Fuel Gauge Reading Low

tlslack

Bronze forum user
Messages
11
The Jaeger fuel gauge on my 65 Tiger reads about 3-4 gallons low (8 gallons when full). I checked the voltage at the input terminal of the fuel and water temp gauges, with the ignition on and engine not running. Both were at 12 Volts. Voltage at the battery is 12.5. The other terminal of the fuel gauge read 7 volts with the gauge reading 4 gallons. I also checked voltage at what I think is the instrument voltage stabilizer (it is oval shaped) and found voltage at 12 volts. Any ideas as to the cause and solution for the low gauge reading?
 

Cal44

Gold forum user
Messages
428
I am certainly no expert but many times it's the sending unit. That poor thing, stuck in a corner......in gasoline.

There are no repos around that I am aware of and NOS is far and few between.

There are shops that can check your gauge and sender and do a rebuild.

Palo Alto in California for us on the "nut case left coast".
http://paspeedo.com/

If it is any solace, my gauge reads low as well. Better that than thinking you have a full tank.
 

0neoffive

Gold forum user
CAT Member
Messages
2,853
The OHMS have it

We could play tech-no data all night and measure resistance (blah blah). BUT, I keep a spare sender just to plug in the tank wire and move the critter up & down in a gas safe environment. It takes a while for the resistance to do it's job, but the readings and a little level optics for guesswork will let ya know if the sending unit has a hiccup. I agree with stuck sender/sinking float/etc.etc. The correct voltage under the dash is there, so you've eleminated all but the business back at the wet fanny. Unless your willing to open an agressive cardio surgeon's gap in the fender's top, removing the tank sender will eat up some family quality time.
When mine got sick many moons ago, I determined that the float was floating fine. Sooooo, I simple bent the float arm lower until we got a more trusting feeling from the gauge readings. Give or take a pit stop or two . . . . . . .honest honey, it'll go another 50 miles easy!!
 

tlslack

Bronze forum user
Messages
11
If the fuel gauge input voltage is allways 12, does the output side of the gauge vary beteen 1-12 volts over the 1-12 gallon tank level? If so, then it would appear that my gauge is out of cal, since it has an output of 7 volts with 7 gallons in the tank, but the gauge reads only 4 gallons. Is there an easy way to recal the gauge or move the needle 3 gallons upward?
 

wag123

Gold forum user
Messages
130
The instrument regulator does not actually regulate the voltage the way that you think. It sends the full 12v to the output in an on-and-off fashion. You can connect a 12v test light to the output and see this happening. If the gas gauge is reading low regardless of how much gas is in the tank (6 gallons read as 2 gallons, 8 gallons read as 4, 10 gallons read as 6, etc), then the regulator could be lazy and not giving enough "on" cycles. This being the case, your temp gauge is also reading low. If the temp gauge is reading correct then it is the fuel gauge itself, not the regulator.
If you take apart the regulator you will find a bi-metallic arm wrapped with resistance wire and a set of points. You can CAREFULLY bend the fixed point closer to the bi-metallic arm and it will give you more "on" cycles. This will raise the reading on BOTH your fuel & temp gauges.
My experience with the fuel tank sending unit is that the spring wiper arm gets lazy with age and doesn't contact the resistance wire consistently throughout it's travel. This can be measured with an ohm meter. To fix the problem you can CAREFULLY bend the spring wiper arm in such a way as to give it better contact with the resistance wire.
 

tlslack

Bronze forum user
Messages
11
Fuel guage reading Low

Thanks for the info. The temp gauge does appear to be reading correctly, and I susoect the level/sending unit in the tank is good since it was repaced when restored during the 1990's. That leaves the gauge itself. Can the gauge itself be recalibrated or taken apart and to move the needle upward by 3-4 gallons?
 

0neoffive

Gold forum user
CAT Member
Messages
2,853
WARNING WARNING

More WARNING: If you decide to bend the contact arm inside the blowproof cover of the tank sending unit, please note that it is specifically designed to make contact with more than 2 (multiple) coil wraps at all times. This is a redundant requirement to prevent an ARC (boom!). Pay Attention & live to drive away happy. randy
 

wag123

Gold forum user
Messages
130
Thanks for the info. The temp gauge does appear to be reading correctly, and I susoect the level/sending unit in the tank is good since it was repaced when restored during the 1990's. That leaves the gauge itself. Can the gauge itself be recalibrated or taken apart and to move the needle upward by 3-4 gallons?
Just because fuel sending unit was replaced does not mean that it isn't malfunctioning. Remember, we are talking about Lucas electrical components here.
Is this something that just started happening recently?
 

tlslack

Bronze forum user
Messages
11
Fuel Guage Reading Low

I bought the car recently and it was reading low when delivered. The question I STILL have is whether the Jaeger guage can be recalibrated, or simply taken apart to move the needle upward.
 

cadreamn67

Platinum Forum Member
Messages
608
This is not the answer to your question, but is something you might try to confirm whether or not the gauge is properly working. If you have the CAT shop notes, check out page N18. Nice explanation of fuel and temp gauge operations. Includes a way to test the gauge.

