Some Electrical Basics and Troubleshooting

Many people are stumped by automotive electrical systems. It really doesn’t have to be very mystifying. I hope to cut through the fog of confusion with this and other posts to help you figure out electrical issues with your early Z car.
Let’s start with the basics.
Most of the electrical system of the S30s consists of a source (battery or alternator), conductors (wiring), switch, and load.
For most of the electrical diagnostics you will need to perform, you need to be familiar with Ohm’s Law:
V=I*R
or
I=V/R
The second equation emphasizes that current is proportional to voltage and INVERSELY proportional to resistance. Therefore, as the resistance (load) increases in the circuit, the current drops.

This brings up an interesting point. When you think about the fuses and their ratings, you probably think that they are in the circuit to protect the component (headlight, radio, etc.). Actually, the fusing is protecting the wiring. Unless you have a problem with the voltage regulator, the voltage in the circuit will be about 12 to 14.5 volts. The smallest fuse in the fuse box is 10 amps. If the fuse was rated to protect the component, the rating would imply a load of a little more than 1.2 to 1.45 ohms. A typical high beam headlight of the vintage is about 60 watts. By using the equation below, that gives an impedance of around 2.4 ohms (assuming 60 watts is at 12 volts). I can guarantee you from past experience that a headlight will blow at 17 volts or so. That is only about 7 amps. However, short circuits are a bigger worry. If a component shorts to ground, the fuse needs to blow before the wire melts. Therefore, consider the gauge of the wire for the fuse rating.
P=V*I
I=V/R
P=V2/R
R=V2/P
R=122/60
R=144/60
R=2.4 ohms

So if you’re still with me, let’s talk about measurements.
Voltage can only be measured across a source or a load.
Current has to be measured through a single conductor.
Resistance is measured across a load or can be measured along a conductor to ensure a continuous circuit (continuity).
A meter that measures voltage, current and resistance is referred to as a multimeter. When using a specific function on the meter it is common to refer to the meter by the operational function, such as voltmeter, ammeter or ohmmeter.
Here’s a link to a video on using a multimeter. If you search Youtube, you can find a lot more videos on the subject.
For basic diagnostics, an inexpensive meter will work, but when you are trying to diagnose problems with your charging system, you will probably need a higher quality meter. I have never had problems with Fluke meters. Just be ready to pay a decent price for one.

Phantom draws:
I want to touch upon a common problem I have seen in various forums. Many people have tried to diagnose why their battery keeps running down when the car is off. Many people refer to this as a phantom draw. The most irritating thing about this problem is that one of the most common diagnostics proposed is to pull fuses until the draw stops. It is a very unsystematic approach to solving the issue. The problem can be addressed through a much better approach using a good understanding of electrical principles.
Keep these things in mind:
1. A source can be a load to a source with higher potential (voltage).
2. Remember that for the drain to happen, you have to have a closed switch (or no switch) and a load.
3. A switch can fail closed.
To diagnose the problem, the first thing you want to do is put an ammeter in series with your battery.
1. Put your ammeter on its highest setting. (Warning: You will probably need an ammeter that can measure at least 2 amps.)
2. Disconnect the positive cable of your battery.
3. Connect the positive cable to the negative terminal (common) on the ammeter. (Be sure not to let this touch the chassis of your car.)
4. Connect the positive terminal of the ammeter to the positive terminal of the battery.
5. Make sure the car doors are closed, so the dome light is off.

Now you have a baseline for the amount of current in the circuit draining your battery.

