troubleshooting C1 fuel gauge
07-29-2005, 08:53 AM
#1
Intermediate
Thread Starter
troubleshooting C1 fuel gauge
I put this together from what I worked out with info from several sources and my own trial & error. I hope it might be useful for someone when they search the archive one day.
It took me a while to work out how the C1 fuel gauge works. I’m not an electrician but have a good understanding of basic electric circuits.
Inside the C1 fuel gauge there are actually 2 circuits that fight each other to produce a reading on the gauge. Inside the gauge there are 2 coils at 90 degrees to each other with an armature at the intersecting point of the coil axes. The needle is fixed to the armature. One circuit has full current all the time when the ignition circuit is on and earths through the housing of the gauge. This circuit pulls the needle up to full. The other circuit earths via the brown wire connected to the “S” terminal on the back of the gauge and then through the fuel tank sender. This circuit pulls the needle down to empty. The current in this circuit is determined by the resistance of the sender unit. This should be 0 ohm at empty and about 30 at full, so there is maximum current at empty and minimum at full. They both get their power through the pink wire that connects to the back of the gauge. I believe that the advantage of this balanced type gauge is that the reading does not depend on the voltage in the system. If the voltage drops, both circuits have the same relative drop in current so the needle doesn’t move. I can’t see any other advantage.
To work it out follow this sequence.
1. Disconnect the brown wire from the “S” terminal on the back of the gauge. This simulates a full tank (zero current). Turn the ignition on and the needle should show full. Turn the ignition off again.
2. With the brown wire still disconnected, connect a jumper wire from the “S” terminal to an earth point (somewhere on the steering column is easy). This simulates an empty tank (max current). Turn the ignition on and the needle should show empty.
If this test is OK, there is a problem with the sender unit circuit. To work this out you really need to get the sender out of the tank and measure the resistance at empty and full. This is a bit of a job. You need to take the cover off the tank and undo the 5 screws holding the sender in. You will need a new gasket because the old one will break for sure. You can get a gasket for a few bucks from any of the usual parts people. Alternatively you could measure the resistance to earth on the brown wire at the gauge (after disconnecting from the gauge) when you know the tank is full and again when the tank is near empty.
Hope this is useful to someone sometime. If it doesn't make sense just ask me.
John
blu61
http://www.users.bigpond.com/blu61/
It took me a while to work out how the C1 fuel gauge works. I’m not an electrician but have a good understanding of basic electric circuits.
Inside the C1 fuel gauge there are actually 2 circuits that fight each other to produce a reading on the gauge. Inside the gauge there are 2 coils at 90 degrees to each other with an armature at the intersecting point of the coil axes. The needle is fixed to the armature. One circuit has full current all the time when the ignition circuit is on and earths through the housing of the gauge. This circuit pulls the needle up to full. The other circuit earths via the brown wire connected to the “S” terminal on the back of the gauge and then through the fuel tank sender. This circuit pulls the needle down to empty. The current in this circuit is determined by the resistance of the sender unit. This should be 0 ohm at empty and about 30 at full, so there is maximum current at empty and minimum at full. They both get their power through the pink wire that connects to the back of the gauge. I believe that the advantage of this balanced type gauge is that the reading does not depend on the voltage in the system. If the voltage drops, both circuits have the same relative drop in current so the needle doesn’t move. I can’t see any other advantage.
To work it out follow this sequence.
1. Disconnect the brown wire from the “S” terminal on the back of the gauge. This simulates a full tank (zero current). Turn the ignition on and the needle should show full. Turn the ignition off again.
2. With the brown wire still disconnected, connect a jumper wire from the “S” terminal to an earth point (somewhere on the steering column is easy). This simulates an empty tank (max current). Turn the ignition on and the needle should show empty.
If this test is OK, there is a problem with the sender unit circuit. To work this out you really need to get the sender out of the tank and measure the resistance at empty and full. This is a bit of a job. You need to take the cover off the tank and undo the 5 screws holding the sender in. You will need a new gasket because the old one will break for sure. You can get a gasket for a few bucks from any of the usual parts people. Alternatively you could measure the resistance to earth on the brown wire at the gauge (after disconnecting from the gauge) when you know the tank is full and again when the tank is near empty.
Hope this is useful to someone sometime. If it doesn't make sense just ask me.
John
blu61
http://www.users.bigpond.com/blu61/
Last edited by blu61; 07-29-2005 at 09:06 AM.
