Fuel gauge problem
 

The fuel gauge on my 91 Camry is stuck on full. Is this more likely to be a connection problem (fuse or wiring) or a malfunction of the sender gauge itself? If possibly a fuse issue, where is the fuse? How can I locate the source of the problem? I'd rather not remove the fuel tank unless absolutely necessary. Thanks.

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The fuel tank sender can be accessed by removing the rear seat lower cushion, the cushion pulls up from the front. Reach under the front of the seat at the sides and pull up.

Removal of the seat will expose a small access plate bolted to the chassis. Remove the plate to reveal the access port for the fuel pump, sender and wiring.

To test the sender disconnect the wires going to the sender inside the fuel tank (Yel/Blue). Once disconnected turn on ign key and determine if the fuel gauge now reads empty.

Find a 3.4 watt light bulb and connect between the Yel/Blue and Brown wires. Turn on ign switch, gas gauge should move slowly to full indication.

Measure ohm resistance between the above wires, should be approx 3 ohms when tank is full and 110 ohms when tank is empty.

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Thanks!

I think I neglected to mention that this is a station wagon. The rear seat bottom (which is split into two) seems to be hinged at the front. There seems to be a catch at the back, but I don't know how it works, and can't release it. Any ideas? Do I want to lift the left, the right, or both parts?

I got the seat up. There is a strap you pull on. Duh!

The gauge goes to zero with the wires disconnected and ign on, and I measured the resistance between Y/Bl and Br, and it is 1518 ohms. Apparently the sender is shot. I presume it's just a matter of removing the 5 or 6 bolts and replacing the unit.

I've pulled the sender out. I notice that besides the float that controls the variable resistor, it also has some electrical component encased in a plastic shell, which is immersed in petrol. Does that activate the "gas low" light?

Out of curiosity I opened up the sender. The variable resistor is fine, with a resistance that varies from about 0 to 110 ohms. The signal is not getting out to the connector, though. Since I can't trace a direct connection from the Y/Bl wire to anything inside the sender, I'm wondering if the resin-encased lump on the top contains some active electronics, which I can't do anything about. Alternatively, it could be just sealing the unit to make it air-tight, in which case there must be a broken conductor somewhere.

Yes, the lump is a thermistor. This changes resistance based on temperature. This is what signals the low fuel light on the dash. The part changes resistance with temperature.

The fuel gauge should work even if the thermistor fails. The part has no active electronics.

Yel/Blue goes to the fuel gauge. Brown is ground.

Yel/Red to the thermistor. White/Black is ground.