Strange Fuel Gauge Readings

[1949 Wraith]

I am working on my recent '42 Fargo. The previous owner stated that the fuel gauge reads empty when full and raises to full as you use fuel. OK I figure an easy fix just swap the wires around.

 

Well I drain the tank figuring to get the old gas and as much crud out of the tank as I can. I'm filling up the tank with about 4 gallons and check the gauge, she reads about a quarter full, great. Add another 4 gallons and she reads around a half tank. Add another 4 gallons and it has dropped back to around the quarter full again. I am still doing brake work so not driving yet but i suspect if I fill the tank It will read empty like the PO stated.

 

I would imagine if the float can go to half a tank when it is half full, it should at least stay at a half when the tank is full. Wiring should be correct for it to work correctly up to half a tank. Leaving connections that might be poor or mis-calibrated fuel gauge?

 

Has anyone come across this issue before with thermal fuel gauges?

 

Regards

[Merle Coggins]

Sounds like a faulty sender to me. The fuel tank sender is a variable resistance device. When the tank if full, and the float is high, the resistance is very low which causes the magnets in the gauge to pull the needle towards the "F" mark. When the tank is empty, and the float is at the bottom, the resistance is higher which weakens the magnetic pull in the gauge and the needle moves towards the "E" mark. 

By your description the gauge seems to work at first, then starts going backwards. This tells me that as the float goes higher there comes a point where the resistance starts going up again instead of continuing to drop. You may need to pull the fuel tank sender and inspect it further. It can be tested out of the tank with an Ohm meter. 

[glasspacks]

I agree with Merle, pull the sender and inspect the resistance wire where the slider moves along over the winding. A multi meter on the ohms scale should tell you if the sender is operating normally. Good luck.

Glasspacks.

[1949 Wraith]

2 hours ago, Merle Coggins said:

Sounds like a faulty sender to me. The fuel tank sender is a variable resistance device. When the tank if full, and the float is high, the resistance is very low which causes the magnets in the gauge to pull the needle towards the "F" mark. When the tank is empty, and the float is at the bottom, the resistance is higher which weakens the magnetic pull in the gauge and the needle moves towards the "E" mark. 

By your description the gauge seems to work at first, then starts going backwards. This tells me that as the float goes higher there comes a point where the resistance starts going up again instead of continuing to drop. You may need to pull the fuel tank sender and inspect it further. It can be tested out of the tank with an Ohm meter. 

Thanks Merle I will check it out

[dmartin_egroup]

I note several are suggestion using a multi-meter.  Do any of you have a meter that will give accurate readings below say 200 ohms?  A fuel sender is in pretty low ohms range.  Just saying....

[Merle Coggins]

If your meter won’t read below 200 Ohms it’s time to upgrade to a new meter

[50mech]

Being a thermal guage there are no magnets in the guage. 

It's a heater wrapped around a bimetal strip.

It also sounds like your sender is working correctly since you get a response through the whole range.

Sender failure in these usually means anything above a certain point is just empty..not a gradual drop back down.

 

I'm not positive about the Fargo guage but I know on Willy's and several others there is an adjustment for the empty to half range and then the half to full range.

The half to full adjuster is a flat spring that rests on sort of a 90 deg drop off plate.

If that adjuster is too high it stops it halfway and then  the further curving of the strip sends the needle back instead of forward.

Lowering it to the right point means it kicks under at half and sort of flops the strip over.

 

If it's this design there will be two small holes at the gauge rear. The one on the high side is for high. You can just see teeth through them. A little precision driver can be used to slide the adjustment, in this case you would need to go toward the center. Power off, a bit at a time each try.

 

Unfortunately each adjustment also affects the other so dropping the high range will mean youll end end up needing to drop low also. Finnicky.

Edited August 15, 2020 by 50mech

[50mech]

Also the adjustment swings on a rivet, if it's out now it may not be able to hold itself tight. If it goes back out you can pull the gauge apart and replace it with a screw and washer.

Edited August 15, 2020 by 50mech

[Merle Coggins]

50mech,

The truck fuel gauges are not thermal devices. They are magnetic devices. Here is an image from the service manual showing the dual coils. One constant coil that pulls to “E” and a variable coil, that connects to the sender, which will pull to “F”.  As the resistance changes in the sender it alters the magnetic strength of the variable coil and the needle will balance between the two. 

[50mech]

I know how EM gauges work.

The OP stated a thermal gauge.

 

Here they are in the truck manual

 

Edited August 15, 2020 by 50mech

[50mech]

Although looking at the mechanical diagram it appears to be set up slightly different and may or may not have the adjusters.

The high side heater may just be burnt out.

I would run the meter between post 1 and ign post on the gauge to see if there is any continuity there.

Or more simply check for voltat T both sender terminals to ground.

 

 

Edited August 15, 2020 by 50mech

[Merle Coggins]

I didn’t notice the thermal gauge comment from the OP. However, B-series trucks don’t use that type of gauge so maybe that’s why I over looked it. The image I posted comes right from the B-Series Shop manual. 

[B1B Keven]

Pre '48 fuel gauges need to have a load put on the two lower posts and power to the top.

Vic has a piece of test equipment set up to adjust these fuel gauges.

They aren't like the B series gauges.

 

[Merle Coggins]

Hmm...  I guess I skipped over the model year part too. Feel free to disregard my comments. 

[MBF]

I’d remove the gauge, and then with an analog ohm meter (not digital) move the sending unit arm through its full range of motion slowly and look for a smooth sweep of the needle on the meter.  If it jumps, or drops that would confirm a bad spot in the rheostat on the sending unit.

[Jerry Roberts]

It has been my experience on the 1939 - 1947 Dodge trucks that both the tank sending units and the dash gauges are very prone to failure . The new replacement sending units only work for about 1 1/2 years and even the NOS dash gauges often need their points to be cleaned before they will even work . These points are very fragile . There is a new style replacement sender on the market with the float that swings up from the bottom in stead of swinging down from the top . You might have better luck with that model than I did . I fought with mine for a very long time and ended up throwing the  #*!  thing away . 

[bkahler]

On 8/15/2020 at 7:52 AM, Merle Coggins said:

50mech,

The truck fuel gauges are not thermal devices. They are magnetic devices. Here is an image from the service manual showing the dual coils. One constant coil that pulls to “E” and a variable coil, that connects to the sender, which will pull to “F”.  As the resistance changes in the sender it alters the magnetic strength of the variable coil and the needle will balance between the two. 

 

Merle, I realize this is somewhat of an old post but it sort of hits home for a problem I'm having.

 

My B3 Series service manual published in 51 does not have any info on the fuel gauge.  It covers the horn, lights, ignition, etc. but does not cover the fuel gauge.  

 

Does your service manual have any details on how to adjust the fuel gauge for proper readings?

 

Thanks,

Brad

 

 

 

[Merle Coggins]

[bkahler]

Merle, thanks for posting the page out of the manual.  It would appear there is no adjustment of the gauge.  So things either work or they don't

 

Thanks again.

 

Brad