How do you bench test a two wire fuel sender? P14 Plymouth

[Joe-Bob]

I have a '42 Plymouth and I'm tired of running out of gas.  I have a NOS gauge that I believe is in working order.  I have a trouble-shooting guide that I found, and with a full tank of fuel my gauge reads FULL.  If I ground the #1 wire it reads 1/2 full.  If I disconnect the #2 wire the gauge reads EMPTY.  If I reverse the #1 and #2 wires the gauge reads EMPTY.  Now that the tank is about 3/4 full the gauge still reads FULL.

 

I rewired the car several years ago with an aftermarket wiring kit and changed to 12 volt negative ground.  I used a runtz thingamajig to drop the voltage to the fuel gauge.  To be honest I don't remember if the gauge worked when it was still wired as a 6 volt positive ground.  Today I disconnected the battery and pulled the fuel sender out, hooked up a multimeter to it and tried to figure out if anything changes when you move the float up and down.  I know NOTHING about multimeters.  I set it to ohms and shoved the red probe in the #1 contact and the black probe in the #2 contact.  I think the meter read "1".  That didn't seem right, so I left the red probe in the #1 contact and grounded the black probe.  That set a little buzzer off and caused the meter to go up and down when I moved the float.  I think the range was from 1 to around 67 or so, but the readings weren't steady; they wavered up and down but in a general direction of lower to higher and back.  I thought (hoped) I jiggled something loose and maybe the fuel gauge would move lower when I reinstalled the fuel sensor, so I put it back together.  I even ran a separate ground wire from the sender to the body hoping that might make a difference.  The gauge still reads FULL.

 

I'm sure I'm missing something simple, because "I'm" simple...  If anyone can help me out with this I will be very appreciative.  And then we'll move on to the odometer which also doesn't work.  That's the other reason I keep running out of gas...

 

I apologize in advance if I'm in the wrong area, or this subject has been worked to death already.  I tried to find some answers using the search function and was able to get some ideas, but I can't seem to find anything in my manual or on here about bench testing the fuel sender.  I don't mind buying a new one, but I would be a bit P.O.'d if I did and that wasn't the issue.

 

 

[Plymouthy Adams]

if you have the book on your car..the section on the fuel gauge is a tremendous help and explained very well...if not...maybe someone can scan the pages for you...it is a long write otherwise

[Joe-Bob]

Thanks for the response.

 

I've read and reread my manual.  The first sentence starts out with "The most convenient method of testing for a faulty gauge is to use a spare tank unit known to be in good condition."  I'm afraid I don't have a spare tank unit.  Then it explains how to connect this spare tank unit to the gauge and if the gauge works, then you know that the original tank unit is faulty and needs to be replaced.  There's also an explanation of cleaning the points in the gauge if it fluctuates and how to calibrate the gauge if it reads incorrectly.  Mine just stays on Full.  There is nothing on how to test the original tank unit. 

 

I'm pretty sure that I read something somewhere about how to bench test the fuel sender with a multimeter.  It's supposed to register a certain ohm value when the float is up and another value when it's down.  But I'm not sure how it said to hook everything up and what the readings should be.  For all I know I accidently did it correctly, but I don't feel like I did.  I'm just hoping someone can point me in the right direction. 

 

I scanned the repair manual but have no idea how to upload or paste it to this topic, in case someone could point out what I'm missing...  Sometimes I just stay in frustrated mode.

[greg g]

Run a jumper wire to ground the sending unit to one of the screws that retains the cover to the trunk floor. Many times this will address the situation you are having.  Also check up under the dashboard and make sure nothing is in contact with the terminals on the gauge.  I had a defrost duck drop down and the metal reinforcement in the duct was causing an inadvertent ground of the gauge causing the reading to vary if and when the duct was contacting the terminal.

Edited May 24, 2015 by greg g

[Plymouthy Adams]

two main items here in the repair...being familiar you will wish to see that the gauge when connected to battery will regulate it to the 5 volts per the built in voltage regulator..second is to ensure you have 30 ohms across the two post that is the sender in the tank and verify that each leg to ground (float arm is the path to ground) in any position  of the float divides the 30 ohms equally. If leg 1 has 10 ohms, leg two should show 20 and vice versa....AND that the division of the 30 ohms is smooth and equal across the full scale of the wiper (float arm movement)  If so...then good grounds and clean connections of the leads and proper lead orientation gauge to sender ( 1-1 and 2-2) and Bob's your uncle..

