Fuel gauge not working

[Mertz]

All the gauges in my truck are working very well except the fuel gauge. I have a new tank and sending unit installed per instructions. I checked and I have 6v to the gauge.  I checked for continuity of each of the wires from the tank to the gauge. One terminal showed continuity from the wire to one terminal only. The other wire showed continuity to both terminals. The gauge does not move with 4 gallons in the tank. It seems to me that the wire showing continuity to both terminals means a short in the gauge. Would that be a correct assumption?  I have never heard of a thermal fuel gauge before but that should be no surprise I’m only 71. 

[Ivan_B]

Are you using OEM gauge and OEM sending unit, aftermarket parts, or a combination of both? No idea how the aftermarket stuff works. The OEM sending unit is just a resistor with a float. If you disconnect it, the gauge should show full tank. If you "ground" the gauge wire to body, it should read empty (or the other way around ). This is how you can usually confirm that everything is working properly. All the readings in between are controlled by the float position.

If you have something else installed, what is it?

[joecoozie]

 

Did the gauge work before the new tank/sending unit?

Make sure the sending unit is well grounded.

Running an extra ground wire from the sending unit to the frame (or any other part of the body) makes for a good ground.

[TodFitch]

19 minutes ago, Ivan_B said:

Are you using OEM gauge and OEM sending unit, aftermarket parts, or a combination of both? No idea how the aftermarket stuff works. The OEM sending unit is just a resistor with a float. If you disconnect it, the gauge should show full tank. If you "ground" the gauge wire to body, it should read empty (or the other way around ). This is how you can usually confirm that everything is working properly. All the readings in between are controlled by the float position.

If you have something else installed, what is it?

It is not clear from the original post what year Plymouth car or truck the question is for.

 

Their info says they have a 1940 Plymouth truck. In 1940 the cars used a two wire fuel gauge. Not sure if the truck also used that or not (the truck and car part numbers are different for the dash unit).

 

Your advise would be correct for a 1949 and up Plymouth. And backwards with respect to reading for the early 1930s Plymouths that used a single wire sender. And wrong for the era that used a two wire sender.

[Marcel Backs]

Try the troubleshooting guide in a copy of Motors Auto Repair Manual. The guy who designed the fuel gauge soon went to work for the Furd Motor Company LOL! M 

PS check the past posts on this matter because this topic comes up frequently.

[Sniper]

11 hours ago, Ivan_B said:

The OEM sending unit is just a resistor with a float

 

Not with the thermal gauge setup.

 

From my 46-54 FSM, 40 truck should be similar.

 

 

[Mertz]

The sending unit and tank are new and I tested the sending unit by seeing if it shows resistance with about 4 gallons in the tank. The gauge is original to the truck and the wiring looks good. On the new sending unit the terminals were labeled gauge and ground. I switched wires back and forth and never got a full reading. I believe I have a good ground from the frame to the tank but since the sending unit is isolated from the tank it should not ground it. I don’t believe the wires are grounded because I get continuity.

Edited August 12, 2023 by Mertz

[Sniper]

Wrong sending unit, there is no gauge and ground label on the factory sender, it is labelled 1 and 2 and connects to the same terminal labels on the gauge.

[Plymouthy Adams]

The sender................ground comes into play through the wiper running across the resistor on the sending unit as it takes each leg (A and B or 1 and 2 depending on how marked) to ground.  The gauge is drawn in both directions at the same time resulting in dead accuracy with no fluctuation of the needle, the needle is so slow to respond to change that it prevents flicker of the needle.   The resistance of the sender is approx. 30 ohms across these terminals readable with your meter.  IF your wiper is mid of the resistor, then approx. 15 ohms is on each leg.  If at one end of the resistor, the one leg will be zero and the other will read the approx. 30.  (it is a divider)  Anywhere along the wiper the total of the two outputs should add up to 30 ohms.  All this is simple check using ohm meter.  This says the wiper is working within the sender stating your cup (sender body) to wiper connection is good and not erratic and that the wiper is contacting the resistor full travel with no opens.  From there, you MUST have a good ground from the sender to the tank and of course tank to chassis ground.  

[Mertz]

I did some checking with the ohm meter. (Wire 1 is on the right side of the gauge and 2 on the left)

Fuel Gauge wire between 1&2 46.6 ohms 

Sending unit wire between 1&2 61.6 ohms 

Sending unit to ground both wires open

Gauge wire 1 to ground 846 ohms key on

Gauge wire 2 to ground  832 ohms key on

Wire 1 from gauge 0.9 ohms to terminal 1 48.6 to terminal 2

Wire 2 from gauge 0.9 ohms to terminal 2 48.7 to terminal 1

Changing sending unit wires to gauge had no effect

Grounding wire 2 moved the needle to full. 
 

