Can Somebody Help Explain The Fuel Sensor Circuit?

[durtkillon]

So I'm waiting on a stator to come and thought I'd work on some of the other little nit pick stuff on the bike while I have the tank off and harness exposed. One thing that on the bike that I suspected doesn't work correctly are the low fuel and possibly the FI indicator lights. Neither light has ever come on. If I unplug the fuel level sensor, it does not illuminate. If I bridge the circuit, it still does not illuminate.

When I test the fuel sensor with a full tank, the switch is closed (makes contact to ground). I have not however tested the opposite condition as I currently have a very full tank. When looking at the diagram, it seems that indicates the switch is failing. I suspect it should be normally open when the tank is full and closed to illuminate the LED when the tank is nearly empty.

The diagram below is confusing to me. The power seems to come from the ECU, but then is passed to both sides of the diode. If I trace the path of least resistance, I'm not sure why the diode would ever light up, since you can close a circuit to ground through the 65ohm resistor without ever passing through the diode that has a 750 ohm resistor on it.

I do not understand this circuit. I believe the level sensor is failing (closed to ground), but I suspect that the LED is also bad. I've tried test them, but they do not illuminate with my diode tester. I have not tested the 65ohm resistor, but I do not understand it's purpose.

 

[Trenchcoat85]

BTW, the 68 ohm resistor is infamous for breaking the leads.
It may be hiding in your wire loom close to the regulator. I'm not sure about TXCs.

I don't get it either. Mine has never worked very good- right out of the crate. Comes on with 70 miles left now.

 

[Trenchcoat85]

My FI LED flashes once about 0.5 seconds after the bike dies.

 

[durtkillon]

My FI LED flashes once about 0.5 seconds after the bike dies.

Trenchcoat, Does you level switch open with a full tank or is it closed?

 

[Trenchcoat85]

Trenchcoat, Does you level switch open with a full tank or is it closed?

I dunno- kinda wondering myself. I assumed the reed was closed at high fuel levels but that doesn't jive with the schematic. (btw, there very well could be an error in the schematic... I found a few).

let me go out into the smoke (lotsa fires around here) and take an ohm reading. I think Johnrg has one on the bench, too.

edit: with 14 miles on a previously full tank, I measured the unpluged reed switch as closed (0.4ohms; shorted meter leads are 0.3ohms). My low fuel LED was off when I started it up.

 

[durtkillon]

I

I dunno- kinda wondering myself. I assumed the reed was closed at high fuel levels but that doesn't jive with the schematic. (btw, there very well could be an error in the schematic... I found a few).

let me go out into the smoke (lotsa fires around here) and take an ohm reading. I think Johnrg has one on the bench, too.

edit: with 14 miles on a previously full tank, I measured the unpluged reed switch as closed (0.4ohms; shorted meter leads are 0.3ohms). My low fuel LED was off when I started it up.

Hmmm. Does the light come on if you start it up with the reed switch unplugged?

It looks like the 68 ohm resistor has current whenever the bike is running and not empty. Add engine and rectifier/regulator heat and I can see why they cook.

KP

 

[Trenchcoat85]

I


Hmmm. Does the light come on if you start it up with the reed switch unplugged?

It looks like the 68 ohm resistor has current whenever the bike is running and not empty. Add engine and rectifier/regulator heat and I can see why they cook.

KP

weird that you asked this 'cause I just came back from running the test again with the bike upright this time (not just on the kickstand). Same results (full tank, led off, 0ohms).

However I noticed that there is only one internal lead- and the "body" is used for the other conductor, and its black lead has a ring connector on it. so I grounded the body- no LED. I disconnected the ring connector and the LED lit up. I grounded the ring connector against the frame and the LED went off. hmmmmm.

Also, to answer your question: Yes, when you unplug the reed switch the LED comes on. ground the black wire and it goes off.

The 68ohm resistor is unsupported and vibration breaks the lead off- at least, this is the more common failure mode.

 

[R_Little]

I believe mine broke off once. I switched the pump housing so I don't remember.

Anyway, when it worked the light came on 1 mile from running out of gas anyway.

Now it seems to come on with about 8 miles left.

 

[durtkillon]

Both resistors tested fine and the level sensor seems to be OK. The LED's are both blown on my bike. Perhaps a previous owner applied 12V to them? I ordered new ones.

 

[Trenchcoat85]

a little late (nice to be back, too!) but you pretty much could use almost any LED if you could jury-rig it into the rubber doohickey.