At my wits end, good ole fuel gauge troubles

[FlamesAreWicked]

The short and skinny of it. 1968 Dodge Dart

Sending unit is giving me a reading of 66 ohms. It’s a new sending unit. There’s about 4-5 gal in the tank. This reading was using my multi meter on the stud of the sending unit and the metal line that comes from upstream on the car. I applied ground briefly to the plug that goes on the sending unit stud using a power probe and the gauge in the cluster does rise as it should. With everything connected as it should, I added a supplemental ground with alligator leads from the metal line coming out of the sending unit to the metal line from the car. My gauge doesn’t budge off empty

I have a new solid state limiter in the cluster along with new circuit boards. I’m getting 5v at the plug when checking with my power probe.

At this point, only thing I can think of is maybe the plug isn’t getting a good connection to the stud. When I checked it with the power probe, since the end is pointy it’s touching at the very deepest point of the plug where the stud isn’t. I dunno, I’m just spit balling at this point.

The brand of sending unit is The Right Stuff and has a range of 6-78 ohms. Accuracy isn’t the goal.. but to have some function going on would be great.

 

[Dana67Dart]

With 4-5 gallons in the tank I would not expect much more than that from any aftermarket sender.

The curve (or lack there of) on these senders is full for 3-4 gallons, 1/2 for about 8 gallons then empty for the remaining 3-4 gallons.

Look at the graph in post 4.

Product Review: A100 Fuel Sender For 67 Dart

 

[FlamesAreWicked]

With 4-5 gallons in the tank I would not expect much more than that from any aftermarket sender.

The curve (or lack there of) on these senders is full for 3-4 gallons, 1/2 for about 8 gallons then empty for the remaining 3-4 gallons.

Look at the graph in post 4.

Product Review: A100 Fuel Sender For 67 Dart

I did see the the graph in another thread trying to read up on tips that I may have missed. Guess I can dump another 5 in and see what happens. When I replaced the tank and sending unit, all I had was a 5 gal can to pour in and I did take one little trip in it to check another thing I was working on.

 

[MadScientistMat]

Gauges read pretty close to empty at 66 ohms. I did a video of testing two '66 gauges with a set of resistors:

 

[FlamesAreWicked]

I poured some more fuel in. My resistance reading on the sending unit stud is a little bit over 50 ohms. Gauge is still showing empty.. passed empty even.

 

[Dana67Dart]

My resistance reading on the sending unit stud is a little bit over 50 ohms. Gauge is still showing empty.. passed empty even

But if you short the connector at the sender the meter moves?

 

[Dana67Dart]

If the nut is still on the connector may not be touching the post.

You could clip a lead from the post to the inside of the connector for testing.

Btw at 10 gallons if the sender was OEM it would be reading close to 23 ohms that it's reading 50 is also very close to empty on the gauge.

 

[Dana67Dart]

Your first reading with 5 gallons in the tank you got 60 ohms that's the equivalent to empty

50 ohms is like 3 gallons but you have 10 gallons in the tank.

You are going to need to get 13-15 gallons in (3/4 tank of gas) for it to read 1/2 tank

 

[melcardona]

With 4-5 gallons in the tank I would not expect much more than that from any aftermarket sender.

The curve (or lack there of) on these senders is full for 3-4 gallons, 1/2 for about 8 gallons then empty for the remaining 3-4 gallons.

Look at the graph in post 4.

Product Review: A100 Fuel Sender For 67 Dart

just inst a new one...6 gal said empty...but full works fine...like you said as long as it sorta works!

 

[FlamesAreWicked]

But if you short the connector at the sender the meter moves?

Yep applying ground via the connector makes the gauge move

 

[FlamesAreWicked]

just inst a new one...6 gal said empty...but full works fine...like you said as long as it sorta works!

It is a new sending unit

 

[Dana67Dart]

Yep applying ground via the connector makes the gauge move

I would try the jumper I mentioned from the sender to the connector

Post some photos of the inside of the connector and the post on the sender.

 

[FlamesAreWicked]

If the nut is still on the connector may not be touching the post.

View attachment 1716493463

You could clip a lead from the post to the inside of the connector for testing.

Btw at 10 gallons if the sender was OEM it would be reading close to 23 ohms that it's reading 50 is also very close to empty on the gauge.

I did trim a little off the plug because I thought maybe the stud wasn’t going inside the plug enough..it’s bottoming out on the base of the stud, or so it appeared. It’s going on fully. It does have a flat at the top of the stud as the one with the arrow pointed to it.. it doesn’t have that little extra nub sticking out beyond the threads.. since the power probe has a pointy tip, it made me think that maybe the stud isn’t getting full contact.. just spitballing at this point

 

[FlamesAreWicked]

I would try the jumper I mentioned from the sender to the connector

Post some photos of the inside of the connector and the post on the sender.

