gas gauge question

[billfish]

The gas gauge on my stock 51 ford 6 volt poss ground system stopped working today. It does not move off empty. I thought I would start at the sending unit wire in the trunk and take a volt meter reading from the wire to ground with the key on to see if there is a signal being sent to the gauge from the sending unit. What kind of a reading should I look for? Does this make sense or is there a better way to isolate this problem?

 

[JSeery]

This is a general diagram, it is similar to the oil pressure and temperature units. They use a bi-metal arms that is heated by the current passing through a coil. The current is controlled by contact points, so it is a type of pulse system. Variation in the condition (in this case fuel level) is measured by changing the pressure on the bi-metal arm what changes the current required to make / brake the contact points.

Added some better diagrams from a post by 19Fordy on the Barn. These diagrams are for a 51 Merc.

Do not believe you can test these circuits like you are thinking. It is a current device, not a voltage device. The last attachment has a test to isolate the problem between the sender and the gauge.

Also might read post How to calibrate oil pressure gauge? same issues, same type of gauge.

 

[big job]

Make sure the tank is grounded, momentarily ground the sender wire, the gauge
will move (if the gauge and wire is good) my bet is the brass float is gas logged,
I just did mine it was full of gas just a pin hole.

 

[jon laing]

I have the same gas gauge problem on my 1939 deluxe ford sedan. Do these comments also apply to a 39? Jon Laing

 

[JSeery]

Ford gauges work the same through most of these years.

 

[billfish]

Thanks for the information-I grounded wire at sending unit and gauge worked. Will order new sending unit. I also need to remember to use my green book before I post. Thanks again---Bill

The sending unit had what looks to be a condenser connected to the sending wire on top of the unit. I do not see this in the wiring diagrams, any idea what it's for ( radio interference) maybe)

 

[19Fordy]

The 40 Ford gas gauge works on the same "bi-metallic" principles as pointed out in posting #2. I think 1939 is also the same. If you are having trouble with your gauge, remove the sending unit from the gas tank and move the float up and down to simulate "full" and "Empty' to check it out. But first attach the hot wire from the dash gauge to the little screw on top of the fuel tank sending unit and then run a ground wire from the metal sending unit body to a ground' like the trunk latch. Also check to see if there are any hair line cracks in the float where gas has leaked in. Here's what the 40 sending unit looks like. In 1940 they cost $1.75 new. The median income for a man in 1940 was $956. Seventy years later, the median income was $33,276. Women in 1940 earned 62 cents for every dollar a man earned. In 2010, women earned 74 cents for every dollar a man earned.

 

[JSeery]

Thanks for the information-I grounded wire at sending unit and gauge worked. Will order new sending unit. I also need to remember to use my green book before I post. Thanks again---Bill

The sending unit had what looks to be a condenser connected to the sending wire on top of the unit. I do not see this in the wiring diagrams, any idea what it's for ( radio interference) maybe)

Are you sure you need a new sender if the ground is the only issue? The ground problem could be several other issues, such as the fuel tank not making a good ground. The sender grounds to the tank and the tank to the frame, etc. Might check grounding the sender to the tank and then grounding the sender to the frame, and so on to see if you and locate the grounding problem.

 

[billfish]

I put a jumper wire from sender wire screw to screw holding sender mounting plate to tank. Gauge moved to full. I think this proves the ground is good? I also than removed the sender from the tank and using an ohm meter and moving the float up and down saw very little change in reading. Does this prove sender is bad?
Any idea what the condenser is for?

 

[big job]

Ya sounds like it (ground the sender body and move float by hand) & not work.
At that time viabrator/vac tube radios had condensers or suppressors all over the
place on the generators, oil senders gas tanks whatever and where ever usually by
the dealer to stop the annoying whirring out of the speaker. Strangely some vehicals
had these and some did not. Then in 1957 the (fomoco Philco) radios were transistor
power supplied but still had to warm up (vac tubes) still used, but transistors ended
the rf whine ending the need for all those suppressors. Has anyone come across the
spiraled springs in front wheel bearing covers?? Thats another radio suppressor.

 

[TJ]

Did you check the float to make sure it is not full of gas from a pin hole?

 

[adileo]

Mine never goes above half full. Used drake replacement and it shot up to full and never came down. So I put new cork on old one, re-installed and still only shows half full - I now keep track of mileage @ at 100 I fill tank

 

[rotorwrench]

King Seeley made a damn good system but after many many years they are bound to fail at some point. The little breaker points will eventually get crusty or the hair thin heater coil wire will finally break.

You can pop the top off the fuel quantity senders and sometimes repair them but it is always a crap shoot.

