ok so I've been having this issue for a little bit now and its killing me. I was a lead tech at a mechanical company and I definitely know wiring schematics and diagrams so when I need wiring help something is wrong.

Car has no taillights, no brake lights, no interior lights, no dash lights, no blinkers, no hazards and no power door locks working. For some reason the reverse lights do work though, I think this is because its getting power straight from IG1 on the ignition switch instead of from the battery.

Cars headlights, radio, gauges, and warning lights work no problem.

I jumped from the NO wire on the TNS relay and the tails light up but nothing else works still.

I jumped from the battery straight to the wire out of the BTN fuse and nothing, no lights at all.

Only thing I can see that can somewhat stop all these things from working is that BTN fuse yet its fine. Unless a wire broke off in between the BTN and where that wire connects to everything. I just don't know where so I can try to jump it out.


Anyone knows where the wire out of the BTN fuse meets a junction point that separates into everything else??

 

Check fuses, bust out a meter and check for continuity if you think its a broken wire.

 

The overall block diagram is a good start. Make sure you remove each fuse and check for opens with a digital multimeter (DMM). I also recommend looking at any work that was done to the car. Maybe a connector was inadvertently left off.

Additionally, you may have to look at the detailed schematics of a couple of circuits. You may discover a component to check that wasn't in the overall block diagram. When electrical gremlins arise, take a look at the Body CPU.

 

I've checked the fuses and they were all good. I checked other components like the tns relay and they check out.

 

Based upon your findings thus far, the brake/turn/hazard and interior lights have CPU #2 in common. The power door locks also receive an input from CPU #2.

As indicated earlier, you will have to verify each specific schematic section. The block diagram only shows basic component interfaces. So, the Brake lights, turn signal, and hazard lights are in section F1. Interior lights are in section I2. The power door locks are in section K2.

I used my 94 electrical manual. There may be some differences between your 93 and my 94 FD. I also assume that your car is stock. Yes, I know what assume means too!

There is one other thing to inspect: Ground points. Refer to Section Y for ground point locations. Record ground points from each section and see if there is something in common. Perhaps your car has a loose ground or the ground was intentionally cut/relocated.

Verify these findings using your 93 Body electrical manual and report back to the thread.

Welcome to the world of FD wire troubleshooting. Best of luck on your quest!

 

Do you also have an issue with the factory alarm not setting properly? The body CPUs have a tendency to develop cold or broken solder joints. The NYC roads are harsh on our cars. I replaced my body CPU because the alarm wouldn't set properly.

This problem is an extension of the FC Body CPU. After replacing one on a Vert, I noted the tell-tale signs on my FD.

 

Good info, I haven't checked anything about anything cpu related. I didn't realize they have any effect on these things.

 

FYI, the Body CPU is different from the ECU (PCM). The ECU is for the engine, located in the passenger side kick panel and the body CPU is next to the fuse box in the driver side kick panel.

 

Yeah I know that. Seems like the wire that leaves the BTN fuse is broken or grounded somewhere in between the BTN fuse and the Joint Box where the fuses are. I jumped from the BTN fuse to the W/R wire terminal in JB-05 plug and everything works except now I have a problem with the Stop Lights always staying on. They never turn off. Any ideas? I'm going to go through the diagram tonight and see if I can figure it out.

 

2Rotors, Have you made any progress or found the problem?

Could you answer these questions to clear some confusion that I may have from your last post:
1. What wire is broken between the BTN fuse and JB-05 (fuse box)?
2. Is there a specific fuse in JB-05 that you attempted to read - did the wire read open?
3. What wire did you jump from BTN fuse to what component?
4. Did you make that jump wire a permanent fix?
5. What mods do you have in the car?
6. Did you have any recent mods done to your car?
7. Disconnect the body CPU; anything change for the better - interior lights, stop lights, headlights, etc?

Here are some recommendations when diagnosing an electrical gremlin:
1. Print out the wire diagram and take notes at the points you measure. A color code system works well (e.g. green ink/pencil for good reading, red ink/pencil for bad reading).
2. Take measurements in one circuit at a time, such as the Interior Lights circuit. Then move on to another circuit.
3. Start from the circuit input then work towards the output.
4. Disconnect parallel circuits via fuse box (JB-05).
5. When reading resistance from 2 connectors far from each other, ground one pin to the chassis then connect the black lead (of DMM) to the chassis and the red lead (of DMM) to the pin in question.
6. Be mindful of the many harnesses a wire may travel through - a signal can travel between the front body harness to the engine harness to the dash harness. Refer back to #3 for parallel circuits awareness.
7. Switches and relays are typically drawn in their "normally open" or "NO" position.
8. A relay is powered by a coil and will read a near short between those 2 leads (good reading).
9. When measuring resistance across a diode, polarity matters. The "arrow" points to the negative material and that's where the black lead goes. When the leads are reversed, a considerably higher resistance is measured.

My apologies in advance should my questions/recommendations offend your expertise. That is not my intention.

 

Haven't even had a chance to check the car at all. I'm going to mess with it tomorrow, going to check the stop light switch and see if it's faulty or stuck in the closed position. And no offense taken at all, I appreciate the help.

 

Ok so problem is solved, it was a broken wire between the btn fuse and the fuse box by the driver's side footwell.


The brake light being on no matter what issue was the plastic piece that the brake light switch makes contact with to let you know the brake pedal is pressed and depressed was gone and there was only a hole where it should be on the brake pedal. Put a short bolt and nut in the hole and everything is good.

Everything is good to go now. Will be tuning and driving my car for the first time in about 3 years.

 

Glad to hear that. Well done on finding the broken wire!

Where exactly was the broken wire - near a connector or somewhere in between? Do you know what caused the wire to break?

Be wary of wire chafing in the middle of a wire bundle (or connector for that matter). The goal should be to "cure the disease instead of relieving the symptom(s)." What I mean is, understand how the wire broke to prevent it happening again.

Again, well done on fixing the broken wire! Enjoy tuning your Rex and certainly enjoy the drive after a 3yr slumber!!