I’ll get some photos soon but the sending unit stud looks just like the one above with the arrow pointed to it.. the plug is the orig rubber 90° slip on connector. There isn’t a nut on the stud.. tempted to cut the end off and put an eyelet on it with a nut holding it on.

 

[ProjectBazza]

I did trim a little off the plug because I thought maybe the stud wasn’t going inside the plug enough..it’s bottoming out on the base of the stud, or so it appeared. It’s going on fully. It does have a flat at the top of the stud as the one with the arrow pointed to it.. it doesn’t have that little extra nub sticking out beyond the threads.. since the power probe has a pointy tip, it made me think that maybe the stud isn’t getting full contact.. just spitballing at this point

I may have missed this (you may have done this already), but I ran into this when I messed around with the sending unit in Bazza's VG (an Oz-built Dart, if you will), and one thing that helped me was testing the circuit with a potentiometer (100 ohm, if memory serves). In other words, I put a pot in place of the sending unit, turned it until the gauge was on empty, recorded the value of the pot, then went to half tank, did the same thing, and then full. Turned out the sending unit was ok, and the problem was somewhere inside the wiring harness, under the dash. Been too long to remember exactly what it was though, sorry.

 

[RustyRatRod]

REMOVE the sending unit from the tank. Use a jumper wire to ground the sending unit to a good, clean ground on the car. Plug the sending unit wire from the gauge into the sending unit. With the ignition on, move the sender float from empty to full and see what the gauge does. Might need two people.

Keep this in mind. NEW and GOOD are in different places in the dictionary because they don't mean the same thing.

 

[Dana67Dart]

the plug is the orig rubber 90° slip on connector

Before you do that let's make sure the connector is ok.

Another thing that can happen is the wire inside the connector can break. And in some positions it is fully attached and others it's not.

So any testing you should manipulate the connector and look for reading changes while moving it.

To me the issue is at the connector OR you just don't have enough fuel in the tank to make the needle move. There is additional resistance on the wire that adds to the sender compounding the issue

 

[FlamesAreWicked]

Before you do that let's make sure the connector is ok.

Another thing that can happen is the wire inside the connector can break. And in some positions it is fully attached and others it's not.

So any testing you should manipulate the connector and look for reading changes while moving it.

To me the issue is at the connector OR you just don't have enough fuel in the tank to make the needle move. There is additional resistance on the wire that adds to the sender compounding the issue

I’m thinking the same thing.. my next break from work, ill see if i can get someone to watch the gauge to see if playing around with the wire while plugged in makes anything happen.. its vexing lol. While accuracy isn’t the ultimate goal.. knowing before I run out would be nice. Since that portion of the wire is exposed to the elements, no telling what’s going on inside the sheathing. Thankfully, the wiring in the car is in fairly good shape overall. No signs of it ever catching fire in the usual places like the firewall block.

I did double check that the incoming hard line is giving me continuity for ground since the underside is undercoated.

But yep, either I need more fuel in the tank or there’s something going on with the plug/wire. Just happenstance that when I checked voltage and applying ground to test the gauge, it worked.

 

[FlamesAreWicked]

Before you do that let's make sure the connector is ok.

Another thing that can happen is the wire inside the connector can break. And in some positions it is fully attached and others it's not.

So any testing you should manipulate the connector and look for reading changes while moving it.

To me the issue is at the connector OR you just don't have enough fuel in the tank to make the needle move. There is additional resistance on the wire that adds to the sender compounding the issue

It’s definitely a thought.. I’m soon gonna be pulling the motor and I was hoping to have this sorted out prior but.. may just take a break if after my next couple of attempts result in failure. Wait till after the engine swap and revisit this rubix cube

 

[FlamesAreWicked]

Before you do that let's make sure the connector is ok.

Another thing that can happen is the wire inside the connector can break. And in some positions it is fully attached and others it's not.

So any testing you should manipulate the connector and look for reading changes while moving it.

To me the issue is at the connector OR you just don't have enough fuel in the tank to make the needle move. There is additional resistance on the wire that adds to the sender compounding the issue

Makes sense though that if I’m getting 50ish ohms of resistance at the sending unit plus more resistance in the wire connection from internal corrosion or whatever… the added resistance would mimic the resistance reading of an empty tank… we shall see soon hopefully.

 

[cosgig]

Sooo, did you ground the sending unit wire to make the gauge go full like was suggested above by Dana? First things first, make sure the gauge even works before you go doing other things.

 

[MadScientistMat]

Grounding the gauge will at least show that it works, but my gauge passed the grounding test and flunked testing with resistors. I just bought that pack of assorted resistors on Amazon for a few bucks.