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The A/c compressor clutch was not working and I was testing underhood fuses. When I reached the #2 fuse for the Rear window defogger relay I heard a click. Compressor clutch relay was bad and i replaced it and the clutch is working fine now. There were no complaints about rear defogger but I am just trying to understand what is happening and why. Alldata says that the #2 underhood fuse name is "RR Defroster coil" and the circuit or component protected is the Relay Control Module. If I would have used my snap-on LED test light I would not have noticed anything. It lights up perfect with that one cause I checked it but with my bulb test light its really dim and you can barely see the coil in the bulb lighting. If I take fuse #2 out, the side with power is still dim. If I take the relay out and check power on the control side it, my test light is bright. If I take the relay out of the equation and jump a wire in between the 2 legs of the control side (pins 3 and 4) and then put my test light on the fuse, it lights up good. When I push the rear window defogger switch I hear the relay click on and off every time and the indicator lights up. But with the switch on I test my fuse with both test lights and they dont light up but as i push the switch on/off the relay still clicks! Just trying to use this as a learning experience cause it's my uncles and he doesn't need it right away but nothing is making sense about what is happening. Does this mean that my problem is resistance? Thanks for your time and I am learning so much from your channel.
with the relay issue it is common for the contacts to get damaged from corrosion and arc strikes on the contact pad. After a while of the arc strikes it no longer will make good contact and will effectively choke the current flow to a circuit. i.e. higher resistance in the relay due to these effects. As for your two test lights lighting up or not lighting up there are two sides of the relay one is controlled by the switch to turn on/engage the other half of the relay. a relay can be bad and still make an audible click.
This may be reading to much into it I don't have schematics for that car but do have some electrical experience. If I am understanding your story right when you touched the fuse with your test light it clicked. Very important what was your test light connected to? From your story I'm assuming ground so the the question is, what kind of relay is being used?
It could be that it is a normally closed which means that power is used to shut it off. If so when you touched the fuse with your light you took the power away so the relay was then closed. And if it is normally closed then you could seem some voltage feed back when it is closed.
Purely just guessing but you can take the relay out and determine what type of relay it is and if your learning do some tests with a multimeter to see whether it's a normally open or closed.....sometimes it has diagram right on the relay that will also tell you.
It sounds like the fuse is in the ground side switched leg of the relay control circuit like this.
Power>Relay Coil>Fuse>Switched Ground.
When you stick the test light to the fuse your circuit becomes this.
Power>Relay Coil>Fuse>Test Light>Ground.
The relay coil and test light are two resistances in series so you'll have some voltage drop across each of them. Probably 8ish volts across the coil and about 4 volts across the light.
Yeah you are right. Control side has power coming into the relay from a 7.5 A fuse then goes from relay into Fuse #2 which is a 30A fuse and then to ground. I havnt seen a wiring set up like this before. One fuse is before the relay and one after the relay. The one I am getting the dim light out of is the one after the relay just like your saying. So my test light goes out on fuse # 2 because once the ground is switched on all my voltage drop is used by the coil of the relay? Am I understanding this right? Ive never seen a fuse after a relay before but im pretty new to reading wiring diagrams.
Thanks for the input from everyone i did go back to check it is ground controlled and I swapped this relay with another one to see if it was bad and had the same results. I really appreciate everyone's comments.
Alopez wrote: So my test light goes out on fuse # 2 because once the ground is switched on all my voltage drop is used by the coil of the relay? Am I understanding this right?
Yeah you've got it. When the control applies ground to the circuit all your voltage drop is across the coil.
Alopez wrote: Ive never seen a fuse after a relay before but im pretty new to reading wiring diagrams.
It's not common but sometimes OEMs use fuses as jumpers to configure a fusebox to the vehicle.
