One thing that happened when I took the car in for its inspection was we saw the front turn signals were not working. It felt a bit like they both failed together. They had been mostly working before, but now there was nothing. The driver's side had been ok and the passenger side did not work originally. I had replaced all the bad bulbs last October when the car originally arrived and the passenger side worked some of the time. A local shop had recently suggested this broader failure may be the column switch (the switches are notorious it seems) so I started out looking for something involved. More on that in a few moments.
I had pulled the passenger side side marker housing apart previously and cleaned the contacts a little bit, replaced the bulb, and still got nothing. So I ordered a new housing. But you need to remove the front bumper to replace the housing; something I don't think I can do alone without messing up the paintwork. I have been waiting to take on that job but had not gotten to it yet.
Now neither side worked, but in different ways which was not obviously apparent. What was wrong? Most car people I meet are better mechanics than I am, often far better, but electrical stuff usually scares them off. I have dabbled with electrical stuff for many years so while cars are a bit odd in some ways, they are much simpler beasts. I thought maybe walking through how I tested to find my issue might be helpful, if a bit humbling.
Here is where we started:
Driver's side parking light & turn signal are both dead. Side marker is ok.
Passenger side marker light is dead. Parking light is ok. Flasher does not seem to work.
Rear parking lights and turn signals are all fine. Phew.
All of these circuits, all lighting, ignition, you name it, are all loops, going from the battery to some device and then connecting to the chassis (ground). Sometimes there is a switch in the loop, sometimes there is a fuse as well. You know they are parallel if for no other reason than different things are on different fuses and can be on/off or broken and they do not affect anything else. Like in your house - same idea. There is a nice series-parallel intro here: (https://www.swtc.edu/Ag_Power/electrical/lecture/parallel_circuits.htm
No matter how much stuff shows up on the drawing, it's just a bunch of loops like this one:
It's all loops, in parallel. (drawing: Nissan Academy)
That thick bar on the bottom is the chassis of your car. All the body panels, as well as the engine block and transmission casing are kept at ground potential.
OK, so here we go.
Step 1 - Do you have a schematic?
This should always be task #1 - make sure you have a schematic of the thing you are going to troubleshoot, otherwise you will not know what you're looking at.
Here is the 1988-89 exterior lighting diagram. The front turn signal wiring is largely down the left side. Battery + is up top, and the circuit "flows" to ground at the bottom of the page. So read it top to bottom to help understand what order to troubleshoot in. Your friendly neighborhood physicists may pedantically insist that electrons flow the other way. But they aren't here. Follow it however it makes sense to you. I have spent a lifetime going from + to ground, so that's what we're doing.
All of the switches are shown in their "Off" positions in the diagram. This is a pretty normal convention.
In our case we're lucky because Porsche & Bentley have included a block diagram that shows where everything runs to ground. (Below) The lower half of this diagram shows your lights all sharing ground G106 (again, bottom of the page). The idea here being if your ground connection was bad, you could possibly see issues in any or all of the systems that use it like your horn, radio, front lights, fog lights, etc. Since it is the negative terminal on the battery, the whole car could be "dead" if this ground was really bad. Since we don't have these issues, this main ground is probably fine. It's easy to find and measure at any rate.
Next, we need to know where those fuses are from the schematic. Fuse Box 1, fuses 9 & 10 are used for the front signal lights. Here that is labelled 'Luggage Compartment fuse/relay panel 1' which is really in the middle of the assembly. You can see a pair of 5 amp fuses labelled "turn signals front (left)" and "turn signals front (right)".
Finally, locate the ground diagram in your manual. Your chassis is going to connect to the negative terminal on the battery somewhere. In this case, it also happens to be where the front lighting circuits go as well. Modern cars all use what is called a negative ground wiring scheme. Yes, there were very old cars that used positive ground, so we still specify. Here you can see ground point G106 is up front by the battery. It is where the battery's negative terminal's strap is connected to the chassis. Simple to check, but probably ok because the other systems it serves are ok. The grounds under the car and in the engine bay can be problematic, just something to file away.
