Modern timing lights and dwell meters almost always work with older, Negative Earth cars. 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. (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. 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. This means bringing a second battery close to the car so the timing light can get power from it. If you use a second independent battery to power the timing light, it will sometimes work. Sometimes even this won’t work, but a trick might work. 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. If the car is set up to be Positive Earth, the spark will run the other way. So, we put the induction pick-up onto the plug wire with the arrow pointing toward the spark plug. Most of these induction pick-ups have an arrow on them showing the direction the spark is supposed to be traveling. When a spark passes through the plug wire, the induction pick-up detects it, and tells the timing light to flash once. They look like a clothes pin that simply clips onto the #1 spark plug wire. The challenge comes in the inductive pick-up most modern timing lights have. 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. Timing lights appear to be constructed a few different ways. 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. 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. In a Positive Earth car, that wire will be positive. In a Negative Earth car, that wire will be negative. (Tachometers usually do the same.) The points go to earth. The dwell meter is also connected to the small wire that goes from the coil to the points. 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. Since the block is earth, and the car is Positive Earth, the red lead gets the positive it wants. 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. (This technician has about a dozen small scars on his chest from connecting a timing light to a battery in a running car. Batteries produce hydrogen gas which is explosive. This is done to avoid the possibility of a small spark over the battery when the second lead is connected. 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. For battery tests we should always connect directly to the battery. Many of us do not like to connect our test gear directly to the battery. The polarity of the car is a non-issue at this moment. Red still goes to the positive side of the battery and black still goes to the negative side of the battery. 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. They simply will not work if we try to connect them up in the reverse of what is called for.Īs 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. The root of the matter is the circuitry of these electronic tools is polarity sensitive.
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