Hello,
I have a strange problem with a fuse from my motorbike. It has stopped working but when i check the voltage past the fuse its the same as on the battery, however when i connect a bulb through that fuse the voltage immediately drops to 0.
When i check for resistance of that fuse it sometimes gives a minimum reading and sometimes very high reading.
Anyone can explain that and give any idea why has this fuse not blown completely? It happened twice so it wasnt only a bad fuse and its size was right as well.

 

Replace it...the element is obviously making an intermittent connection.

 

It did not sweep through completely, there is a residue left that your high impedance meter will measure through, but not support any significant current.
Max.

 

Hello,
I have a strange problem with a fuse from my motorbike. It has stopped working but when i check the voltage past the fuse its the same as on the battery, however when i connect a bulb through that fuse the voltage immediately drops to 0.
When i check for resistance of that fuse it sometimes gives a minimum reading and sometimes very high reading.
Anyone can explain that and give any idea why has this fuse not blown completely? It happened twice so it wasnt only a bad fuse and its size was right as well.

Probably corroded fuse clips.

 

Thanks for the answers.
I have checked the regulator/rectifier with multimeter and when i put positive terminal from my meter to positive on the rectifier and negative to any of the output pins the resistance keeps slowly growing forever, it was 2.5kohms and growing. Is that normal or that indicates problem with the regulator/rectifier?

 

The regulator may include a capacitor, which gets charged by your meter and results in the reading change.

 

I've had that problem before and it's annoying.

Measuring the voltage "across the fuse" can sometimes help. On blade type fuses those points are accessible on the top of the fuse.

If you assume the circuit is ON, the voltage across the fuse should be small. OFF, usually zero.
ON and blown - full battery voltage.

 

Measuring the voltage drop across a fuse (so I hear) is a good way of determining a parasitic drain on your battery. The voltage across the fuse should be very very small. If you read a large drop across the fuse then you know the fuse is blown (or bad in your case).

Bottom line, if the fuse is suspect - just replace it. They're not expensive and easy as heck to replace. So just replace it and be done with the problem. Oh, and toss that bad fuse in the trash so it doesn't bite you again in the future.

 

Probably corroded fuse clips.

That would be my guess. Motorcycles or motorbikes have their wiring and fuse blocks more exposed to the weather and elements. In addition to corrosion connector terminals can work their way loose. As to:

I have checked the regulator/rectifier with multimeter and when i put positive terminal from my meter to positive on the rectifier and negative to any of the output pins the resistance keeps slowly growing forever, it was 2.5kohms and growing. Is that normal or that indicates problem with the regulator/rectifier?

To really answer this one would need to know the bike and how rectifier / regulator are configured. Meaning a good electrical circuit drawing of the bike.

Ron

 

If it is a used bike or any vehicle for that matter, they can have some strange things done to the wiring over time. So a back feed wouldn't be out of the question on one.

 

As alec described at #6, this is very typical of a circuit with a capacitor.

All modern ohmmeters measure resistance by applying a known constant current to the resistor under test and measuring the voltage across it. Depending on the meter design the current may change for each range or one current may be used for more than one range. The current is typically less than a milliamp even for the lowest resistance range and may be less than a microamp for the high range.

The following applies to trying to measure the fuse in-circuit where there is a stable DC supply voltage on one side of the fuse and a circuit with a capacitor to ground on the other:

When there is a capacitor of significant size, the current from the meter will take some time to charge the capacitor. If the meter leads are reversed while there is still charge on the capacitor the meter has to start charging all over again - but first it has to discharge the cap. It can take tens of seconds for the reading to stabilize. If a specific range can't be set, this is just an annoying "wait til it quits changing" scenario. All power must be disconnected prior to attempting to measure the resistance, otherwise the full supply voltage will be applied across the meter if the fuse is blown. The meter won't like this.

In any case, a reading above a few ohms means any fuse rated at more than a few milliamps (and very low current fuses are rare and expensive) is blown.

If you measure the voltage across a blown fuse in a circuit with a capacitor, the voltage will first be the full supply voltage then gradually decline in a DC circuit. Most DMMs have input resistance of 10 megohms, so it again takes a very long time for the reading to stabilize. It may still be changing after minutes. But if you even briefly see more than tens milivolts across the fuse, it is blown. This measurement requires that the power be connected normally, so care must be taken to avoid shorting anything or contact with the circuit if the voltage is high enough to be dangerous.

 

Thanks again for the answers, i have checked the fuse after taking it out so i knew there was something wrong with it, was just curious what.
Unfortunately i cant find a wiring diagram for this model but I have done a similar check to a different rectifier and the resistance was stable so i decided to order another rectifier as they are not expensive

 

Hello,
I have a strange problem with a fuse from my motorbike. It has stopped working but when i check the voltage past the fuse its the same as on the battery, however when i connect a bulb through that fuse the voltage immediately drops to 0.
When i check for resistance of that fuse it sometimes gives a minimum reading and sometimes very high reading.
Anyone can explain that and give any idea why has this fuse not blown completely? It happened twice so it wasnt only a bad fuse and its size was right as well.

I have read all of the answers in the thread and I think you have been provided with some good ways to proceed. I am noticing this part of your post

It happened twice so it wasnt only a bad fuse and its size was right as well.

.

Are the fuses that you are using rated for AC or DC?

 

Fuse is DC as it is between the battery and rectifier