Step 2: Devise tests
Whenever you are troubleshooting, you need to isolate systems and test them. Where to start? In this case I wanted to see if the column switch was at fault first, I was thinking I lost the lights and blinkers at once. In this case we're lucky because the hazard switch operates with or without the key and it bypasses the column switch. So if the blinker unit works and the wiring & bulbs are all good, the hazards will work even if the switch is bad. That's a good test because it gives us some clear answers. And it's quick and easy.
Why does that work?
Hazard flashers are designed to work in almost any circumstance, for safety reasons. Looking at the schematic, the hazard switch is top left. When you press it, all of those "arrows" swing to the left. The one that matters most here is on the far left side - that "always hot" lead gets connected to pin 49 on the blinker unit. Hot means energized or connected to the battery. That is how we know this does not need an ignition key or the column switch. We're going to energize this circuit without any of that stuff being involved. Pin 49A is where the blinker power comes from - the 12V for the lights that is turning on and off. Pin 31 needs to go to ground. There is no front or rear flasher, it is for everything. The blinker either works or doesn't, as far as I know.
I like to make copies of the big diagram and then draw on them to help me visualize this stuff. It's a little messy but you should get the idea. The bulbs with 2 filaments at the bottom are the parking light/turn signal combos. The single bulb is the side marker lamp.
I pushed the hazard button in and .... nothing. Only the rear blinkers worked, and flashed at high speed. This told me that the rear wiring is good, and that the flasher unit is working. The column switch could be bad, but if it is, it is not alone. Even if it's bad, it would not stop the front flashers. The fast flashing tells us we have some bulbs out of the circuit.
To check the column switch you can engage it with the ignition on. The indicators should blink. My fronts didn't blink, (more on that below) as expected.
If you test without a key the parking lights will all light - but not blink. (the blinker filament is not engaged in the bulbs) Without a key each of my working parking lights turned on and stayed on for whichever side I selected, which seemed to confirm the column switch (at least 1/2 of it) and the wiring to the bulbs.
This works because if you look at the column switch there are two sections. The left side handles blinking. The right side runs back up to the ignition switch to a different "hot at all times" connection, and then the connections run out to the light switch to light the parking lights in all 4 corners, 2 at a time.
Try and follow the wiring and imagine what the switch is doing. It just takes practice.
Additionally, the turn indicators inside my tachometer were blinking at double speed when the blinkers were engaged (key in, or hazards on). This again confirmed that the blinker unit worked, but also confirmed that we have power & blinking all the way to the top of the fuse panel. It also indicates we have bulbs out of circuit (possibly blown or disconnected) since it is going fast. If you follow the wires on the schematic, you can see the column switch, front indicators, rear indicators and tach indicators are all connected to the same general area just above the fuses. It is odd that the front indicators are fused and the rears are not, but hey, what do I know. ¯\_(ツ)_/¯
Next I measured the voltage at the fuses, top of the fuse and bottom - in case the fuse is bad. The upper side of your fuse panel is where the power goes into the fuse and the lower side is where the power goes off to whatever it is powering. It's just a convention in this car.
Here it is helpful to have an analog volt meter because if the flashers are on, a digital meter will probably just blink "OL" while it tries to auto-range on a signal that won't stop changing. An analog meter will swing back and forth
which will tell you what is really going on. You don't need fancy meters, a $20 Harbor freight digital meter is fine for the garage and you can find used analog meter deals on ebay.
So, I had flasher signal voltage on both fuses, top and bottom so it looks like my wiring is good to the fuse panel. If you followed along, we have gone from the top of the page to about halfway down the page. The only thing left really are the light housings, bulbs, and their wiring & grounds.
Which gets us back to the whole "I replaced the bulbs" thing. Upon further inspection, some of my bulbs were not working properly, and some of the sockets were in bad shape. The big dual filament bulbs (the front turn signal + parking light) have spring loaded contacts, and they not only were corroded but the contact plates were sticking so they were not pushing on the bulbs very well. The passenger side one despite lighting up was only lighting up on the parking light filament - the turn signal filament was not working. So it looked OK with the cover on, but it was not OK. I replaced it and had to work on the socket a bit. That, combined with the driver's side socket being in bad shape meant I had no front turn signals.
