Fuel Gauge Voltage

[JPP]

I am trying to get my fuel sending unit and my fuel gauge to talk to each other.  I have a B2B 6V Pos Ground. Lets start simple.  I have a NOS gauge from Vintage I want to bench test.  I have new wiring from Rhode Island wire.  I have a new tank from Tanks Inc and their Mopar fuel sending unit.  So I have wired the left side of fuel gauge to ignition switch and the right side of switch to the fuel sender in tank.  The fuel sender is grounded to the frame.   I measure 3.7 volts at the fuel sender at the tank.  The gauge is on emplty and the tank is half full.  How do I bench test the gauge, I have a 6 Volt bench power supply that I can tune to what ever voltage 0-15V.  Thanks in advance for your help and support.

[B1B Keven]

Pull one of the wires off of the fuel gauge. Read between the two posts with a multi-meter set on ohms. Should read between 25 and 45 ohms.

If it does, it's good.

Start checking for good contact between the gauge and housing, housing and dash. Sometimes paint can screw up the contact.

Check the voltage at the sending unit again. This time rock the truck to 'slosh' the gas around. The voltage reading should move around.

[Merle Coggins]

A quick check, if everything is assembled, is to use a jumper wire and momentarily connect the terminal on the sending unit to a good ground while watching the gauge. If you ground the terminal the gauge should peg out at full, hence the momentary thing. That will tell you that the gauge and wire are good. Then you need to check the sender and/or ground connection.

[B1B Keven]

45 minutes ago, Merle Coggins said:

A quick check, if everything is assembled, is to use a jumper wire and momentarily connect the terminal on the sending unit to a good ground while watching the gauge. If you ground the terminal the gauge should peg out at full, hence the momentary thing. That will tell you that the gauge and wire are good. Then you need to check the sender and/or ground connection.

Even better!

[maok]

What I would be testing/checking is the resistance values (using your multimeter set in the ohm setting) of the sender unit, as you move the sender unit, not the voltage.

Essentially, you need to match the resistance values of the sender unit to the original sender unit for that particular gauge.

[JPP]

My son, recent electrical engineer is tasked with solving this issue. Any insight and direction is respected, valued and appreciated. You guys are good!!

[Watash]

I have to jump in here.  My gauge is pegged at full after installing a new sending unit.  I think the ground is good and the lead is good.  I check continuity from the lead disconnected to frame and it was good.  I am at a loss as what to do next.

[JPP]

I got mine figured out had to get good ground and reverse gauge wires. All working splendidly. 

[Watash]

A thought just occurred to me.  Does the gauge itself have to be insulated or should it be grounded as well?

[B1B Keven]

55 minutes ago, Watash said:

A thought just occurred to me.  Does the gauge itself have to be insulated or should it be grounded as well?

Grounded to the gauge cluster and it in turn needs to be grounded to the body.

[Watash]

Okay here I go again.  I have a 52 B3B converted to 12 volt.  Coming off of a voltage resister, I have 7.25 volts going to the left terminal on the gauge (facing the back of the gauge) and the gauge ground is good.  I ran a new 16 ga wire from the right terminal of the gauge to the terminal on the sending unit.  The sending unit is new and identical to the one I removed only with a better float. Sending unit ground is good.  Voltage at the sending unit terminal is 3.7 volts.  Tank is approximately half full but the gauge stays pegged on full.

I removed one lead from the gauge and checked the resistance between terminals.  It measured 25 ohms and should be okay.  The only explanation I can come up with is the new sending unit must be bad.  Any thoughts are appreciated.

Edited September 7, 2017 by Watash

[B1B Keven]

I've seen bad 'new' sending units before.

[maok]

To test the new sender unit, pull it out of the tank, connect your ohm meter on its terminal and its case (ground). Now move the float up and down to see if the resistance value changes with the movement. It should have an approximate range of 10-100ohm, but could be less or more . The sender unit is just a simple varying resistor.

If its consistent and smooth then your problem lies else where.

Your 3.7 volts seems too low at the sender, I would have thought that there shouldn't be that much voltage drop from before and after the fuel gauge.

[Watash]

Thanks I'll give this a try tomorrow.

[Ruderhaus]

Hmmm.. I'm experiencing the same exact problem. It started a week ago. Replaced the sending unit. Still pegs the gauge to full. Everything has been working for a few years. The sending unit is working fine when tested with an ohm meter. I read 26 ohms across the gauge when isolated.

[Watash]

I read the fine print.  Mine is not for trucks but looks identical to the old one.  I think car and truck must have different ohms.  I may try a variable resister to see if I can real it in.

[johnsartain]

I thought I might include this tidbit on safety that someone gave me a long time ago. I never questioned as to how true it may be, I figured better safe than sorry. It went like this...

Fuel sending units are a varying resistor which is typically a contact sliding from point to point across a resistor coil in side the tank. Even though sparks could occur they are of on consequence as the amount of fuel vapor in the tank is too rich to cause an explosion should a spark occur. The safety part comes in when testing the sending unit. Best practice is to completely remove the unit from the tank and the tank area and to wash the fuel from the sending unit and allow to dry completely before testing. 

The unit that Tanks inc sells for early Ford and Mopar is a 73-10 ohm unit which I believe the ohms is 73 when empty (lower float position) and 10 when full (upper float position).

This page on Tanks inc. give their trouble shooting tips ( they probable apply to most other units as well)...http://www.tanksinc.com/index.cfm/page/ptype=results/category_id=159/home_id=-1/mode=cat/cat159.htm

[Watash]

Quote

Thanks for the link and comments.  One thing I noticed is the sending unit does not care about voltage.  If full is less ohms, I wonder if I am sending a reduced voltage to the unit would affect the ohms.  In other words, I will send 12 volts and see if it reacts differently.

 

[Merle Coggins]

41 minutes ago, Watash said:

Thanks for the link and comments.  One thing I noticed is the sending unit does not care about voltage.  If full is less ohms, I wonder if I am sending a reduced voltage to the unit would affect the ohms.  In other words, I will send 12 volts and see if it reacts differently.

If the needle is pegging to the full side more voltage will only make it worse. We need to look at Ohm's Law. Voltage = Amperage X Resistance (V = I X R) The gauge moves based on how much amperage flows through the electromagnet inside. More amperage = higher level reading.

If the sender is 10 Ohms at full and 70 Ohms at empty, let's do the math. 6 volts / 10 Ohms = 0.6 Amps.  6 Volts / 70 Ohms = 0.086 Amps.  12 volts / 10 Ohms = 1.2 Amps.  12 Volts / 70 Ohms = 0.19 Amps.

So, if your gauge is pegging to the full side when the tank is much less than full, you need more resistance in the circuit, or lower voltage.

Edited September 12, 2017 by Merle Coggins

[The Oil Soup]

Here is a link to build an IC voltage regulator. You don't need the capacitors or heat sink. https://www.jalopyjournal.com/forum/threads/how-to-build-a-voltage-regulator-for-3.448038/ 

This is the integrated circuit I used.

[Watash]

Thanks to all responses.