Hey Scotty! I recently bought a 2004 Mazda 3 as a beater. It’s starting to develop an electrical issue where both the running taillights and the dashboard lights will not turn on. All of the other lights work normal as they should; the headlights, brake lights, reverse and turn signals. Being a YouTube mechanic, this is out of my jurisdiction. Can you help shed some light on what can cause a complex electrical issue like this to occur, and how to fix it?
also, thank you very much for your years of videos showing how to maintain and fix one’s vehicle properly. It’s especially helped save me tons of money in the long run, so you are forever appreciated by DIY YouTube mechanics like myself! Thank you again and I hope to hear back from you soon!
You have circuits which are protected by 3 seperate fuses which aren't working.
In these cases it's best to pick just 1 of the circuits and troubleshoot it.
I chose the Right Hand tail light, Right Hand parking light, and the License Plate lights circuit.
They're all on the same circuit and are protected by the "TAIL R" fuse in the passenger compartment fuse box.
I highlighted and labeled that part of the circuit. Here it is:
The way this part of the circuit works is when you turn the Light Switch to the 1st position (TNS) it completes the Ground (through the wire highlighted in Green) and energizes the TNS relay.
When the TNS relay is energized Power goes (through the wire highlighted in Red) and through the TAIL R fuse to the RH tail light, RH parking light, and license plate lights.
So how can a guy who isn't into automotive electrical stuff start diagnosing this circuit?
1st, when the Light Switch is in the 1st position (TNS) you should have Power from the TNS relay on both test tabs of that TAIL R fuse in the passenger compartment fuse box so grab a test light and see if that's happening. (Like this)
If you don't have Power there then you can test the Light Switch to make sure it's providing a Ground (in the TNS position) to energize the TNS' relay coil.
It looks like this:
So you can do the continuity test with a multimeter when the Light Switch in the TNS position while making sure that the connector has a good Ground on the "E" terminal.(if you don't have a multimeter you can use a test light to check for Ground on the A, B, E, and K terminals in the TNS position while back probing the connector.
This seems like a lot of trouble, right?
Why can't we just test the TNS' relay socket terminals for power on both the Control Side and Load Side, and a Ground on the Control Side when the Light Switch is ON?
It's because your TNS relay isn't replaceable. It's soldered onto the printed circuit board inside the PJB (Power Junction Box) ((Passenger Compartment Fuse Box)).
Yours on that '04 Mazda 3 is probably like this one.
So I hope that's helpful.
One more thing. Sometimes if there's a bad solder joint or a stuck PCB relay you can smack the side of that PJB (Power Junction Box) ((Passenger Compartment Fuse Box)) a few times and the relay starts working again.
Just consider that this isn't just a fuse box. It's a microprocessor so if you do decide to smack it a few, use the appropriate amount of force to persuade it without killing it.
Possible issues with grounding. Starting at the battery, check the grounding connections for corrosion and looseness. Each fused circuit on the car has a ground connection, that if loose or corroded will cause the circuit to fail. Everywhere you find a wire attached to a metal part of the car, or frame is a grounding connection. Check all that you can find.
well I'm no Youtube mechanic, but the 1st thing I do when I have electrical problems, is check my fuses & relays.
I already did that. That’s why I’m bringing this up, because I have no knowledge of working on anything electrical pass the fuses and relays.
you will need to learn electrical troubleshooting. Circuit basics, reading an electrical schematic, using a multimeter etc..
or,
give your car to someone who can.
what can cause a complex electrical issue
The problem need not be complex. A nicked wire, loose grounding point, corrosion in a connector etc.
But pinpointing the fault, without spending a lot of money replacing a unnecessary parts, will require a methodical approach.