| Recently bought a 91 300ZX 2+2 to play with. Although I specialize in mostly Italian, German and rare cars I think the Z is the real deal for a sports car, pretty impressive design. The first problem to tackle was the lighting circuit. I think many of you are familiar with the no taillights, no low beams or dash lights and high beams stuck on high. This short can occur almost anywhere, so it can be very frustrating if you don't follow the right procedure. I didn't find a good troubleshooting guide so I thought this might be helpful. Here's how I found the problem and fixed it in one day. With a test light and multi meter. You can check with a test light by connecting the alligator clip to positive and probing wires. A multi meter is more effective. I use the diode test to get a tone so I don't have to look at it. This is what to do when you have no idea what the problem is. Symptom: No tail lights, dash/illumination, fog light or low beams. High beams come on when H/L switch is activated. Illumination/Marker light fuse blows when parking lights are activated. Brake lights and turn signals function. Diagnosis:
1) Remove all non-factory wiring and repair all cuts and splices.. (Yes the massive stereo system and alarm)
2) Repair cut and spliced wires in trunk and remove manual tail light switch installed by P.O. to bypass lighting circuit.
3) Check illumination fuse for short to ground: Found short to ground from fuse to H/L switch. (Red/L. Blue wire) DISCONNECT BATTERY you are no longer testing for power. 4) Test fusible links: All good. (In front of battery)
5) Test H/L switch, dimmer switch, fog light switch, high beam switch and unplug radio: No change. All check good. The blue/red wire at the radio is your clearance light circuit and should not be grounded (Changes from green/red to red/blue). Red/ Yellow is illumination. 6) Unplug H/L switch, test circuit to ground: Failed. Side marker/Tail light grounded (Green/Red), Illumination grounded. (Red/Yellow) The red and blue wire is power from fuse 9B. When the switch moves to park lamps it sends the ground from the green/red wire to the red/blue wire-this is your dead short.
7) Unplug Tail lights, Side Markers and Head lights: No change.
8) Check tail light sensor unit: 2 terminals burned. Replaced unit, no change.
9) Remove H/L dimmer relay and test: Relay functional. H/L Switch/ Side marker wire shorted to ground. (Green/Red)
10) Test low beam wiring at connector: Both wires 12v with H/L on.
11) Test low beam relays L & R and high beam relay: All test good.
12) Remove all light bulbs, test connectors: All side light/ taillight wires shorted to ground on both terminals.
13) Unplug all switches in car (wiper, hazard etc.): Illumination wire shorted to ground. (Red/Yellow)
14) Cut or remove pin for illumination (red/yellow) and side marker wire (green/red) at H/L switch: Illumination fuse 9B no longer shorted to ground. Side marker light circuit grounded. No other change. Keep in mind that you are now testing the disconnected green/red wire at the headlight switch for short to ground throughout this process. 15) Unplug rear harness at fuse panel: No short in tail light or side markers circuit. Test the green/red wire at the tail light sensor (unplugged) If the sensor is plugged in, this wire will be grounded. Ignore it.
16) Unplug front lighting harness at fuse box: No short. Test from bulb sockets to ground. 1 should ground, the other should not.
17) Unplug dimmer relay harness at fuse box: No short. Test the terminal closest to the radiator.
18) Unplug light timer control: No Change. Now all harnesses have been eliminated from the short. 19) Isolate problem to main harness.
20) Unplug power mirrors, mirror heating, lighter and shifter illumination: No change.
21) Remove glove box light and switch: Not plugged in.
22) Remove glove box: Note 2 wires plugged together and taped with similar connectors to glove box light.
23) Disconnect wires: Short eliminated at green/red wire to headlight switch.
24) Reassemble fuse box, kick panels, lower dash covers, heater ducts, sill plates, rear interior covering, radio, console, glove box, light bulbs, splice headlight switch wires, replace fuse and test: All functional So even though this was a simple short, it took a process to isolate. It's almost always something foolish that causes these problems. In short, isolating which harness the short is in will get you where you need to go. If you unplug all connectors from a harness and test the terminals to ground, you shouldn't have any grounds except for body grounds. Now you have isolated that harness and can trace the wires to find the short. This is actually uncommon unless power has been shorted to ground without being fused which is usually pretty obvious because you will have melted wires. NEVER BYPASS A FUSE, find the short and repair it or it could end up in disaster. This can be a full day process but it is doable if you are patient and systematic. This is just a step by step of what I encountered. This process will work on any wiring problem. Always do this procedure with the battery disconnected after you have verified where the power loss is. There's too much potential to fry something if you don't and contrary to what most think, you do not need power to test most electrical circuits. Questions are welcome.
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01:00:55 07/13/15
