jshireaz

OK. I'm stuck.

Vehicle is a 1998 Honda Civic LX.

The low fuel light is not coming on.

Fuel gage is working fine.

All the other lamps, that are supposed to, come on during the initial check when the key is moved to ignition.

Lamps for turn signal, charging system, seat belt, oil pressure, check engine, SRS all work fine.

What I've done so far:
* Followed the factory service manual to verify the fuel sending unit is working.
* Tried the low fuel lamp in another position, it works fine.
* Tried another lamp in the low fuel lamp position, it doesn't come on.
* Removed the sending unit from the fuel tank and simulated a minimum fuel condition (move the float all the way down. While doing this, I tried several lamps in the low fuel lamp position, none of them work. They work in other positions. Measured the voltage across the contacts for the low fuel lamp. ***** ~ 12V *****
* Cleaned the contacts on the circuit at the low fuel lamp position
* Attached two leads to the lamp and connected them to the low fuel lamp position terminals on the cluster. Moved them around to try and make contact in case something is wrong with the pad.

For the life of me, I can't understand why there is 12V present, with a good bulb, and no light.

Any ideas? I'm baffled.

Thanks much,
- J

deschlong

iTrader: (8)

Hmm, this response isn't going to help you right away, but I am going to chew on it for a bit ...

One thing that occurs to me right away is that I don't believe it's the float location that sends the signal for the low fuel. I was under the impression that it uses a thermistor - and the light comes on when its temperature is high (not surrounded by fuel)

Aha - just checked the ETM:
"A thermistor is mounted in the fuel tank unit. When
the thermistor is cool, its resistance is very high."
.... etc., etc., see the ETM page 75

D16SiHatch

op, this should help ya out

jshireaz

Well, that would explain what the other doo-dad was besides the potentiometer. I was wondering about that. Problem still remains though. The terminal has 12 V but the lamp isn't lighting.

jshireaz

D16SiHatch, thank you very much for taking the time to post these instructions. This is basically what I meant when I said I followed the factory service manual instructions. The sensor seems to be functioning correctly as there is 12V across the low fuel light lamp terminals but for the life of me, I can't figure out why the bulb isn't lighting.

D16SiHatch

you should always have +12v there. the thermistor is what provides the ground to complete the circuit.

"A thermistor is mounted in the fuel tank unit. When the thermistor is cool, its resistance is very high. When the thermistor's temperature increases, its resistance decreases. Fuel in the fuel tank transfers heat away from the thermistor fast enough to keep it cool so the thermistor's resistance stays high and the low fuel indicator light does not come on. When the fuel level drops below the thermistor, the thermistor's temperature increases. Without the fuel to cool it, the thermistor's resistance decreases, allowing current to flow through the low fuel indicator light and the thermistor to ground, and the low fuel indicator light comes on."

deschlong

iTrader: (8)

OP: Have you checked for connectivity b/w the GRN/YEL at the sending unit and GRN/YEL *wire* at the gauge cluster?

The fuel light lamp is a special bulb/connector with a built-in resistor (IIRC) ... if this resistor has failed then ... well it should probably still work I suppose, but it also might cause problems, too. This may sound weird, but I have seen odd behaviour with my multimeter at the lamp connectors behind the gauge cluster before, but found that all my bulbs worked properly ... so, yes I agree it's strange that you had 12V across the lamp connector but not so much that I wouldn't try other avenues to try to locate the problem.

deschlong

iTrader: (8)

Scratch that about the 12V ... what D16SiHatch said makes sense.

jshireaz

Click. OK. Now it's making sense. There is always going to be 12V but if the resistance is high, there will not be enough the current flow to light the bulb. I need to re-examine the thermistor.

Unfortunately, dear readers, you'll have to wait till next weekend for the exciting conclusion.

jshireaz

Good detective work gentlemen. It was indeed the thermistor. Always reads open circuit.

deschlong

iTrader: (8)

Great. Now that you've diagnosed it -- do you want to buy my fuel meter? It's from a 1996 Civic. I paid $40 for it ... turned out not to work in my '94. I have no use for it, and no reasonable offer will be refused. I can include the original bill of sale.

jshireaz

I found one for $50 but my wife said it wasn't worth it. How about $40 including shipping for yours? Are you confident the '96 and '98 are compatible?

Cheers,
- J

deschlong

iTrader: (8)

Great! Let's discuss further through Private Messages.