Low Fuel Warning Light Out - Wrist Twisters
 
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post #1 of 11 Old 12-04-2012, 09:43 PM Thread Starter
Why's everything on fire?
 
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Low Fuel Warning Light Out

Discovered mine wasn't working the hard way. Looking at the parts fiche seems to indicate that the light source is an LED, not a replaceable bulb. Anyone have any suggestions as to what I should look at first or how I should troubleshoot this?

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2002 Honda 919
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post #2 of 11 Old 12-04-2012, 10:42 PM
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Looking at 19-18 in the service manual, it states to use a jumper wire on the fuel pump plug and if the light comes on, replace the fuel pump. If it doesn't then you'll have to replace the combination meter (gauges).

Lets hope it doesn't come to that. However, if it does, you can crack open the gauges and see if you can do a desolder, swap, and resolder.

Idk what type of LED bulb it would be. Sorry I'm not too much more help.

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post #3 of 11 Old 12-05-2012, 01:06 AM
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How about just keep an eye on mileage and call it good.

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post #4 of 11 Old 12-05-2012, 01:49 AM Thread Starter
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Yeah, I could do that, but I prefer for all systems to be operational unless there's a very good reason to go otherwise.

Further, I'm suspecting that it's probably the instrument pod. In fact, I was already looking for one because last month I had to replace a burned out backlight bulb on the thing and found this. The outer covers were scratched but not cracked or anything, but this is what was inside:







Anyone know if the 04-07 pods will work with the 02-03 computer and wiring?

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post #5 of 11 Old 12-05-2012, 02:03 AM
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Oh that sucks CB700s... Wonder if any of the circuit board printed paths are broken due to that. Hard to tell from the side we can see.

I remember there being a wiring snafu relating to the koso gauges coming from an 02/03 to an 04+. If memory serves me right, the 04+ has an extra wire for the OEM plug which might of been the ground wire? Sorry, its 1am here, my brain is overloaded on caffeine, deprived of sleep, and stressing about graduate finals. I'd suggest looking for the thread that talks about the wire differences in a thread relating to the Koso and go from there. I'd try to find it for you but I can't afford to get side tracked for too long lol.

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post #6 of 11 Old 12-05-2012, 02:17 AM
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Screw it, I did some searching because I need a break. My current assignment is just pissing me off.

I had the wiring backwards apparantly. the 04+ has 1 less wire than the 02/03 as per: https://www.wristtwisters.com/forums/...tml#post528999

Bout all I could find at this point.

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post #7 of 11 Old 12-05-2012, 02:24 AM Thread Starter
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Just throwing out a wild guess, I'd bet that extra wire is probably related to power or ground for the clock and eliminated a duplicate or something.

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post #8 of 11 Old 12-05-2012, 08:38 AM
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The low fuel warn system is quite simple -- there is a thermistor in the tank heated by battery voltage (through a ballast resistor on the instrument cluster board) and cooled by contact with fuel. Under those conditions the voltage reading at the Brown / Black wire should be about 2 volts or so less than the system voltage. When the fuel drops sufficiently to expose the thermistor to air it heats up, decreasing it's resistance. Under those conditions the voltage will drop to roughly 1/2 of the system voltage, which will trigger the low fuel warning LED. This function is performed by the processor IC in the instrument cluster and is not servicable.

To troubleshoot this disconnect the brown instrument connector in the headlight shell and measure the resistance of the thermistor between the Brown / Black wire and ground on the main harness side of the connector: at an ambient temperature of 25C (77F) it should read 1000 ohms plus or minus 5%. If it does not read in this range check it at the fuel pump connector to be sure there is not an open in the wire between the two connectors. If it reads normal the problem is almost certainly in the instrument cluster and there is little you can do past checking for broken traces or for a fried ballast resistor.

Rob

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post #9 of 11 Old 12-05-2012, 10:29 AM
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Great info Rob! I have a couple of questions.

Could one replace the ballast resistor if that indeed was bad?

What about repairing a broken trace? I've done this on printed circuit boards and is not difficult to do provided there's enough room to work with and you're careful to not short anything out. Would it be a different situation due to the constant vibs of being on a motorcycle?

How would one test if the LED is still good (I know the odds of it failing are very low) when its still connected to the board? Would the only ideal solution be to desolder the led and test it separate from the board?

Could one figure out which chip the LED indicator is processed through and replace the chip or are there too many variables?

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post #10 of 11 Old 12-05-2012, 12:18 PM
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Quote:
Originally Posted by Pvster View Post
Great info Rob! I have a couple of questions.

Could one replace the ballast resistor if that indeed was bad?
The ballast resistor (actually, two 36 ohm 3 watt resistors in series) are standard axial leaded types -- easily replaced by clipping the leads as close to the resistor body as possible and twist / solder the leads of the replacement to them. There may be some sort of coating on them which will have to be removed before soldering.
Quote:

What about repairing a broken trace? I've done this on printed circuit boards and is not difficult to do provided there's enough room to work with and you're careful to not short anything out. Would it be a different situation due to the constant vibs of being on a motorcycle?
It will not be a problem. Vibration will not have any more effect on a repair than on any other traces on the board. That is as long as it is properly done.
Quote:

How would one test if the LED is still good (I know the odds of it failing are very low) when its still connected to the board? Would the only ideal solution be to desolder the led and test it separate from the board?

Could one figure out which chip the LED indicator is processed through and replace the chip or are there too many variables?
The LED can be considered eternal as relates to the life of the motorcycle it on. There is the possibility of physical damage, but that will be easy to see once the board is removed from the housing. Forward and reverse resistance checking the LED can be done in circuit without risking damage to other components, but given the number of other parts attached to the circuit I can't say whether the check would provide useful information.
There is also the driver transistor (TR22) near the ballast resistors that may have been damaged.

As to replacing the processor -- first, it is a proprietary unit mask programmed for its application that would be just this side of impossible to obtain, and even if you could find it replacing the 100 pin surface mounted part is not a job for the average hobbyist.

BTW, if the thermistor is bad it may not be necessary to replace the entire fuel pump assembly -- many later cars have a similar setup and if you can obtain a fuel pump from, say, a late '90s honda from a salvage yard it has a similar thermistor attached. It may take some creativity to mount it up, but would save a considerable amount of cash. Remember to resistance check it to be sure it's 1000 ohms at 22C.

Good luck.

Rob

If it has already been done, it is safe to assume it is possible to do it.
On the other hand, if it has not been done never assume it is impossible to do it.
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post #11 of 11 Old 12-05-2012, 02:25 PM
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Quote:
Originally Posted by nd4spdbh View Post
How about just keep an eye on mileage and call it good.
That's what I have to do!

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