[Jerry Roberts]

The points on your dash gauge might not be making good contact . Sometimes even the NOS dash gauge points won't work unless cleaned . They are very fraglie . 

[Plymouthy Adams]

your reading are all off the chart......PLUS......you do not read ohms with KEY ON....

 

what OHM reading do you get across 1 and 2 of the sender.....should be approx. 30

 

to do a full swing check of the sender it is easier out of tank for reading full deflection of the wiper and the splitting of the resistor .....leave the gauge out of the equation till you confirm the sender.  

 

After confirming the sender confirm sender to tank ground when mounted, tank to body, body to chassis.  Then you confirm with ohm meter (key off) the wire from the gauge 1 to tank 1 and gauge 2 to tank 2....with both leads NOT CONNECTED to the sender.  Each should read zero which is continunity.  Now read to see if either is going to ground best done if not connected to the back of the gauge.  If either read to ground in this manner, repair for shorted wired to chassis.

 

 

Edited August 12, 2023 by Plymouthy Adams

[kencombs]

2 hours ago, Plymouthy Adams said:

The sender................ground comes into play through the wiper running across the resistor on the sending unit as it takes each leg (A and B or 1 and 2 depending on how marked) to ground.  The gauge is drawn in both directions at the same time resulting in dead accuracy with no fluctuation of the needle, the needle is so slow to respond to change that it prevents flicker of the needle.   The resistance of the sender is approx. 30 ohms across these terminals readable with your meter.  IF your wiper is mid of the resistor, then approx. 15 ohms is on each leg.  If at one end of the resistor, the one leg will be zero and the other will read the approx. 30.  (it is a divider)  Anywhere along the wiper the total of the two outputs should add up to 30 ohms.  All this is simple check using ohm meter.  This says the wiper is working within the sender stating your cup (sender body) to wiper connection is good and not erratic and that the wiper is contacting the resistor full travel with no opens.  From there, you MUST have a good ground from the sender to the tank and of course tank to chassis ground.  

 

34 minutes ago, Mertz said:

I did some checking with the ohm meter. (Wire 1 is on the right side of the gauge and 2 on the left)

Fuel Gauge wire between 1&2 46.6 ohms 

Sending unit wire between 1&2 61.6 ohms 

Sending unit to ground both wires open

Gauge wire 1 to ground 846 ohms key on

Gauge wire 2 to ground  832 ohms key on

Wire 1 from gauge 0.9 ohms to terminal 1 48.6 to terminal 2

Wire 2 from gauge 0.9 ohms to terminal 2 48.7 to terminal 1

Changing sending unit wires to gauge had no effect

Grounding wire 2 moved the needle to full. 
 

The two red statements above seem to point to an issue.   Sound like the sending unit is a mismatch to original equipment.

 

Nevermind, Plymouthy answered while I was typing.  

Edited August 12, 2023 by kencombs

[Mertz]

I bought the sending unit and the tank together and it was advertised for my truck. I understand there are different sending units and wasn’t sure which was correct. I assumed since it was for my truck it would be correct. It would be very difficult to change out the unit without dropping the tank. I will try improving the ground to the tank and hope the screws holding the unit in make a good ground. 
 

I thought that the ohm reading from the sending unit varied with the fuel in the tank. My ohm meter is a cheap one and reads 1.6 when probes are touched together. Other than being over 30 the rest of conditions listed appear to match. 

 

[Plymouthy Adams]

as you have not checked specific points as requested and reported back.....I am not sure how anyone can assist you further.....guidance is being offered but no answers are being returned to those trying to assist....it is a process of tests made in a systematic manner...

[soth122003]

I just changed the sender in my tank 3 days ago. The tank was empty and before I installed it I checked it with the 2 wires hooked up and GROUNDED. It was spot on read empty and as I moved the sender, read it went to full. Installed the sender and it wouldn't read past 1/4 of a tank. Installed a ground wire from on of the sender mount screws to the cover plate in the trunk and the sender read 1/2 tank with a full tank. I need to adjust the float arm down about 1 inch to fix this, but the tank is full to the max, so I have to use about a gallon of fuel to remove the sender without spilling gas everywhere. Point being is the sender needs to make a good ground to the car to work right.