The short version is to connect a 9 volt battery to the gauge, Plus side of battery to contact on meter and Negative side of battery to mounting screw. Fuel gauge should read about 8 gallons (2/3 of scale). Might give that a try before taking it apart and trying to move needle. BTW, same 2/3 of scale reading if you apply same procedure to the temp gauge.

Cheers, Gene
 

wag123

Gold forum user
Messages
130
Just short to ground the terminal on the back of the gauge that goes to the fuel level sending unit. You should get a full reading on the gauge. If not, then it is the gauge. I personally have not attempted to adjust/calibrate a fuel gauge.
I did have the same problem as you and found that the fuel level sending unit was not functioning properly. I had to bend the spring arm inside it so that it would make contact with the resistance wire consistently throughout it's travel. Problem solved. Before you do this you should pull the connector off of the sending unit and short it to ground to see if the gauge reads full, it could be that you have resistance built-up in the connections and/or wiring that is causing the gauge to read low. This is another typical Lucas problem.
 
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tlslack

Bronze forum user
Messages
11
Fuel Guage Reading Low

I shorted out the terminal to the sending unit and got ~10.5 gallons. Same thing when shorting out the sending wire in the trunk. The last mark on the guage is about 11.5-12 gallons. Hooked a new 9 volt battery to the same terminal and got 6 gallons instead of the expected 8. I also tried the other terminal from the stabilizer and got 2 gallons. The guage normally reads negative 2 gallons when the ignition is off. Not sure if it should read zero or not. The temp guage reads the expected 200 when applying 9 volts. So it appears the fuel guage is reading about 2 gallons low, which is not so bad, but I would consider adjusting the calibration if it is easy to do myself. Anyone know if, or how it can be done?
 

cadreamn67

Platinum Forum Member
Messages
608
Please understand what I am about to say is all conjecture. As I understand it, the temp gauge and gas gauge are really just volt meters. You will note that both gauges have little dashes at 12 o'clook. I understand those are calibration marks. I am guessing that is where the needle should be when 6 volts (half of 12) is applies across the terminals. (If only one terminal, the mounting stud is also an instrument case ground, or negative, terminal). I am guessing the accuracy of the voltmeter movement is pretty poor so they just "tune" them to the mid-point of the scale, for obvious reasons.

If they are like the old style volt meters I grew up with, there should be a variable resistor (potentiometer) somewhere inside. Most likely it would look like something with a slotted screw driver tab in the middle. By inserting a slotted screw driver tip in that slot and turning it one way or the other, you vary the resistance, which affects where the needle reads.

Again, I have never opened up one of these so I am just guessing. If you want to have a look, proceed at your own risk. If you do and find what I am describing, I would say hook up a 6 volt battery as you did the 9 volt one with the gauge on a workbench. Rotate that potentiometer a little from side to side and try to get it to read at that 12 o'clock dash mark, remove battery, reassemble gauge, and install.

CAUTION, that potentiometer may be frozen in place after all these years and you may not be able to move it or end up breaking it trying. Patience here is a virtue. If it will not move under gentle persuasion, disconnect the battery and perhaps discretely apply a bit of PB Blaster or similar product to free it up after "soaking" for a while. Soak up any spilled liquid off the circuit board with a q-tip. Not sure if what you would be using is electrically conductive or not and we do not want any shorts! As I said this is all conjecture, but you could at least have a look to see. It is what I would do but that is me not necessarily you. My 2 cents. Cheers, Gene
 

wag123

Gold forum user
Messages
130
The fuel and temp gauges are not actually voltage meters in the traditional sense as they are not magnetic. When voltage is applied, a bi-metallic strip is heated and the hotter it gets the more needle deflection you get. Between the stabilizer (which also uses a heated bi-metallic strip) and the gauge you get both voltage AND temperature compensation. In the old days, this was a VERY common method car manufacturers used to achieve reasonable accuracy at a variety of temperatures and input voltages as well as to slow down the gauge's response to rapid fluctuations in fuel level while driving. It was a simple, reliable, effective, and cheap system.
On the back of both gauges there is a small hole in the metal case and inside the hole there is a slotted adjustment mechanism. This is not a variable resistor (or potentiometer), it physically moves the entire internal mechanism back and forth. I suspect that this is used to zero out the gauge reading at rest but if I were you I would short to ground the wire on the fuel level sending unit and adjust the gauge to read "full" instead of the 10.5 gallons it is now reading (this should also bring the reading close to zero at rest on your gauge). If the fuel level sending unit is functioning properly, you SHOULD now get a fairly accurate fuel level indication.
The way that I verified the accuracy of my gauge was to drain and then run the fuel tank(s) down to empty, then I purchased two 2-gallon cans of gas and one 4-gallon can of gas (because I have two 2.5 gallon gas cans and one 5 gallon gas can). First I dumped 2-gallons of gas in it and checked the gauge, then I dumped another 2-gallons in it and checked the gauge, finally I dumped the last 4-gallons in it and checked the gauge. This gave me a good idea of just how accurate my fuel gauge is up to 8-gallons, particularly at the more important lower 2 and 4 gallon fuel levels. Alternatively, since it is much more important that the fuel gauge is accurate with a lower amount of gas in the tank, you might want to consider adjusting the fuel gauge to read 2 gallons with exactly 2 gallons of gas in the tank (this actually makes more sense if you think about it).
 
