Show whole topic
Jul 07, 2014 8:42 am
Jul 07, 2014 8:42 am
Subject: Re: fuel gauge
This job took me much longer than I thought but finally the solution and explanation for the problems:
On the sketch the upper diagram shows how an original (3 connector) tank sender (a potentiometer) would have worked with the still original fuel gauge in my car. The fuel gauge, when connected to 12V and ground only, shows half full. The signal from the sender is between negative (ground) and +12V. (Works accordingly on cars with positive ground). This pulls the needle up or down accordingly. It seems all modern available tank senders only have 2 connections: one is ground and the second signal, no connection to +12V. Might be for safety reasons not to have power from the battery directly on the sender if it shortcuts by fault.
The sender I found in my car is of the modern type: a tunable resistor (potentiometer) against ground only. This would work fine with other fuel gauges like the one from MGA and other cars but not with the one in my Lagonda. Connected directly to the original Lagonda fuel gauge it would display from half full to empty only when it goes from top to bottom in the tank. But there was another problem with this sender: mechanically when in low position the tank would be still half full because the drop arm is so short and a longer one can not be fitted because the fuel tank is divided in compartements leaaving not much space for the sender drop arm. And the tank was not grounded properly, so the signal would be erratic.
The solution: I found a universal tank sender where the potentiometer can be adjusted to sit in the middle of the fuel tank. This allows to have a total travel of the 26cm needed with a drop arm length of only half of that. It came with a fuel gauge that would work fine with it but looks to modern (see parts not needed on the right side of the image "fuel gauge replacment"). So it needed an electronic that converts the signal to fit to the old gauge. This is shown in the lower part of the sketch: an operation amplifier with adjustable gain (defined by the 100k resistor and 10k potentiometer). The gain is set once to match the sender unit to the fuel gauge reading. The current through the sender is only 2.5mA and limited by the 5k resistor. I prefer to have low currents in the tank. I also have put a fuse of 32mA in the connection to the tank sender, just in case something goes wrong. I also had to make a new holding plate for the new sender because the one that came with it has 5 holes not six like on my fuel tank.
It works fine tested on the bench, the reading should now quite well correspond to the real fuel level. Also works in the car but I have not tested it yet under real driving conditions.
I will not tell you how many hours this took me but I now understand why so many drivers are using a wooden dipstick to check the fuel level.
Attachments:
This job took me much longer than I thought but finally the solution and explanation for the problems:
On the sketch the upper diagram shows how an original (3 connector) tank sender (a potentiometer) would have worked with the still original fuel gauge in my car. The fuel gauge, when connected to 12V and ground only, shows half full. The signal from the sender is between negative (ground) and +12V. (Works accordingly on cars with positive ground). This pulls the needle up or down accordingly. It seems all modern available tank senders only have 2 connections: one is ground and the second signal, no connection to +12V. Might be for safety reasons not to have power from the battery directly on the sender if it shortcuts by fault.
The sender I found in my car is of the modern type: a tunable resistor (potentiometer) against ground only. This would work fine with other fuel gauges like the one from MGA and other cars but not with the one in my Lagonda. Connected directly to the original Lagonda fuel gauge it would display from half full to empty only when it goes from top to bottom in the tank. But there was another problem with this sender: mechanically when in low position the tank would be still half full because the drop arm is so short and a longer one can not be fitted because the fuel tank is divided in compartements leaaving not much space for the sender drop arm. And the tank was not grounded properly, so the signal would be erratic.
The solution: I found a universal tank sender where the potentiometer can be adjusted to sit in the middle of the fuel tank. This allows to have a total travel of the 26cm needed with a drop arm length of only half of that. It came with a fuel gauge that would work fine with it but looks to modern (see parts not needed on the right side of the image "fuel gauge replacment"). So it needed an electronic that converts the signal to fit to the old gauge. This is shown in the lower part of the sketch: an operation amplifier with adjustable gain (defined by the 100k resistor and 10k potentiometer). The gain is set once to match the sender unit to the fuel gauge reading. The current through the sender is only 2.5mA and limited by the 5k resistor. I prefer to have low currents in the tank. I also have put a fuse of 32mA in the connection to the tank sender, just in case something goes wrong. I also had to make a new holding plate for the new sender because the one that came with it has 5 holes not six like on my fuel tank.
It works fine tested on the bench, the reading should now quite well correspond to the real fuel level. Also works in the car but I have not tested it yet under real driving conditions.
I will not tell you how many hours this took me but I now understand why so many drivers are using a wooden dipstick to check the fuel level.
Attachments: