Fuel Gauge and New Sending Unit - Only Registering 1/4 tank

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goat_72
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Fuel Gauge and New Sending Unit - Only Registering 1/4 tank

Postby goat_72 » Thu Oct 12, 2017 12:11 am

Apologies if this topic has been covered ... since getting my 72 Ford F100, the fuel gauge has never registered a reading. This article has been a huge help and very well written and basis of my troubleshooting below - http://www.fordification.com/tech/fuel- ... -units.htm

I tested the original sending the unit by unplugging the sending unit cable and grounding it to the cap of the sending unit aka sending no resistance and it registered a "full tank". This told me potentially the sending unit itself was shot, so I ordered a new one. Taking out the old unit the float indeed was in poor shape.

I replaced the old sending unit with a new one. I now get a reading of about quarter tank, when I estimate to have 3/4. Unfortunately, I did not test the sending unit manually with full / empty positions before installing the new one.

Using my multimeter, I can see the sending unit is pushing out about 20 ohms which is inline with a 3/4 tank of gas.
I also used the multimeter to test the power cable which I believe is coming from the instrument panel voltage regulator (IPVR) and is regulated to 5v. Testing the power cable again with my multimeter I get a reading of about 1.5 volts.

Could this suggest the instrument panel voltage regulator is perhaps faulty? How does this correlate to resistance being pushed of 20 ohms out and 1/4 tank? Does this suggest perhaps there is too much resistance for 1.5 volts being supplied or does the sending unit operate on a different voltage to that of the gauge itself and the gauge only reacts to the resistance?

Other things that I notice when I switch on lights and the like and pull a draw on the battery, the instrument lights on the dim a bit. I suspect the IPVR is supposed to keep this in balance?
'72 f100 swb
'17 audi allroad
'15 toyota highlander
wife & 2 kids

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basketcase0302
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Re: Fuel Gauge and New Sending Unit - Only Registering 1/4 tank

Postby basketcase0302 » Sun Oct 15, 2017 4:50 am

"goat_72"]Apologies if this topic has been covered ... since getting my 72 Ford F100, the fuel gauge has never registered a reading. This article has been a huge help and very well written and basis of my troubleshooting below - http://www.fordification.com/tech/fuel- ... -units.htm

I tested the original sending the unit by unplugging the sending unit cable and grounding it to the cap of the sending unit aka sending no resistance and it registered a "full tank". This told me potentially the sending unit itself was shot, so I ordered a new one. Taking out the old unit the float indeed was in poor shape.

I replaced the old sending unit with a new one. I now get a reading of about quarter tank, when I estimate to have 3/4. Unfortunately, I did not test the sending unit manually with full / empty positions before installing the new one.

Using my multimeter, I can see the sending unit is pushing out about 20 ohms which is inline with a 3/4 tank of gas.
I also used the multimeter to test the power cable which I believe is coming from the instrument panel voltage regulator (IPVR) and is regulated to 5v. Testing the power cable again with my multimeter I get a reading of about 1.5 volts.

Could this suggest the instrument panel voltage regulator is perhaps faulty? How does this correlate to resistance being pushed of 20 ohms out and 1/4 tank? Does this suggest perhaps there is too much resistance for 1.5 volts being supplied or does the sending unit operate on a different voltage to that of the gauge itself and the gauge only reacts to the resistance?

Other things that I notice when I switch on lights and the like and pull a draw on the battery, the instrument lights on the dim a bit. I suspect the IPVR is supposed to keep this in balance?


You mentioned that the float was in poor shape-yet you didn't mention anything about replacing it? And yes testing the sending unit out of the tank will tell you if the IPVR is holding voltage to the sending unit.
On the instruments lights I'd first go to the source of the power drain, (the lights) which is always a known culprit on our bump era trucks.
1) How much does your voltage drop to, (and from) when you turn the lights on?
2) Have the existing headlight bulbs been changed to halogen or any other newer style bulb?
Jeff
viewtopic.php?f=22&t=46251
SOLD-71 F-350 dually flatbed, "little 302" (some folks here call them) .030 over V-8 with a "baby"C-6, B & M "ghetto" truckshifter, Dana70/4.11 ratio, intermittent wipers, tilt steering, full LED lighting on the flat bed, and no stereo yet (this way I can hear the rattles to diagnose)! SOLD!
A lot of "oddball" Ford parts collected from working on them for 34 years now!
2008 Ford Escape 4 x 4

goat_72
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Re: Fuel Gauge and New Sending Unit - Only Registering 1/4 tank

Postby goat_72 » Mon Oct 16, 2017 12:53 am

Hi

Yes I replaced the sending unit but didn’t check full / empty positions before installing. I filled up the tank and I now get a reading of just over 1/4 tank on the gauge. When I tested the power to the sending unit with it unplugged from the sending unit that is when I saw 1.5v.

As for lights - need to double check that but I see parking lights and stuff have been updated.

As for charge with engine not running I get about 12.5v
Engine running with lights on it can be as low 11.94. Engine on no load I can get up to 14.28. But that said it’s quite variable.

Should I consider getting g the IPVR looked at as I understand this needs to control that 5v is sent to sending unit.
'72 f100 swb
'17 audi allroad
'15 toyota highlander
wife & 2 kids

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Russell J
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Re: Fuel Gauge and New Sending Unit - Only Registering 1/4 tank

Postby Russell J » Mon Oct 16, 2017 6:35 am

This troubleshooting option is simplistic and I apologize if you've already checked it, but I noticed that you still haven't mentioned whether or not you've replaced the float; you've only said you replaced the sending unit.

My point is that I have had at least 2 bad floats in the the past. They will fill part way with gasoline and therefore, hang low and not level with the surface level of the gas in the tank. This will register a low tank when it is actually full.

Remove the sending unit from the tank and remove the float from the arm. Shake the float. If you hear gas in there, replace the float. If you are confident it is empty (as it should be) check this item off the list of potential problems.

goat_72
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Re: Fuel Gauge and New Sending Unit - Only Registering 1/4 tank

Postby goat_72 » Tue Oct 17, 2017 10:51 pm

Hi Russell

Ok, I understand now ... so yes to confirm when I replaced the "sending unit" I actually replaced the entire kit, including sending unit, arm and float. I would hope with this being brand new it would be sealed tight. To that avail, I will take it the out and check the ohms with full and empty arm positions together with the integrity of the float.

I will take a look this weekend and report back. Thanks again for the tuition ...
'72 f100 swb
'17 audi allroad
'15 toyota highlander
wife & 2 kids


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