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tlslack

Bronze forum user
Messages
11
Fuel Guage Reading Low

Thanks for the info. I tried your suggestion to check the accuracy at about 2 gallons, and found it to be very close. For now I think I will leave well enough alone. One other issue I forgot to mention is that the fuel guage sometimes swings wildly while driving, so fast that the needle is a blurr. Thought it might just be engine vibration, since the rear view mirror is a blurr on the highway. Seams to be worse when the fuel tank is low. Maybe a sending unit issue? What do you think? Also, are new sending units still availabe? I did see one on Ebay a couple months age for about $20.
 

Cal44

Gold forum user
Messages
428
New sending units are not available unless someone is hording an NOS.

Palo Alto Gauges does sending units and you could send you fuel gauge at the the same time for calibration to the sender. They have a website.
http://paspeedo.com/
 

wag123

Gold forum user
Messages
130
Thanks for the info. I tried your suggestion to check the accuracy at about 2 gallons, and found it to be very close. For now I think I will leave well enough alone. One other issue I forgot to mention is that the fuel guage sometimes swings wildly while driving, so fast that the needle is a blurr. Thought it might just be engine vibration, since the rear view mirror is a blurr on the highway. Seams to be worse when the fuel tank is low. Maybe a sending unit issue? What do you think? Also, are new sending units still availabe? I did see one on Ebay a couple months age for about $20.

I think that the gauge itself could be bad. Because of it's design, you should not be having any kind of wild/fast swings. The heated bi-metallic strip reacts very slowly to changes in input.
 

drbill

Gold forum user
Messages
124
Water temp

This is not the answer to your question, but is something you might try to confirm whether or not the gauge is properly working. If you have the CAT shop notes, check out page N18. Nice explanation of fuel and temp gauge operations. Includes a way to test the gauge.

The short version is to connect a 9 volt battery to the gauge, Plus side of battery to contact on meter and Negative side of battery to mounting screw. Fuel gauge should read about 8 gallons (2/3 of scale). Might give that a try before taking it apart and trying to move needle. BTW, same 2/3 of scale reading if you apply same procedure to the temp gauge.

Cheers, Gene
Gene,
My water temp gauge reads low (fuel gauge reads fine). This 9-volt test method, when you say plus side to "contact on meter," do you mean the connector on the back of the gauge itself? When I saw that in the Shop Notes, I wasn't sure about what he meant by "meter."
Also, can the test be done with the gauge removed from the car?
Thanks,
Bill
 

cadreamn67

Platinum Forum Member
Messages
608
Hi Bill,

My gauge is currently out of the dash so I tried the test. Could not get the gauge to read anything. Maybe my gauge is bad too. But when I put the nine volts across the back of the two terminals, the gauge read full scale. I have not looked at the circuit wiring in the car to see if having it in dash with the instrument voltage regulator in the circuit with the key off would make a difference.

There is another test (I believe not in the Shop Notes) that you can do with the gauge in the car. The sender in the intake manifold is a resistor whose value changes with temperature. You take the wire off the sending unit and connect a resistor of certain values between the wire and a ground on the manifold. Then check what the gauge reads with the key on. There is a table to tell you what the gauge should read. If the gauge reading matches the table, the sender is bad.

I can look for that if you would like. Or perhaps someone else on the forum has it and can post it. For some reason, I am thinking a guy by the name of Mark Olson first published it.

Gene
 

drbill

Gold forum user
Messages
124
Hi Bill,

My gauge is currently out of the dash so I tried the test. Could not get the gauge to read anything. Maybe my gauge is bad too. But when I put the nine volts across the back of the two terminals, the gauge read full scale. I have not looked at the circuit wiring in the car to see if having it in dash with the instrument voltage regulator in the circuit with the key off would make a difference.

There is another test (I believe not in the Shop Notes) that you can do with the gauge in the car. The sender in the intake manifold is a resistor whose value changes with temperature. You take the wire off the sending unit and connect a resistor of certain values between the wire and a ground on the manifold. Then check what the gauge reads with the key on. There is a table to tell you what the gauge should read. If the gauge reading matches the table, the sender is bad.

I can look for that if you would like. Or perhaps someone else on the forum has it and can post it. For some reason, I am thinking a guy by the name of Mark Olson first published it.

Gene

Gene,
I'll try again using both terminals. I have a spare water temp gauge and only connecting to one terminal with the 9 v, I got nothing, just like yours.
The problem is probably in the sender, but I hate to just swap out parts before testing.
Thanks,
Bill
 
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