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- What To Do With a Pocket Bike
- What to Do When You Fail an MOT
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Most circuits will be operated by some form of a switch. This can be either manually operated by the driver or automatically by a sensor launch x431 v plus. A relay may be introduced into the system, in order to provide more power than a conventional switch could handle. Relays generally control the ground side of the circuits. In order to diagnose a problem with a circuit, you must first understand how the circuit should work; then you can determine why it doesn't work.

Let's start with a simple case, a taillight. For example this case power goes to a switch, then back to the taillight bulb by way of a fuse, a few connectors, through the bulb, and to ground. If any one of these components are not working right, you will lose a taillight. Now since the taillights are not switched with ignition they can be turned on without the key in, we know that power goes from the battery to the switch, through the correct fuse, then out to the taillights. In this case the switch is operating in the hot side of the circuit; if it is not working, or if it is off, no power will be applied to the bulb. The bulb is always ground, and will light up at any time that it gets power.

So now a look at another possibility, the fans for the radiator. These are switched by a temperature switch whenever the radiator gets hot enough. On cars with factory AC Autel MaxiDiag MD808, they can also be turned on by turning on the AC, but we will look at a simple non-AC car. So how does it work? In this example, power from the battery is always available to the fan so that it can run even when the car is turned off. However, because of the resistor in the circuit, once power is drawn by the motor, the voltage will be less than full battery voltage, causing the fan to run more slowly. This is what will allow the fan to turn on even when the car is parked. There is also a relay that will supply full power to the fan whenever the ignition is on - the normal mode of operation.
Once we get past the power source - relay or resistor - this circuit takes a very different turn (as compared to previous examples). Power goes directly to the fans. After the fans, it goes to the temp switch, then to ground. As a result, diagnostic attempts on this circuit will yield rather different results than the taillight circuit. The taillight bulb should be grounded all the time. The fan, however, will not be grounded until the radiator is hot enough! Therefore you must test for a ground at the temp switch, not at the fan. At the fan you should be looking for power, and continuity (0 Ohms resistance) to the temp switch. If you short out the terminals of the temp switch at any time, the fans should turn on - if they work.

One last example, a variation on the above theme, as a comparison; this one, the horn circuit. As before, we have the arrangement of power going, in turn, to the fuse, relay, horns, and ground. The main difference here is in the switching for the relay; whereas the above circuit is switched by ignition, this circuit is switched by the horn switch (H, on the steering wheel). The relay is turned on using a ground switch just like the temp sensor; in this case, power goes through the control circuit on the relay, through a wire to a contact on the steering wheel and up to the horn pad. When you press on the horn pad, the wire is grounded to the steering column, completing the circuit, turning on the relay, and providing power to the horns.
Now you can see why so many electrical problems arise in these cars when grounds are not in good shape; switching of all the circuits is completely dependent on having good grounds!


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