I pulled all of the bulbs out, cleaned things up as best I could and shot WD40 into the spring loaded contact plate area. I freed them and moved them around with dental picks. It was really jammed in there. Accumulated dirt & rust I guess.
Then the passenger side marker light got completely disassembled and I polished all of the contacts with a Dremel and white (fine) polishing compound. This included the ground connection, the socket body & blade, and the retainer that holds it in place and makes the ground. Now it works well.
The dual purpose lights are working but if we pull the bumper off for anything I will replace both housings. They are still pretty sketchy even after some work.
The moral I suppose, is that the trickiest situations to fix are the ones where more than 1 thing is wrong. To compound that I made poor assumptions - that the bulbs I had installed were working properly (on both of their filaments & on both sides of the car!) and that I had lost these lights in the same event, but I had not.
I made up for those mistakes by being organized about testing and eliminating possible causes. It is always good to do the tests before you take any action. The tests are usually quick to do; the contact cleaning took the most time in this nearly free repair. (2 bulbs, plus time)
If I had this to do again I would pull off all of the lenses (covers) and see which filaments were lighting etc. and if I was getting a signal (voltage) at the contacts. Check the bulbs, wiggle the bulbs, etc. With hindsight, the housings & sockets are the areas with the most exposure to the elements and are intuitively the most likely place to have issues. Never assume, always start with your basics. When someone brings me a guitar amp to fix, the first thing I do is ignore is their diagnosis so I can go in with a clear head.
Someday I may remember that when working on cars.
The contacts for the driver's side dual-bulb. Corroded, and that disk they are mounted to was stuck - it was not springing against the bulb contacts. Also worth mentioning that even though these bulbs can go in 2 ways, they only work if you install them such that the 2 contacts on the bulb are vertical (like that bit in the socket) after you rotate the bulb to lock it in place, so in practice it only works one way.
All better on this side!
No wonder these didn't work... yuck.
That eye mounts beneath the retaining screw to make the ground connection. We polished up the screw and this contact as well as the brass contacts and base.
the bulb base got cleaned up with 0000 steel wool. This is where the ground connection is made from the brass straps.
The blade (+) contact also got cleaned up with 0000 steel wool
The straps and screw got polished using a Dremel and some white polishing (fine) compound. Nice and shiny.
The light housings are relatively expensive ($150+ ea) and you need to remove the bumper to get to them and their wiring so it is definitely worth the time to polish contacts and so on to avoid this labor. If we do remove the bumper for something else someday I will likely replace the 2 dual-purpose housings as they are both in rough shape from age and are more critical for safety. If I had more patience/time I would soak the contacts in vinegar prior to cleaning. It is pretty effective for getting off corrosion. A good approach left over from my time in restoration work is to always use the gentlest thing that will get the job done.
Aside: Why do the flashers speed up when a bulb (or bulbs) are out?
The most basic rule in DC electronics is probably Ohm's law.
It states that Volts = Current X Resistance. Commonly stated V=I*R
Here in your car, the voltage is nearly always 12, so changing resistance in something will change the current flow (since V can't change).
When you have 2 bulbs in parallel (the front and rear signal on a side) they draw more current than if a single bulb is working because the parallel bulbs have a lower resistance. (R is lower so I has to be higher to keep the other side, V, equal to 12). This should make intuitive sense because the most current you can draw is a "short circuit"; one where R is close to zero and I is as big as it can get. V doesn't change. (see the series/parallel link up above)
The old style flashers worked using a bi-metallic strip that heated up when electrical current was run through it. They connect the lights when cold (notice you always start with the light on) and heat up from the current flow which changes their shape until they disconnect and cool off, only to reconnect once they are back to their original shape. In repeating this cycle they turn your lights on and off at a given cadence.
With half the current (one bulb, or 2X what R is normally) it does not heat up as much and does not change shape as much. As a result the heating/cooling cycle happens faster because they have not gotten as hot, and you get the double time blinking.
Modern circuits have replaced these bi-metallic strips and use a current sensor but the idea is the same. It's designed to tell you there is a problem.
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