thefiatek

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.

zxguy1986

Thanks for a great post! Your process is the basic (right) one followed by good mechanics who understand that aftermarket wiring by well-meaning 'repair' guys and owners can really screw up the factory electrical.

Normally you could have gotten PDFs of the '91 factory service manual from the Xenon site - XenonZcar.com - but they've been 'fixing' the site for a while. Looks like that manual isn't available now. The FSM has complete electrical wiring diagrams which can be a real help in tracing harness problems and other trouble.

Thanks again.

thefiatek

A wiring diagram would have been handy! Sometimes there is only one way to skin a cat. It got a little tricky under the dash when I realized that the green/red wire changed colors a couple of times. Had to resist opening that harness up. These actually have a pretty good wiring system that minimizes amperage draw. The con is all the flippin' relays make it not so straight forward. The Italians on the other hand just like to wire it straight and fry everything when something goes wrong and Mercedes.....well, don't even want to go there.....

zxguy1986

Since you own that Z now, you should get hold of the FSM for it. Loads of great inspection and troubleshooting pages. I don't have the '91 Z32 FSM here but somebody out there does. Why not post here and ask if someone can email you the PDF files. All they will need is your email.

My guess is all the Z32s would be similar so maybe another year would be ok...

thefiatek

Yep. Probably going to need it at some point. Runs a bit rich so I'm on to the fuel injection this week. Already found some broken F.I. wiring so I'm going to have to TS that and there's obviously a lot to it. My '95 525i will blow the doors off this thing and that shouldn't be so. It has 30 less HP and weighs 1000 lbs. more. Definitely like the car and the miles are low. It was someones daily driver and it looks like "maintenance" meant change the oil. Already knocked out the brakes all the way around, going to do the T-belt, trans service etc. Solid driver for sure. Now I just have to turn it back into a sports car

Twocycler

I bought a 90 TT Auto that had been sitting for a long time. The car turns over but will not fire. I replaced a very rusty fuel pump and level sensor but still it will not start. I checked the fuses and found that the 10A shift lock fuse I the engine bay was blown. I replaced it and it immediately blew again. I read that I should check the cigarette lighter for a short. When I checked the lighter, I saw that the green/red 12v supply wire also had continuity with ground. The resistance was 0.8 ohms. I checked the wiring diagram and started looking for shorts. When I disconnected both tail lights, I still had continuity, but resistance increased to 80 ohms. So there is definitely a bulb short here.

I went back to the fuse block and checked continuity of the shift lock fuse again. Now I got infinite resistance. I think great, maybe the short is gone. I hook the battery back up and when I checked resistance in the fuse location I get perfect continuity with no resistance. Any advice or ideas would be appreciated!

thefiatek

Sounds like you're headed the right direction. (Be sure to check for ground faults with the battery disconnected, you're looking for continuity to the body.)

If you haven't found the location of the short, I would proceed as follows:

1) Plug all the lights back in and verify the short.
2) Remove the left lower kick panel, locate the rear wiring harness and unplug it.
3) Check the short at the lighter and the shift lock fuse.
4) No short: In rear left tail light panel, remove the tail light sensor (don't unplug yet) Check the G/R wire to ground. You may find continuity (normal)
5) Unplug the tail light sensor and test the G/R to ground. No short: Replace tail light sensor. Short to ground: Unplug tail lamps, side markers, license light and third brake light while checking G/R. One of these will be your short. If it remains shorted, then you have a damaged wire (Not very common) and will have to trace it. From the rear to the front.
6) Reassemble and test.

For your no start:
1) With the battery connected, check the fusible link in front of it.
2) Verify power to the fuel pump relay (key on and start position)
You should have voltage to 3 terminals. The other is the reference to the ECM.

Then proceed to the fuel pump control module.

Have you checked the ignition system yet? I usually check that first because it's pretty much all right in front of you under the hood.

^

Twocycler

Thanks Fiatek,

You are both knowledgable and helpful. I will get to work on your suggestions tomorrow after I get home from a car event with my son and his R32 Skyline.

One question for you. Why does connecting the battery establish a ground on the secondary side of the fuse socket?

Thanks again,

Rocky

thefiatek

When you have a system with relay controlled circuits. When you power them, some of the relays and or modules may have circuits that are supposed to switch to ground, even a fuse. It depends on the entirety of the circuit. If it doesn't blow the fuse it's working properly. This is why I test with the battery disconnected, you can be chasing a false ground.

On there other hand if a fuse to a relay blows, the relay is probably faulty internally.

Make sense?

Twocycler

Here is what I have found so far. If I say something that makes no sense, please bear with me.


I get 0.8ohms between the green/red and the black on the cigarette lighter. If I disconnect just the individual tail lights with the green/red wire, I get 79 ohms. If I disconnect all of the rear main harness plugs in the drivers footwell, I also read 79 ohms. I seems to me I have bulb shorts in both tail lights, but I am confused as to why ave partial continuity of 79ohms between green/red and ground when the other main harness plugs are disconnected.


Following up on your suggestion to check the fuel pump, I found a few things on that front. When I connect the battery, you can not hear or feel the relay closing at all. I removed the relay and if I used the red ire as ground (relay #5) I get battery voltage on both the white/black wires (relay lines #2 & 3). The wire that goes to what is pin #1 on the relay has no voltage.


I bench tested the relay by applying power to #1 & #2 and you can clearly hear it close.


I still have a dead short on the shift lock switch fuse in the engine bay.


Thanks,


Rocky

thefiatek

Let's work with the short first and get it fixed before moving on to the fuel pump.

Check the G/R to body ground not the black wire. Unplug the green tail lamp sensor and check the G/R there. It's ok if you have resistance plugged in. If you unplugged the rear harness, your short is elsewhere. 79 ohms is a relative ground so you still need to get an infinity reading.
Have you unplugged the radio? If you haven't, do so and also unplug the H/L switch and check the lighter. If you still have resistance unplug the black connector on the top left of the fuse box (It's a P.I.T.A.) this is your front light circuit. The remaining plugs can also be unplugged from the fuse panel if you still get ground. The second one back on the top (blue) is the H/L dimmer relay. If you still have ground on the G/R you have now isolated it to the main harness. You may as well unplug the shift lock plug while your in there with the lighter.

Don't forget to unhook the battery and check to a body ground.

Sounds like your relay is fine.