 

Joe Lee

[Mertz]

The truck has the sending unit under the cab and impossible to reach. I checked and the tank seems grounded. I checked at the mounting bolt on the frame and the access cover on the tank. I got continuity at both locations. Still might need a better ground.

 

The grounds are as follows: number 0 wire from the battery to the engine and number 0 wire from the engine to the frame. If you ever owned a Jaguar you would know the importance of grounding. Then they’re all the bolts holding on the sending unit to the tank. I will improve the tank ground by adding another wire. 

Edited August 13, 2023 by Mertz

[soth122003]

This might seem like silly question, but did you check under the seat for an access cover to the sending unit? I mean they put them in the cars so you wouldn't have to drop the tank to troubleshoot wiring problems. Make sense they would put them in the trucks as well.

 

Joe Lee

[Young Ed]

7 hours ago, soth122003 said:

This might seem like silly question, but did you check under the seat for an access cover to the sending unit? I mean they put them in the cars so you wouldn't have to drop the tank to troubleshoot wiring problems. Make sense they would put them in the trucks as well.

 

Joe Lee

39-47 trucks didn't get an access hole for the sending unit in until 47. My 46 doesn't have one but the 47 does

[Mertz]

I thought about cutting one in but the tank is pretty close to the floor and don’t want to hit the tank. If it’s the wrong sending unit I will have to drop the tank and then I might cut the hole or just add a good ground wire. 

[Mertz]

I’ve been working with the seller of the sending unit. The id on the box has a part number 317135 75-10 Ohms early ford and mopar. Is that the right resistance for my 40 truck?

[Mertz]

Old post but still not resolved. I’m going to have to go through great efforts to add a ground to the sending unit cover. I have about 1/4 tank of gas and no reading on the gauge. Does anyone know what the ohm reading should be at 1/4 tank?  I will also check to make sure there is power from the ignition switch to the gauge. I talked to the supplier and they insisted the sending unit is the correct one even though the terminals read ground and unit instead of 1 and 2. 

[sidevalvepete]

Looks like you are really trying to solve this so my 2c would be to drop the tank, make a decent access/inspection hole and cover. Then you can do any of the above system checks. Including adjustment of the float arm. I had issues with a replacement tank for my sedan that fitted perfectly with the straps and mounting under the trunk floor but it had a different shaped pressed flange to mount the sender unit. It sat a bit lower in the tank. I needed to take it out a few times to adjust the float arm. Still isn't perfect but close enough for me to read.

Point being, you need to be able to read and have adjustment access to all these various factors. Hope you solve it..

[Veemoney]

On 8/12/2023 at 12:04 PM, Plymouthy Adams said:

he sender................ground comes into play through the wiper running across the resistor on the sending unit as it takes each leg (A and B or 1 and 2 depending on how marked) to ground.  The gauge is drawn in both directions at the same time resulting in dead accuracy with no fluctuation of the needle, the needle is so slow to respond to change that it prevents flicker of the needle.   The resistance of the sender is approx. 30 ohms across these terminals readable with your meter.  IF your wiper is mid of the resistor, then approx. 15 ohms is on each leg.  If at one end of the resistor, the one leg will be zero and the other will read the approx. 30.  (it is a divider)  Anywhere along the wiper the total of the two outputs should add up to 30 ohms.  All this is simple check using ohm meter.  This says the wiper is working within the sender stating your cup (sender body) to wiper connection is good and not erratic and that the wiper is contacting the resistor full travel with no opens.  From there, you MUST have a good ground from the sender to the tank and of course tank to chassis ground. 

 

PA's explanation provided previously is correct for a 2 wire fuel sender set-up. I pulled the one in my 47 truck a year or so ago, took it apart and cleaned it up. It worked up till about a few months ago but it was in pretty bad shape. I just purchased a new sender from AB  off E-bay and it tested out just as described above. If your sender is a 10-70 ohm I'm not sure how your gauge readings would react to that. The link below is the one I purchased and installed, $63 +tax and shipping or make him an offer.

 

For 1939 Chrysler DeSoto Dodge Plymouth Fuel Tank Gas Gauge Level Sender Unit | eBay

[Mertz]

I will do some more testing this morning. Looking at some videos I believe the gauge is working but I will retest. I will check power to the gauge and make sure the tank is grounded. The sending unit should be connected to the tank by all the screws. The tank should be easier to ground and test.  I’ll let you know my results.