Induction Timing Lights and Positive Ground cars

Timing lights, dwell meters, hand-held tachometers, and some other modern electronic tools have had problems when being used on some of our Classic British cars that happen to be electrically outfitted to be Positive Earth (Positive Ground).

Timing Light

The root of the matter is the circuitry of these electronic tools is polarity sensitive. They simply will not work if we try to connect them up in the reverse of what is called for.

As an example, if your electronic tool has a wire with a red clip and a wire with a black clip for power, we would normally connect the red to the positive terminal of our battery and the black to the negative terminal. If the car is set up to be Positive Earth, we can often use our meter by simply closing our eyes to the car we are working on. Red still goes to the positive side of the battery and black still goes to the negative side of the battery. The polarity of the car is a non-issue at this moment.

Many of us do not like to connect our test gear directly to the battery. For battery tests we should always connect directly to the battery. However, for other tests some of us have gotten into the safe habit of first connecting our red clip to the positive side of the battery and then connecting the black lead to the block. This is done to avoid the possibility of a small spark over the battery when the second lead is connected. If you find out that you need new a battery, you may order car batteries for sale online or from your local auto supply shop.

Batteries produce hydrogen gas which is explosive. (This technician has about a dozen small scars on his chest from connecting a timing light to a battery in a running car. The battery exploded when the second cable was connected.) In a Positive Earth car connect the black lead from the meter to the negative battery terminal and then the red lead to the block. Since the block is earth, and the car is Positive Earth, the red lead gets the positive it wants.

Now, what happens if we connect a dwell meter to a Positive Earth car? If the meter were completely independent of the car, except for use as a power source, we could get away with it. But that’s not how dwell meters work. The dwell meter is also connected to the small wire that goes from the coil to the points. (Tachometers usually do the same.) The points go to earth. In a Negative Earth car, that wire will be negative. In a Positive Earth car, that wire will be positive. Our electronic meter is expecting to have negative power in the black lead to the battery, and the same kind of power in the lead going to the points. If the car is Positive Earth, the black battery lead to the meter will be negative, but the lead to the points will be positive. The meter will probably not function.

Timing lights appear to be constructed a few different ways. The red lead for the battery can be put to positive terminal, and the black for the battery to the negative terminal, irrespective of how the rest of the car is wired. The challenge comes in the inductive pick-up most modern timing lights have. They look like a clothes pin that simply clips onto the #1 spark plug wire. When a spark passes through the plug wire, the induction pick-up detects it, and tells the timing light to flash once. Most of these induction pick-ups have an arrow on them showing the direction the spark is supposed to be traveling. So, we put the induction pick-up onto the plug wire with the arrow pointing toward the spark plug.
If the car is set up to be Positive Earth, the spark will run the other way. In a case like that, the timing light may not work at first, but will often work if you simply turn the induction pick-up around so the arrow points to the distributor cap.
Sometimes even this won’t work, but a trick might work. If you use a second independent battery to power the timing light, it will sometimes work. This means bringing a second battery close to the car so the timing light can get power from it. Or bringing another car over as if you were going to give the car a jump start, and connecting your timing light to the second car’s battery. (The second car does not need to be running.) This will probably not work on a dwell meter, because it’s looking for a contact with ground through the lead that goes to the points. If the power cables for the dwell meter are connected to another power source, it will have trouble identifying the ground the points are connected to.

Modern timing lights and dwell meters almost always work with older, Negative Earth cars. With a Positive Earth car, an electronic dwell meter probably won’t work. With a little creativity, timing lights will usually work. Of course, if your car was originally a Positive Earth car but it’s been converted to Negative Earth, these issues are moot.

Lastly, since we are talking about batteries and such, let’s talk about jump starting.

Using a modern car to jump start another car means taking a risk. The modern technology in the newer cars is not designed to deal with the impact of a jump start. For that reason we make two suggestions:
1. It is better to use a battery charger to recharge the dead car’s battery than to use your modern car.
2. If you must use your modern car, turn it off. Run your cables. Start the second car. Carefully disconnect you cables starting with the one on the block. Then, you can start your modern car again. If the dead car does not respond quickly to the jump, you should give up the idea of using your modern car for the jump. If the dead car won’t start quickly it will draw a lot of power out of the modern car’s battery. Most modern cars have alternators that can put out over one hundred amps. That brings a lot of current into the electric system to recharge the modern car’s battery after the jump. The battery can easily be harmed. Even if the dead car starts quickly, you should turn the lights on in your modern car when you start it to draw off some of the current for a few minutes as the modern car battery is recharged.

When attempting a jump start, we need to keep two points in mind about placing our cables.

1. We need to connect positive to positive and negative to negative. Run a cable from the positive terminal in one car to the positive terminal of the other car. Do the same with the negative. If one of the cars is Positive Earth, it makes no difference. We still connect positive to positive and negative to negative. What the car does with the positive and negative is not consequential to the jump start.

2. It is best not to connect both cables to the battery. Connect one to the battery and one to a place a foot or two away. The second cable will always make a spark. Batteries explode.
In a conventional Negative Earth car jump starting another Negative Earth car, you can connect the positive terminal of one battery to the positive terminal of the second battery. Then connect the negative cable to the block. The block is negative so the net result is the same. We are just getting that second cable away from the battery and the hydrogen gas. Connect the cable to the block second. A spark at the block is harmless. Disconnect in the reverse order. Disconnect the block, then disconnect the battery.
If a Positive Earth car is trying to jump another Positive Earth car, the principle is the same. Connect the negative terminals of the two batteries together. Connect your positive cables to the block of each engine.
If we are trying to jump a Positive Earth car with a Negative Earth car, turn the Negative Earth car off. Connect the black cable to the block of the Negative Earth car, and the other end to the negative terminal on the battery of the Positive Earth car. Connect the positive cable to the positive battery terminal of the Negative Earth car, and the other end to the block of the Positive Earth car. We’re still going positive to positive and negative to negative. When done, disconnect at the block first.