A bad voltage regulator can easily drain your battery. The voltage regulator acts as a switch between the alternator and the battery. When the regulator fails, the windings of the alternator become a load to the battery when the car is off. If the car has an external regulator, unplug it. If the regulator is internal to the alternator, unplug the alternator. Check the ammeter after this. If the current draw drops significantly, replace the alternator and regulator.
Of course, it is always good to go after the low hanging fruit. If you have less than 1 amp draining from your battery it could be from these likely sources:
1. The glove box door isn’t closed fully, so the light is draining the battery.
2. Your aftermarket stereo constant voltage line is drawing too much current.
3. You (or a previous owner) installed an alarm system, power locks, etc., that is drawing power all of the time. Either trash the added component or just hook the car up to a battery charger when you’re at home.
4. You (or a previous owner) modified the charging system in a 240Z or 260Z by replacing the alternator and external regulator with an internally regulated alternator without considering whether or not the car was wired for an electric fuel pump. The original Nissan designs for an electric fuel pump used two relays. One was energized by the starter circuit, and the other was energized by a wire between the alternator and the regulator. This way, if the engine died, the fuel pump stopped pumping. You’ll have to forgo the electric fuel pump or find an oil pressure switch to wire in (not for the faint of heart).
(Update: I forgot another easy one.)
5. Make sure your brake lights aren’t on all of the time. Sometimes the brake light switch is damaged, and the brake pedal stops making contact with it. The switch is a normally closed switch, meaning that when you press the button on the switch, the circuit opens up. When you release the brake pedal, the arm presses against the switch to turn off the brake lights.

Now, here are some less likely culprits:
1. The contacts in a relay have welded closed. There aren’t too many relays to look for in an S30.
2. The switch in the back of your ignition switch is failing.
3. Corrosion in a switch has bridged the contacts, keep the circuit closed. This isn’t very likely, though.

Do you see how most of these issues revolve around a bad switch or non-existent switch?

If you want to pull fuses, I suggest looking at the wiring diagram in the factory service manual and first pull the fuses that come off of the large white/red wire. In the 240Z, there are only three circuits that fit that description. Isolate and test switches where you can to track down your problem.

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This entry was posted in 240Z, 260Z, 280Z, Electrical, Tech Tips. Bookmark the permalink.

8 Responses to Some Electrical Basics and Troubleshooting

  1. Pingback: 78 280-z battery drain - ZCAR.COM - Datsun : 240Z, 260Z, 280Z, 280ZX, 300ZX, 350Z, 370Z : Nissan

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  5. Andy Downs says:

    I would like to learn what safety devices are intact for the added electric fuel pump on my 73 240Z. I haven’t found a good wiring schematic that I could see and it won’t feed 12v until the car starts. Would like to eliminate my pour working mechanical pump but not sure if the electric pump will come on unless the line is pre primed with pressure. I had to jump 12v directly to my electric pump to prime the system after a gas line repair

    • I’m not going to get much into the details due to the limitations of the comments.
      The electric fuel pump retrofit for the 73 was wired in a manner similar to the fuel pump for the 74. Here is the reference that goes into a little detail: 73/74 Fuel System Modification Plus Manual. Look at page 11. There are two relays added. Relay A’s coil is energized from the alternator. Relay B’s coil is powered from the circuit completed when Relay A is energized. When Relay B is energized, the fuel pump has power. Basically this keeps the fuel pump from being powered if the engine is dead.
      Replacing the mechanical pump is fairly easy to do, so don’t rule that out. To get a better idea of possible solutions, I suggest you join Classic Z Cars if you haven’t already done so. Start a thread in the Carburetor Central forum and be sure to provide plenty of details like what carburetors you have on your car and why you think your mechanical fuel pump isn’t working well.

  6. Andy says:

    I am keeping the mechanical as you suggested. Unfortunately I have AC and will be blanking of the area while getting the original pump rebuilt. I’m still wondering if the electric pump will prime at first start up. I’ve already tried cranking the engine without starting and found it wouldn’t supply power at that point. Usually there is enough gas in the carbs to get it going if I’m thinking correctly. Thanks for the info and I’ll get to tracing the complete circuit soon so I’m educated if future problems arise..

    • The electric fuel pump will not have power until after the engine turns the alternator fast enough so that the alternator can energize the coil on the fuel pump relay. If your carburetors are set properly and functioning to spec, they should have enough fuel in the bowls to get the engine started so the fuel pump relay gets energized. There are some threads on the retrofit kit over at Classic Z Cars.

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