 

[JSeery]

I put a jumper wire from sender wire screw to screw holding sender mounting plate to tank. Gauge moved to full. I think this proves the ground is good? I also than removed the sender from the tank and using an ohm meter and moving the float up and down saw very little change in reading. Does this prove sender is bad?
Any idea what the condenser is for?

How does that prove the ground is good? That would show me the ground is NOT good. An ohm meter would not show a change on this type of sender, it is a "pulse" device, not a resistance device. How are the contact points going to give you variation in the ohms as you move the float up and down? Moving the float changes the pressure on the bi-metal arm connected to the points. This requires more heat to open the points. If it is not grounded with the power turned on how would the points work? And even all connected properly there is not going to be a resistance change. The points vibrate open and closed which produces a current pulse in the circuit. This motion occurs because of heat in the coil around the bi-metal arm. The same current in the circuit is passing through the gauge and produces a corresponding amount of heat in the gauge coil with in turn bends the bi-metal arm in the gauge to move the needle.

You need to understand how the circuits in Ford gauges works in order to trouble shoot them. Don't believe an ohms reading is going to tell you anything.

Look at the diagram of the sender/gauge system and check out the key words:

Contacts
Bi-metal
Heating Coil

This is NOT a resistance system! It is a heating coil system with contact points. They vibrate. It produces a pulse. What is happening in the sender is repeated in the gauge.

 

[19Fordy]

Mr. Seery:
Thanks for explaining so clearly how that sending unit works.
Should be mandatory reading for all FORD old car folks as that
"ohms/resistance" question is so often asked.
Too bad that type of sending unit is no longer made.

 

[billfish]

Jerry
Thanks again for your help. I believe I followed the directions on page 301 in the green book for testing the sending unit. I put a jumper from the sender wire to a screw in the sender hold down plate. The gauge went to full. The book says if this happens than the sender is at fault. I'm assuming this also proves the tank is grounded. Am I still missing something? Thanks for your patience as always.
Bill

 

[JSeery]

I'm at a little bit of a loss here as to what your talking about. Page 301 of the Green Book is about transmissions. The Fuel Level sender is part #9275 on page 407 and there are no test procedures listed.

Now, for the test you describe, there is no way connecting a test ground wire to the fuel level mounting screws is going to make the gauge pointer move to maximum deflection. That is just not going to happen, the sending unit is already grounded so how could an addition ground jumper cause a pointer deflection? You are just testing to see if something is blocking the ground that should already be there. Now, IF the fuel tank was full AND the sender ground was not working THEN providing that ground would enable the gauge to read correctly and it would indicate a "F" full condition on the gauge. This does not indicate a faulty sender, it indicates a sender working as it should with a proper ground.

If what you are doing is shorting the electrical connection on the sender to ground you are grounding the heating coil in the gauge to ground which would heat it to maximum and deflect the gauge to full. I have never read to test this way, but guess it would work if it doesn't damage the gauge.

The repair manuals I have provide two test procedures:

1) to test the gauge, providing 1 1/2 volts to the gauge should move the pointer to a mid-scale reading and 3 volts should provide a full-scale reading. (This has been suggested in this thread several times) This is easily accomplished using a "D" battery (which is 1 1/2 volts), one for half-scale test and two for a full-scale test.

2) The only test they suggest for the sender unit is to check it with a now good gauge.

 

[rotorwrench]

This test is also called out in the electrical section of the Lincoln Mercury 1949 thru 1951 Overhaul Manual but it is a rudimentary test just to see if the gauge or reciever will still indicate. It is just a momentary test since a short to ground will eventually burn the reciever's heating coil out. If there is no response, there could still be an open circuit in the wire between the sender and reciever. If there is a short to gound in the wire or the condenser (if one is mounted) it can burn the receiver coil out with a deflection that indicates over the full mark.

A person might pop the cap off the sender and see if the heater coil, points, or bi-metalic arm can be repaired. I would look at the points first and see if they are still making contact or posibly in need of cleaning.

 

[billfish]

Trying to upload page 301 from ford shop manual, containing gas sending unit test. If this fails maybe someone else can get it done.

 

[JSeery]

Trying to upload page 301 from ford shop manual, containing gas sending unit test. If this fails maybe someone else can get it done.

LOL, now your saying Ford Shop Manual, the other post said Ford Green Book. The Green Book is the parts manual. It posted fine. Yep sounds like it works for a "quick" test if you don't leave it on too long. Well, sounds like the Sending Unit may be the issue. I would still make sure the grounding of the unit and tank were OK, but sounds like you have found your problem. I would guess that the manuals I have do not include that test because of the ease of damaging the Gauge Unit, who knows?