Vdo Fuel Gauge Resistance
Urbano Bozman
The resistance of your fuel sender should match that of your gauge. For instance the resistance of the stock gauge in a 55 Chevy has a value of zero ohms when empty and 30 ohms when full. The following list is for stock gauges and some popular aftermarket brands:
If you have a 0-30, 0-90, and 10-180 Ohm sender then most likely your sender lead wire is grounding out somewhere. Or possibly the center terminal on the sending unit itself is grounding out against the body of the car.
If you have a 240-33 or 73-10 Ohm sender then most likely your sender lead wire is grounding out somewhere. Or possibly the center terminal on the sending unit itself is grounding out against the body of the car.
Our tanks are set up for the standard SAE 5-hole bolt pattern used by all of the aftermarket gauge manufacturers. The 5-holes are not evenly spaced. Two of the holes are farther apart than the other holes. If you are holding the sender with those two holes closest to your body, the hole directly across is known as the index hole. If the float swings to the right, it is a right-swing sender . If it swings to the left, it is a left-swing sender.
Fuel senders have a rheostat that provides a resistance to ground. As the fuel arm moves from the empty to the full position a contact is moved across the board on the rheostat which varies the resistance seen by the fuel gauge. For example a 69 Camaro has a 0-90 ohm fuel sender. When the sender is at the empty position the rheostat is not creating any resistance to ground (zero ohms). When the fuel sender is in the full position the rheostat is creating 90 ohms of resistance to ground. Because the gauge is reading a resistance to ground having proper grounding of the sending unit is essential to get an accurate reading on your fuel gauge.
A fuel sender can be bench tested with a multimeter. With the multimeter set to the Ω setting put the red probe on the post that is the gauge lead and put the black probe on the ground post or on the body of the fuel sender. Record your readings with the fuel sender in the empty position and in the full position. Those two numbers will tell you the ohm range is of your sender.
The first step would be to make sure that you have the proper ohm range fuel sender to match your gauges. For example a 73-10 ohm fuel sender would read backwards on a 0-90 ohm fuel gauge. If you find you have the correct ohm range fuel sender make sure that the float arm is installed on the correct side. The front of the rheostat shows which side the float should stick out from. If the float arm is installed on the other side the gauge will read backwards. Also if the rheostat is installed upside down the gauge will read backwards.
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Did you get the sender in the right 2 pin positions? Did you get it fully plugged in the bottom and locked again? Did you test at the socket at the front fender for ohm signal? Is that socket corroded or damaged?
Also there should be battery voltage at the black/light blue wire at the sender. this voltage comes from pin # 17 at the PDC. The voltage should be less on the dark blue/white wire that comes out of the fuel sender and goes to pin # 26 in the C3 connector at the PCM. The change in resistance of the fuel gauge changes the voltage to the PCM. The PCM then sends the value over the CCD to the instrument cluster circuit board where an algorithm is applied to find the correct needle position. If you do not have voltage at the sender or at C3 pin #26 it may be a bad connection or a bad wire.
The original had the pins from the unit in the middle 2, so I did the same as the original. I'll try the right 2 pins.. Socket to the wiring harness looked okay, the Molex type pins had locked into the inner harness
So there is no voltage at the PCM, even jumping to 2 leads from the harness did not make the gauge move. The harness was connected tight to the sending unit. The truck was taken to a garage, and they traced it to the PCM, So, I need a new PCM? Is there a way to program the PCM and the VIN.
Since the BK/LT BLU wire was at joint connector #1 in the power distribution center I assumed it was battery voltage. After reading this I took another look at the wire diagrams. Pins 15-18 go to ground and that's why there is no voltage. Sorry, that's what happens when I assume.
The voltage does come from the PCM Plug #3 pin #26 dark blue/white wire. I was doing an oil change on the truck today so while I was under there I took some readings at the connector by the left front wheel well.
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My sender took a poop on me, aswell as the gas tank so i have a fuel cell coming, it has a gm style sender 0 ohms= empty 90 ohms =full, i need to find out the ohm range for a 77 280z, any help finding the answer would be awsome, i would measure mine but it doesnt work
I've tested and researched the senders of quiet a few Datsun models. Every one, so far, has been in the same range listed above. There may be others that are different, but I haven't found one. I'll bet a buck every S30 and 510 falls in that range.
Don't know if this helps... I installed a VDO gauge in my 510. Of course it would have read backwards, so I flipped the sender upside down. Simple and works pretty good. I haven't tried this on an S30, but it should work. Just bend the arm to fine tune.
There is a process to do a "custom setup" but that involves setting the empty with an actual empty fuel tank, and then filling the entire tank and setting the full reading. Since I don't happen to have a gas station in my driveway, that would involve draining the tank into gas cans, and then refilling the whole tank.
I'm not much of an electrical guy, but is there a way to trick the gauge into thinking it's reading a 0-90 Ohm sender by simply using a resistor in the sender line (e.g a resistor that reduces the resistance by 33%, I guess)?
It might work out if you pulled the sender (or senders if you have two in your model) and held them right side up to simulate empty and then flipped them upside down to simulate a full level. I think they get their resistance reading from the simple float inside the cylinder so I would try that first. It might only get you 'close enough' but would only take a couple of minutes.
I have been frustrated that this generation of BMW senders are neither the most elegant nor robust bits of hardware ever made. At least, they are blessedly easy to get at... especially in a 4 door with no seats!
I'll note that this is a digital gauge (showing % full) and I'm already driving a car without a working odometer or speedo, so when I'm doing long drives to events I'm already having to calculate speed based on RPM. And since this may also be a stage rally and/or track car at some point, I'd just as well have the gauge read correctly so I don't have to think or remember things, lol.
yeah, I have the paper copy of that sitting on the table in front of me, lol (I bought the gauge new). The "right way" is to do the programming with the full and empty fuel tank (stated in the instructions). I'm just trying to avoid getting a bunch of fuel in cans to do that, since the car isn't terribly mobile right now.
Thanks, Jeff.
The 10K digi pot will only give 4 or 5 set points (on 8-bit, what ever that is) in the 0 - 190 ohm range. Nowhere near sufficient. Imagine that your car fuel gauge only gave, full, 3/4, 1/2 1/4 and empty reading, no value in between.
I am hoping to have at least 30 set points over 200 Ohms.
I gather that it will allow 100 mA through source & drain.
Will a 0 - 5 Volts from a microcontroller not be able to drive the digi pot?
It may be possible by using a PWM signal to sink current. I believe Arduino can sink 40mA. By changing the pulse width the AVERAGE current will vary. If the gauge responds to this in a linear manner you could try this method but make sure the voltage does not exceed 5V. It would be an advantage also to know or measure the source impedance to make sure the microprocessor is not asked to sink more than its capability.
You still have to have some means of telling Arduino or whatever how full the tank is.
For that particular component you would need to provide offset resistors. But there are many varieties of digital potentiometer that are available and it is likely that you would be able to find a device more directly suited to your application. See, for example:
AD5171 Digital Potentiometer
The digital potentiometer requires 3-5v to drive it. If your question is whether the 5v output from a MCU board would be adequate, then the answer is Yes. But the MCU needs a DC supply, and the 12v from a motor vehicle battery is not suitable, as it can range up to 14 volts or more, and is very noisy. Whether it is for the digital potentiometer or for the MCU, you will need some form of power converter.
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