Gas Gauge/Sending Unit problem

Having problems getting gauge to read correctly. I fill the tank and needle goes just over full. As I drive I notice the gauge starts to go toward E. At around 50 miles orso gauge starts moving faster eventually ending below E. I’ll once again fill the tank and it will take around 9 gallons. That’s after around 120 miles.

this afternoon I wanted to check the gauge by ground in the hot wire. The tank was about half full. When the hot wire was grounded the gauge would go from below E to 1/2 tank. I’m thinking the sending unit is the problem. By the way the tank was recently replaced and appears the sender is new.

Grounding the sender wire should cause the gauge to "shoot" way past full, and you don't want to do that for long, only as a "quick test" that the gauge responds

Just for the record, what year/ model are you working on?

Go to MyMopar and download (free) a factory service manual. You may have to settle for Plymouth vs Dodge

Think of the gauge circuits as "end to end" and as a "system"

The cluster gets switched power from the ignition switch to the harness connector on the PC board. Those pins can be a problem, they are crimped. They can corrode and become loose. SO DOES the temp gauge seem to work? IT is fed from that same power

Next, power goes to the IVR, instrument regulator/ limiter. You have a Ralleye cluster? or standard cluster? (No oil gauge) Standard cluster uses an IVR which plugs in. Again it feeds both gauges, so does the temp gauge work OK?

Next, power from the IVR goes to the gauge stud, and they can be loose/ corroded where the nuts tighten on the PC board traces. Loosen/ tighten the nuts a couple times. The sender trace on the board goes to a harness connector pin, down to the kick panel connector for the tail harness, and to the rear of the car. Even the sender connector at the tank can be corroded.

There's about a gigazillion LOL threads on this subject:

67dart273, gauge tester - Google Search

HOW TO TEST. You either need test resistors, or you need to take a multimeter and measure the tank sender resistance with whatever fuel is in the tank. Here is an aftermarket tester with the resistances in YELLOW

Bench Testing a gauge cluster

New senders are junk and do this, the arm is too short and originals are wound non-linear. If you use a A100 sending unit with the long arm it works WAY better but not perfect as no after market unit is wound non-linear. There is a thread somewhere here that explains why the short arm does this. It is obvious once you get it.

Here are a couple of graphs that I just posted on another thread. I pirated them from someone else here awhile back. You can see the difference in performance of the original to the aftermarket gauge. I bought a Meter Match to improve the accuracy on mine. That is about all you can do other than get an oem sender.

MeterMatch.

Here are additional graphs I made with the A100 sender and also added the resistance needed to cause MY gauge to read E, 1/4, 1/2, 3/4 and F.

Notice that the Spectra has 48 ohms at 1/2 tank. 48 ohms should have been at under 1/4 tank. The A100 reads about the same.

That's why your gauge reads 1/4 to E when you have 9 gallons of gas

Additionally I just today found this info in my 67 FSM

Empty is 73 +/- 12 ohms (61 to 85)
Full is 9.6 +/- 1 ohm (8.6 to 10.6)

Very interesting! So, it looks like the A100 is a little better over a 1/2 tank and worse (than the Spectra) below 1/2 tank. Great data!

My methodology was to drain the tank with a syphon hose.

I had 20 1 gallon former windshield washer fluid bottles.

I measured the resistance at the sender (it would be a little bit higher at the gauge for the length of the wire, but to compare the senders it is ok)

I added 1 gallon at a time and bounced the car to allow the sender to move a bit. Waited for it to settle then read the resistance.

The Spectra and A100 both did the same thing with reading empty with 1/4 to 1/2 A tank left.

The later model OEM Mopar looks like the Spectra but the curve is more correct. It has the short arm like the Spectra which might explain the longer reading empty or full at the top and bottom of the scale.


I forgot to mention the gauges have a E and F adjustment in them. They have to be removed from the cluster to adjust it

That’s some fantastic information and lots of work you put into getting it. thanks Dana67.

Anyone have a cutaway of where the float is exactly at a full tank? It is submerged (cant go any higher) for the last 4 gallons? Seems a longer arm could really dial in the actual Ohms and then you could just get a blacked out meter movement and mark where a full tank is, and then every - gallon under that. These 4" arms are useless. Now a 12" arm like an old toilet float would give you less sweep but it would be more accurate as F would really be F and E could really be under 1 gallon.

pishta said:

Anyone have a cutaway of where the float is exactly at a full tank

Click to expand...

This is a close as I have.

As AT's photo of the sender in the tank shows (photo used with permission, thank you Arron

A100 sender


OEM sender



There was a post with a OEM and aftermarket in a cutaway box showing the different movements between the two.

If you grounded the 'hot' wire, then the gauge should read full, not 1/2 full. So sounds like a gauge or wiring problem.

Dana67Dart said:

Here are additional graphs I made with the A100 sender and also added the resistance needed to cause MY gauge to read E, 1/4, 1/2, 3/4 and F.

Notice that the Spectra has 48 ohms at 1/2 tank. 48 ohms should have been at under 1/4 tank. The A100 reads about the same.

That's why your gauge reads 1/4 to E when you have 9 gallons of gas

Additionally I just today found this info in my 67 FSM

Empty is 73 +/- 12 ohms (61 to 85)
Full is 9.6 +/- 1 ohm (8.6 to 10.6)


View attachment 1715854248View attachment 1715854249

Click to expand...

Thanks Dana67, I forgot who had done all this great work.. What is not clear from the graphs is the "feeling" driving the car". The short arm junker that drops faster in the beginning make anyone who does not really understand what is happening very uncomfortable.. the gauge gets down to 1/4 tank or lower on the gauge very fast and even though you know you have more than 5 gallons left you end up counting miles to make sure... The A100 has a much more comfortable feel when driving and traveling. It does not stay full then drop to 1/4 in 40 miles like the short arm so it is not so stressful for the driver. I have the short arm in the daughters Duster and the A100 in the wife's Dart and the difference is noticeable.. The daughter is constantly telling me she is low on gas and I keep explaining the gauge is inaccurate.. the wife only complains near the end of the tank because of that dip right at the end.

There's a sticky thread in the fuel forum about this.

Here is link to sticky
Solved with a Meter Match Fuel Tank Sending Unit/Fuel Float Adjustmet