taking the tank off the sending unit is pretty caked in mud and dirt so I think maybe that can be cleaned up. But also it came to mind that the PO has replaced the fuel pump with an electric so those nice rubber hoses probably weren't doing me any favors grounding through the fuel line. But I still have those 70 ohms I can't account to being added to the sender line so I'm guessing it's a goofy ground between the unit and tank.
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