Me be stupid about ohms as in gas gauge learn me up please

[greg g]

So after running out of gas last night, I have decided that it is time to deal with my gas gauge.  last year in the original tank, it worked but was not accurate in that 1/4 tank indicated was empty.  Then I switched the tank and all it reads is 1/4 tank (previously it would move to full and then decline with fuel use) and just sits there.  It has a separate ground wire.  So I want to check for resistance along the coil with my meter.  Do I simply set the meter for the 200 ohms setting and connect the leads to the terminals (does it matter which lead goes to which terminal) and then check the reading by moving the float through its arch of motion?  If that's it, what reading should I be looking for at full and empty?   P15 2 wire sender.

[Lumpy]

Me stupid too when it comes to ohms and such...I'll be watching this thread as Lumpy never has had a working gas gauge in the fifteen or twenty years I've been driving her. I think my sending unit works, but I'm not sure. I write down my mileage when I get gas, and keep a full two gallon gas container in the trunk, and never have run out of gas. But I'd rather have a gauge. (that worked)

 

Wish the odometer had a trip meter, then it would be just like riding an old motorcycle. Once you figure out how many miles you get out of a tank of gas, you just get gas when you are about 50, 40, 30 miles shy of that mileage.

 

k.

[Oldguy48]

I've got the original sender from my P15. IF I can find it in the garage, I'll see what the resistance reading is and report back.

Wayne

[Mark D]

Greg - one of us is going to have to carry some gas to vermont... From the sounds of it our gas guages work about the same.

[greg g]

First thing to do, ground the body of the sending unit with a dedicated wire, I attached mine from one of the screws holding the sender to the tank to one of the screws on the cover plate.  

 

Also look up under the dash and make sure there is nothing in accidental contact with the fuel gauge terminals.  I had my defroster duct work itself loose and the wire reinforcing the tube was in contact with one of the terminals grounding it thus putting it in a non operative condition.

 

I have been doing the math thing at gas replenishments but I musta got my carries wrong on the last one as I was at least 15 miles short.....fortunately we were close to a gas station.  I usually carry an empty gas can in the car but of course not last night.  So I bought a gallon of water, drank some, dumped the rest, and used it to take gas tot he car.  

[Oldguy48]

I just spent about an hour in the garage searching for that blasted thing, and I can't find it. Hopefully someone else will chime in and give you some answers. I also looked in my service manual, but it gives no information regarding resistance readings of the sender.

Wayne

[DonaldSmith]

Family legend:

 

My dad had a 47 DeSoto Suburban to haul around us seven kids.  We lived some distance from our relatives, so when we went to visit, my dad traveled at night.  Three reasons:  1) He avoided the flashing of the sun and shadows of the trees beside the road, which gave him headaches; 2) Traffic was light, in the country, with no farm implements on the roads, and in the cities, where there were few cars on the streets and the traffic signals would be only flashing; and 3) The kids were bunked down in the back, and quiet if not sleeping. 

 

The problem was that there were few gas stations open, and my dad seemed to ignore the gas gauge.  Once, he ran out of gas in the middle of the night.  He was lucky to find a gas station a few blocks away where they had forgotten to turn off the pumps.  He put some gas in an enameled steel potty which we carried in the car for other purposes, and brought enough gas back to get the car started and to the station.  

 

My dad filled up the car, and slipped the $1.75 under the door of the station.   (Might have been more than that.  17 gallons at maybe 20 cents, OK, he slipped the $3.40 under the door.)  

Edited June 20, 2014 by DonaldSmith

[greg g]

Reminds me of the time I needed to borrow some from a genny powered traffic construction sign.  No door to leave the money under but I did return their jerry can, and it still had some gas in it.   Wonder why they have gone to battery powered ones with solar rechargers??????

[greg g]

Oh by the way I have deduced that ohms is the practical measurment of the effectivness of "jiggle the wires".  I had the sender out, couldn't get any thing but zeros on my meter, put it back in, cleaned up the contacts, put the blue on #1 and the yellow on #2, cleaned the ground wire and screw, and now magically i have just a bit less than half a tank.  Since I put 6.5 gallons in yesterday and drove about 12 miles home, I should have closer to 1/3 tank, as the tank is supposed to hold 17 gallons.  So it will still be close to empty at 1/4 tank reading but at least it is now reacting.

 

So OHMS = the given quantity of jiggles (wires you perverts) over a time of less than ten minutes.  That is my theory and I'm sticking to it.

[janan5243]

When tank is full, float arm is up, should read low ohms. Tank empty, high ohms, maybe 80. shiverlays are opposite.

[Plymouthy Adams]

on a later model those values may be present...but for the early two wire the setup is a bit different..first the voltage is applied to the gauge..the gauge has a built in voltage regulator..the gauge is also a push pull circuit...the needle is being stabilized by proportional difference of the single value resistor in the tank as it goes to common ground through each leg....that is the why for two wires...theoretically the 30 ohm resistor (nominal value as most are 28-32 ohms) is wired to each side of the guage as the wiper moves from full to empty the ohms are split proportionally to ground....if 20 on one leg the then 10 remainly ohms ins on the other..this proves for a very slow moving and accurate gauge that is not affected by fuel slosh..it is also important to ensure that the wires are properly attached to gauge and sender...some are maked each end as A & B and some are referenced as 1 & 2  connect like terminals..both wires must be making good clean connections to properly read..the wiper inside the sender must be clean and making good contact throughout its arc.. also...any low or loose spot between the wrapped wires of the resistor and the wiper pickup will cause an open in the circuit..majority of the sender I have seen have a bad wiper contact to the housing and not bad contact to the wrapped wire wiper/pickup..it is a tension pivot within a cup inside the housing..these usually is the failure point as the metal within the cup can and will corrode...it is not hard to disassemble and clean the sender..if your resistor is good (30 ohms across both terminals) and the wiper reading are smooth to the arm..and not smooth to the cup when amking comparison reading to the  sender housing..then you probably have this corrosion..one can use a jumper here similar to the very nice braided thin wire used in the distributor to make an effective ground.  Do ensure you have a good sender to chassis ground as age and corrosion at the tank flange itself can be questionable..

[Mark D]

Coincidently, someone posted this on facebook this afternoon.

Edited June 20, 2014 by Mark D

[knuckleharley]

Me stupid too when it comes to ohms and such..

I'm not sure,but I think it has something to do with Hare Krisnas.

 

Hope this helps.

[knuckleharley]

When tank is full, float arm is up, should read low ohms. Tank empty, high ohms, maybe 80. shiverlays are opposite.

Is that related to the positive ground versus the negative ground thing?

[Plymouthy Adams]

I believe this has been posted on here a time or two..certainly the troubleshooting tips from the manual has been listed..it is a bit more involved over a single wire as you can one or the other wire fault for some reason or a combination of each...but it is still a rather simple setup when you step back and look at it a bit....On another note..some low cost digital ohm meters do not like the low scales very good and often not very reliable on the very low end of the scale.....an analog usually does better...

 

Good luck Greg...once each leg is checked for tight clean fitting. no shorts to ground in the routing to the trunk..A to A  B to B or 1 to 1 and 2 to 2..verified..good sender connections internally from the wiper to ground..you should be good to go..