Fuse testing without removal

[luv2rallye]

You cannot tell a fuse is ok by just looking at it! It may look fine but actually is blown with a hairline crack (or the fuse wire may be broken near the blade connection) that is not visible. I've lost count of how many hundreds of times people say their fuses are good when all they did was look at them for the obvious wire break and they actually are blown! By using a test light or multimeter you can find blown flat bladed type fuses without removing them. These fuses have 2 contacts on the top of them.

Test light method:
Turn the ignition on (to energize more circuits). Touch the test light from each fuse’s contact to ground. If it lights on both, that fuse is good. If it lights on only one, that fuse is bad. If it doesn’t light on either, that circuit has no power so the fuse may be good or bad and you will have to energize that circuit and retest or remove the fuse and check it's continuity separately with an ohmmeter or a battery & test light.

Multimeter voltage method:
Turn the ignition on (to energize more circuits) and with the multimeter set to read voltage (15 volt scale minimum), measure the voltage across each fuse. If any have any voltage at all, they are blown (since a fuse is supposed to be a short circuit). If no voltage, measure each fuse contact to ground. If there is the same non-zero voltage on each contact, that fuse is good. If both contacts read zero volts to ground, that circuit has no power so the fuse may be good or bad and you will have to energize that circuit and retest or remove the fuse and check it's continuity separately with an ohmmeter or a battery & test light.

Multimeter resistance method:
You must first make sure no voltage is present, in case the fuse is blown, as this will blow your meter. Disconnect the battery and wait couple minutes for any capacitors to discharge before connecting meter. With multimeter set to read resistance (x10), measure across each fuse (should read 0 if good).

Remember, flat bladed fuses can be blown in one of two ways: (1) the common chunk missing/melted case which indicates current exceeded rating or (2) the rarer hairline crack case which indicates thermal fatigue (repeated current on/off). The hairline crack case can be very difficult to see (near impossible while fuse is in place).

Or, you can get this nice little $6 tester that Buss makes for testing fuses in-place. (Note: It isn't 100%, however, since if the fuse is blown but the circuit is shorted between the fuse contacts, or has less than 30k ohms resistance between them, the tester will incorrectly light up indicating a good fuse. Luckily most of the time this isn't the case but just be aware of this possibility if you have a weird fuse blowing situation.)