[Joe-Bob]

Ok, maybe I figured it out...  I went online and studied up a little on multimeters.  I'm not saying I understand very much, but here's what I think I got. Voltage is 12+ volts before my runtz resistor and about 8 volts going to my gas gauge.  If I disconnect the battery and both wires from my fuel sender and do a continuity test, both the #1 wire and the #2 wire test ok.  No shorts there.  Then for no reason, with the fuel sender still installed and disconnected from any wiring, I ran the multimeter from ground to the #1 terminal.  No continuity.  Then I ran from ground to the #2 terminal and there we HAVE continuity.  I believe I have a short circuit to ground on the #2 terminal inside the fuel sender. 

 

Can I get an Amen?  Is my hypothesis correct or am I still on a wild goose chase?  By the way, I pulled the fuel sender, grounded it and moved the float manually.  From fully up to nearly all the way down the fuel gauge does not budge from full.  When you drop the float the last quarter inch or so the gauge does move about an eighth of an inch and that's it.  You'd have to be watching pretty close to notice it before you ran out of gas.

[Plymouthy Adams]

you get no amen when you say short and no continuity...they cannot co-exist in same state....did the resistor measure the 30 ohms across 1 and 2  ????  this is step one of the test....it can be open one end and read one post to ground through the wiper and not to the other post to ground...you must be able to read the 30 across 1 and 2 for a proper resistance test...only when you have that will you be able to divide the resistance of the resistor per each leg to ground which is through the float arm.....please go back and look at your manual again....you must know how it works to troubleshoot what is wrong....

 

to repeat...get the 30 ohms across posts 1 and 2 of the sender...if you cannot get this..one leg is open...

[Captain Neon]

Never mind.

Edited May 25, 2015 by Captain Neon

[TodFitch]

BTW, there is no such thing as a P14.  What you have is a P12.

P14 models were built in the 1942 model year.

[Joe-Bob]

Yup, P14's are 1942's.  Most were built before the end of December 1941.  After Pearl Harbor they manufactured the "black-out" models and I think most of those may have been used as staff cars.  I've owned two over the years and seen less than ten others on the road or at car shows.  You see quite a few on the internet, but I think they are spread out fairly thin across the country. 

[Joe-Bob]

Ok, I'm opening myself up to certain ridicule but my problem is solved and here's my theory.  First of all I admit I'm no mechanic and I know even less about ohms, volts and multimeters, in fact I don't write too good and I mix up my facts when I try... There, we got the disclosure out of the way.

 

Above, when I wrote

 

"Ok, maybe I figured it out...  I went online and studied up a little on multimeters.  I'm not saying I understand very much, but here's what I think I got. Voltage is 12+ volts before my runtz resistor and about 8 volts going to my gas gauge.  If I disconnect the battery and both wires from my fuel sender and do a continuity test, both the #1 wire and the #2 wire test ok.  No shorts there.  Then for no reason, with the fuel sender still installed and disconnected from any wiring, I ran the multimeter from ground to the #1 terminal.  No continuity.  Then I ran from ground to the #2 terminal and there we HAVE continuity.  I believe I have a short circuit to ground on the #2 terminal inside the fuel sender."

 

I stated my facts back-asswards.  I think continuity means you have a connected circuit to ground which means you have a closed circuit with no shorts (check with someone who knows electricity because I could very well be wrong again) so I should have stated "I ran a ground to the #1 terminal. We have continuity.  Then I ran a ground to the #2 terminal and there we have NO continuity.  Anyway, I deduced that the fuel sender was shorting out through most of it's swing cycle causing my gauge to read full and only dropping very slightly as the tank went empty leaving me thinking the tank was full when it was indeed empty.  Whether that deduction is scientifically correct or not is up to the experts to decide, but since the gauge seemed to move from full to empty and back, depending on whether you crossed the wires or not and the wires from the gauge to the fuel sender had continuity I spent the big bucks ($97.00 including shipping) to buy a new fuel sender from Roberts Motor Parts 978-363-5407.  I put it in tonight and voila, my gauge shows 3/4 full. 

 

So, whether my hypothesis was right or wrong, it appears my problem is solved (knock on wood).  So now as soon as it quits raining I'm going to put a few miles on my car and see if the gauge continues to drop.  I WILL make a final report when I know for sure.  In the meantime if I had to pay someone to diagnose this issue I would have spent even more, I'm happy.   And I got a free catalogue with my order that has lots more parts for old Mopars - check them out, they seem very reputable.