Testing a fuse/ removing a fuse.

[Ilovecbrs599999]

When I want to check whether a fuse is blown do I have to disconnect power from the battery before I remove it?

[Royce44]

Depends on what the fuse is protecting.

Wipers? Yeah crack on.

Ecu? I'd disconnect battery.

No idea why in would but my mind is telling me I'll blow the ecu somehow.


You can test a fuse while in place and car is ignition on. Meter across the pins on dc voltage. If it reads 12v then it's blown

[M32Guy]

No need to remove a fuse to test it. Get yourself a test lamp. Find a good earth point on the chassis and touch each side of the fuse. If the lamp lights on both sides then it's good, one side then it's bad, neither side means there's no power to it

[Ilovecbrs599999]

sweet thanks

I also found this video just now too: https://www.youtube.com/watch?v=gKgmIcmp7eo and if you skip to 6:10 he uses a multimeter on a particular fuse and
when putting the black lead on good ground of car and red lead on either side of the fuse he gets the same reading on both sides. He even comments that
there should be the same reading! yet I don't see how:

the fuse must have SOME resistance so when connecting to the end which is furthest from the negative terminal battery won't the reading be slightly less? electron has to travel through the wires to nearest end of the fuse. the eelectron has to lose potential overcoming the resistance in the fuse when it passes through the first end of fuse and reaches the other end. (the one further away) so at that end wont the reading for potential difference/voltage be less?

[M32Guy]

A fuse isn't a resistor. Its designed to let a certain amount of current pass through freely, if that current exceeds the rating of the fuse, it blows. We're only talking about a few mm maybe a cm of metal here. The fuse would need to be several meters long to have even the slightest (to several decimal points) difference to the voltage reading

[oldcynic]

I've had a fuse partially blow, which was a bugger to spot. Most modern fuses are obvious when they've blown - the wire in the middle is visually broken - but this one had turned into a resistor instead.

If there's a particular fuse you think may be the problem then swap it for a new one. They're cheap enough.

[PositronicRay]

oldcynic said:

I've had a fuse partially blow, which was a bugger to spot. Most modern fuses are obvious when they've blown - the wire in the middle is visually broken - but this one had turned into a resistor instead.

If there's a particular fuse you think may be the problem then swap it for a new one. They're cheap enough.

I've had this too, test lights should be used alongside meters, meters can show 12v but thats not the whole story. The old ally bullet fuses can be problematical, cleaning helps.

I think OP is overthinking all this, perhaps if he explains what the problem is someone may be able to provide some relevent advice.

[Ilovecbrs599999]

In that same video the dude said in order to do an ohm's test on a fuse on the other hand- the fuse must be removed .

1) why need to remove for resistance test?
2) do I need to disconnect the power before I remove the fuse?

[Krikkit]

Ilovecbrs599999 said:

In that same video the dude said in order to do an ohm's test on a fuse on the other hand- the fuse must be removed .

1) why need to remove for resistance test?
2) do I need to disconnect the power before I remove the fuse?

1) Because you're testing whichever resistance is lower - the fuse or the circuit->ground it's fusing!
2) Depends on its function, as noted above some items might benefit (especially when they have multiple connections to the various systems, like an ECU) from having the battery removed first.

[GreenV8S]

Personally I wouldn't see any need to disconnect the battery before removing a fuse to test it. It would be extraordinary for any modern car to be designed so poorly that it was harmed by having a fuse fail or blow. Removing power risks causing other problems such as needing to unlock the radio or reset the alarm, or clearing volatile memory in electronic control units causing fault code history etc to be lost. These issues are an irritation rather than a big deal but they're problems you don't need.

[Ilovecbrs599999]

Krikkit said:

1) Because you're testing whichever resistance is lower - the fuse or the circuit->ground it's fusing!

I don't quite understand - if the fuse is blown and we take it out it should read 1 or 0.L to show infinite resistance since that metal inside has melted and doesn't conduct between pins.

If we keep the fuse and test the pins surely it should still read the 1 or 0.L if we suspect it's blown?

[GreenV8S]

Ilovecbrs599999 said:

1) why need to remove for resistance test?
2) do I need to disconnect the power before I remove the fuse?

1) If you perform a resistance test on a fuse that has a voltage across it you will (a) get a false reading and (b) probably damage the meter. You should only perform a resistance test on components/circuits that are un-powered.
2) No, in my opinion.

[Ilovecbrs599999]

sweet:
also for a voltage test why not just measure the voltage between the two terminals themselves on the multimeter instead of bothering to connect one multimeter lead to ground and the other lead to one of the component's teminals (e.g a fuse).

Edited by Ilovecbrs599999 on Monday 11th September 21:11

[GreenV8S]

Ilovecbrs599999 said:

sweet:
also for a voltage test why not just measure the voltage between the two terminals themselves on the multimeter instead of bothering to connect one multimeter lead to ground and the other lead to one of the component's teminals (e.g a fuse).

Edited by Ilovecbrs599999 on Monday 11th September 21:11

If you know that the fuse is completing a circuit, you're right that you can just test for a voltage drop across it. But if both sides are at the same potential then there will be no voltage drop across it regardless of whether it's blown.