Thread: Need help wiring a relay please..

I hate admitting this, but I'm not really sure on the proper use of a multimeter. I just use a test light.

Same thing.. How do you use your test light? Do you put the clamp on a ground and then put the light on each side of the fuse? Please detail..

Yup, I put the alligator clip on whatever unpainted bolt is closest to whatever I'm probing. On each side of the fuse, there's a very small hole in the plastic that exposes metal underneath. I probe that metal using the tip of the test light. I always do both sides just to be sure I'm not just accidentally probing a broken fuse.

black to any ground and red to tops or sides of fuses each fuse has 2 power points on the top of new style plastic blade type ( thats also how you would test for blown fuse by touching both top connections and if 1 side doesn't show power it is blown) once you test all with key off make note of the ones without power and then key on to test for ignition on power. Once you have determed which are key on power now while connected to top of 1 of those power on key on turn to start and verify you have power while cranking. Hope that explains it, pretty simple really. Good luck SNO

And when you are probing with the car off your light doesn't light up, right?

Every fuse I probed was testing 12V with the car off.. I just find that very hard to believe..

Originally Posted by SNOtwistR

black to any ground and red to tops or sides of fuses each fuse has 2 power points on the top of new style plastic blade type ( thats also how you would test for blown fuse by touching both top connections and if 1 side doesn't show power it is blown) once you test all with key off make note of the ones without power and then key on to test for ignition on power. Once you have determed which are key on power now while connected to top of 1 of those power on key on turn to start and verify you have power while cranking. Hope that explains it, pretty simple really. Good luck SNO

Yea, lol. .That's what I thought and what I was doing.. It's just like I said.. They all tested hot with the car off.. WTF! That's why I thought I was going crazy and thought I must be doing something wrong..

There are some that are live even when the car is off, like the headlight and tail light fuses, among others. Definitely not all of them, though.

What type of vehicle we talking about, the under the hood panel looks like it would be hot since those are mostly engine related or major component related. Do you have another fuse box inside the vehicle. I have seen some fuse boxes be all live, It can be frustrating to get switched power that way. SNO

A test light may be better.

The problem with DMMs & voltmeters is that they only require nA or uA for a reading - ie, VERY low current (voltmeters are "high" impedance = resistance).
And some vehicles may have sensing circuits for fuses etc - not enough current to drain a battery, but enough to give a +12V reading.
A lamp (~250mA) or LED (20mA) should be enough load to overcome such sensing +12V connections.


All those center fuses (fuel pump to throttle mtr) should be IGN switched.
Otherwise take power from the meters or IGN coil +12V (unless COP) or wiper etc.
You'd only need to add a fuse to protect the wire to the LEDs - no fuse will protect the LEDs themselves. (If using a relay, then protect the wiring to it etc.)