# Gas Gauge Sending Unit question

#### Johnkatgabe

Mar 21, 2022
16
I'm new here and didn't know where to go or whom to ask, or what category to put this in (my apologies). I would appreciate some help in understanding though.
I have installed a new sending unit in my gas tank(new tank as well) and new gauge in my dash. The sending unit and gauge operate (to my understanding) as follows: 0-ohms is empty, and 90-ohms is full. I filled the tank gauge reads full. I ran out of gas with about 1/8-3/16 reading on the gauge. I pulled the sending unit out and measured the ohms of the sweep. It measures 39-125ohms (empty to full). So i figure the sending unit is bad out of the box. But if that is the case, why would the gauge not have registered closer to 1/2 -3/8 of tank of fuel? The gauge has key hot on one terminal. Sending unit connection, and ground connection. Does this mean the gauge is wired in parallel with the sending unit, and that is why it shows close to empty when the ohms is 40?

#### kellys_eye

Jun 25, 2010
6,514
It could be as simple as the arm carrying the float is slightly bent.

#### Johnkatgabe

Mar 21, 2022
16
I can’t see a way to bend the float arm so that it would alter the resistance reading I’m getting.

#### Bluejets

Oct 5, 2014
6,920
Show details on everything otherwise it's just a stab in the dark that will go on forever.
For example, is the gauge a match to the sender unit?
Is the circuit fed via a regulated supply and if so, at what voltage?
Does this voltage match the gauge and sender unit?

#### Johnkatgabe

Mar 21, 2022
16
“is the gauge a match to the sender unit?” It’s supposed to be. I only know how to test that by looking at the gauge and then measuring the ohms of the sending unit at that time. Is there a better way?
“Is the circuit fed via a regulated supply and if so, at what voltage?” It’s 12 volts from the battery
“Does this voltage match the gauge and sender unit?” They were purchased for this application, so I assume yes, but I’m unsure how to check that either.

#### Bluejets

Oct 5, 2014
6,920
Gauges must be provided with a regulated supply otherwise they do not work as they are supposed to do.
It may be that the gauge kit has an internal regulator but how are we to know as you have still not provided any info in that regard.

I only know how to test that by looking at the gauge and then measuring the ohms of the sending unit at that time. Is there a better way?
Well, yes there is if they don't match.

They were purchased for this application, so I assume yes, but I’m unsure how to check that either.
Once again, supply the info.

#### Johnkatgabe

Mar 21, 2022
16
Gauges must be provided with a regulated supply otherwise they do not work as they are supposed to do.
It may be that the gauge kit has an internal regulator but how are we to know as you have still not provided any info in that regard.

Well, yes there is if they don't match.

Once again, supply the info.
The gauge is supplied 12volts directly from the fuse panel

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#### Bluejets

Oct 5, 2014
6,920
Nothing in the "pamphlet" refers to any wiring or voltage specs.
One appears to be for a fuel pump.
These setups are essentially ammeters and one has to know the meter specs as well as the sender and the applied voltage specs to make any sense of it.

Apparently some installation instructions are included so do you have a copy of that...?

This "eequus" mob seem to be well established company so did you contact them.?

Last edited:

#### Alec_t

Jul 7, 2015
3,590
Is the gauge an analogue one or a digital one?
Does it connect in any way to the ECU or BCM of the vehicle?

#### Johnkatgabe

Mar 21, 2022
16
Nothing in the "pamphlet" refers to any wiring or voltage specs.
One appears to be for a fuel pump.

This "eequus" mob seem to be well established company so did you contact them.?
I

#### Johnkatgabe

Mar 21, 2022
16
Maybe what you’re asking is what I need to figure out. I don’t know how to do that. Is there a way for me to check those things myself. The only voltage regulator I’m aware of on my 85 Jeep is in the alternator. These gauges do not go to or through an ECM.

#### Externet

Aug 24, 2009
891
(to my understanding) as follows: 0-ohms is empty, and 90-ohms is full.
What if you understood wrong ?
Did you read its manual ?
Disconnect the sender and put resistors instead to find out behavior empty-to-full. Do it twice; engine off and engine running.
The voltage regulation comment from bluejets above is also very valid. The gauge can be current driven type if not.

#### Johnkatgabe

Mar 21, 2022
16
Thank you. I will give the resistors a shot

#### Alec_t

Jul 7, 2015
3,590
Show us pictures of the sender and gauge.
Does the sender have only one wire coming from it, or two?
The shape of your gas tank will significantly affect the accuracy of the gauge reading.

#### Johnkatgabe

Mar 21, 2022
16
It has three. One to the gauge, one for the fuel pump, and a common ground

#### Bluejets

Oct 5, 2014
6,920
You have been asked several times for more information which never comes so not possible to assist any further as far as I can see.

#### Johnkatgabe

Mar 21, 2022
16
Bluejets I appreciate the reply. Obviously I’m here looking for help. If I knew those answers, maybe I wouldn’t be here. I’ve asked a few times for someone to tell me HOW to check those things, and I’ve received no help.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,719
Manuals or instructions, pictures? We've seen none of these so far.

#### Alec_t

Jul 7, 2015
3,590
Expanding on what was said in post #12, so that we can figure out what the internals of the gauge might be, can you :
1) remove the sender from the tank and temporarily disconnect it,
2) apply 12V to the gauge, between its +12 terminal (+12) and ground (G),
3) measure the voltage between the sender input (I) of the gauge and G,
4) connect a resistance of 125Ω (or similar known value) between I and G,
5) measure the voltage between I and G,
6) connect a resistance of 90Ω (or similar known value) between I and G,
5) measure the voltage between I and G.
6) connect a resistance of 40Ω (or similar known value) between I and G,
5) measure the voltage between I and G.

The sender can be used to provide the 125Ω, 90Ω and 40Ω in turn..

#### Johnkatgabe

Mar 21, 2022
16
Thank you. Yes I can do this. I’ll order some resistors and see what I get

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