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I want to check if this particular 50 amp fuse on a RAV4 (1998 first generation) is blown or not. Just looking, it's hard to tell because of what I assume are blue-green deposits on copper (this is the view inside it with the cover removed):

enter image description here

Normally I'd remove a fuse and test the two connections with a multimeter set to measure Ohms / resistance, but removing this is a non-trivial task - not only is the bolt holding it in place incredibly stiff, it's also holding the wiring together and I don't like the risk of re-assembling the wiring incorrectly or losing one of the many small parts like washers held with this bolt.

In theory, it looks like I could (after disconnecting the battery!) just use the multimeter's prongs to touch the copper visible on the left and right sides of the bottom of the fuse, and if it gives any reading, in theory that would mean the fuse hasn't blown. Some instinct however tells me that poking inside this fuse like this is a bad idea, or that it might not be as simple as it looks and those might not be points that are connected only by the fuse.

Am I worrying too much? Can I test this fuse without removing it as described?

  • That fuse looks fine to me – Zaid May 2 '16 at 19:23
  • Sometimes fuses develop fine cracks from the heating and cooling effect of years of current cycling. Essentially stress fractures in fuse element near the edge of the fuse not readily visible. Electrical verification removes the guesswork. Circuit breakers also fail for the same reason. – Old_Fossil May 7 '16 at 17:25
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Instead of Ohms checks of fuses I check for voltage with power on. Set your voltmeter to DC volts with the negative lead on the battery negative connector. With the positive lead test both sides of the fuse. If good it will read battery voltage on both sides. Most fuses have exposed metal tabs on both sides of the actual fuse section for this purpose.

On the fuses shown check for voltage at the small legs near the side of the case in the lower section of the picture.

  • Great, I was really hoping they were designed for the purpose! I can see the advantages of using voltage but is there any reason to not use Ohms if the battery is disconnected already? e.g. Could it damage the fuse? – user568458 May 2 '16 at 19:15
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    @user568458 it won't damage the fuse but voltage drop is a better indicator of fuse health and doesn't require removing the fuse/disconnecting the battery to test. – Ben May 2 '16 at 19:26
  • I agree, testing the voltage on the fuse is the best way to go here. You can even put the positive on one leg of the fuse, and the negative on the other with the meter in the volts position. If the fuse is good you will get a very small voltage across it. Much less than one volt (millivolts to maybe a tenth of a volt). If you connect it this way and get + or - 12ish volts, the fuse is toast. – cdunn May 2 '16 at 20:49
  • Agreed. There are some annoying circumstances where there could be a short across the fuse via the body of the car. Usually that's not the case, but not surprisingly, the presence of a short across the body of the car has a curious tendency of blowing fuses.... testing voltage when the power is on will not be fooled by this second path between the two sides. – Cort Ammon May 3 '16 at 3:54
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    Yes, but you can test for that by grounding the negative cable of the tester and making sure there's voltage. Bottom line, it's about understanding what you're doing and why, not following a script. Fred is right on about testing for voltage. There are several ways to do that, and as long as you understand how it works, you'll do the right test. – cdunn May 3 '16 at 14:18
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If you can get to both sides of the fuse, see if you have power on each side. If you want to make sure, remove the battery terminal, and use an ohmmeter to measure the resistance between the battery terminal and the other side of the fuse. Should be zero or close to it. If so, it is good. With this type of fuse, when it blows, there is usually a large gap due to the amperage it takes to "blow" it.

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To remove the fuses take the 3 bolts on the side out that hold the black cables to the fuse box, the loosen with a thin flathead on the front part of the fuse and lift. They come right out.

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