Fuel Light Thermistor Repair

[bikerphil]

Part 1: Removal

Ok, we are going to rebuild the thermistor unit, used on the Specials to trigger the low fuel warning light. The way it works is when the fuel level drops below the level of the component, it heats up and completes the path to ground, then the light will illuminate. When fuel is added, the thermal resistor cools and the path to ground is broken, turning the light off.

First, you will need to remove the assembly from the tank. Unplug the connector, drain the tank and remove the 4 bolts with a 10MM socket. Pop the unit out and put it on the bench. Unsolder the wire that is connected to the small canister but leave the other end alone.



Use a needle nose vise grips to slightly unbend the 2 tabs holding the canister in place and remove it.



Now grab a small block of 2X4 and drill a hole in the narrow side using a 25/64” bit, about 1” deep.
This wood block may be placed in a vise to safely hold the canister during the operation without damaging it.



Now with a small flat headed punch, carefully tap around the lip uncrimping the thermistor insulator from the canister.



Unsolder the opposite end of the canister and pull the thermistor out. The thermistor may have already fried in half, if so, remove the remaining pieces.



Completely remove the old component by unsoldering it from the insulator.

For installation, see part 2. (coming soon)

[bikerphil]

Part 2: Installation

The original thermistor is a axial type. They seem to be unavailable now so we are going to use a radial type with a little creative bending of the leads.

Thermistor type is NTC 1K ohm. A couple places that have them are Digikey (P/N 02-N102-1) or Mouser electronics supply houses.



Bend the thermistor in such a way so it resembles an axial type, making sure neither lead will contact the side of the canister.



Slide the shorter lead in the end insulator cap and solder it using very little solder, just to temporarily hold it. You will still need to solder the insulated wire on there also a bit later.



Slide the assembly into the canister with the long lead protruding thru the hole and solder it. Cut off the remains of the lead.

Put the canister back in the wood block and using the small punch, crimp the lip back down over the insulator.



Install the canister back in it's bracket and crimp the clips back around it holding it securely. It shouldn't be loose in any way, it needs to make a solid electrical contact thru the bracket.

Now, solder the wire back on the end and also make sure the thermistor is adequately soldered.

You can test the unit prior to installation 2 different ways. First you can hook an ohm meter (2K setting) to the wire harness and should get about a 1K ohm reading. Second, you can plug the harness back into the bike's harness and turn on the key. Wait a minute or so and your red warning light should illuminate. If the unit passes these tests, go ahead and install back on the fuel tank.



Thats it, hopefully this is clear to understand. Good Luck!

[bikerphil]

A few extra photos...

[DEEBS11]

Nice repair Phil.

[bikerphil]

Thanks man. I think next I'll do one for the Standards fuel gauge regulator repair. Sorry, no you tube video.

[Snarf2369]

Awesome! Thanks for posting this fix! I can finally have a working low fuel light. Appreciate ya!