13.1.3. Technology of measurements

Voltage measurement

Presence of tension can be proved already by means of a simple control bulb or the indicator of tension. However, in this case is defined only whether tension in general moves. To determine the size of the enclosed tension, it is necessary to connect the voltmeter. First of all, when using the voltmeter it is necessary to establish the range of measurement in which presumably there is a measured tension. As a rule, tension in the car does not exceed 14 V. Isklyucheniye the system of ignition makes: here the secondary tension of system of ignition can reach up to 30 000 Century. This high voltage can be measured only by means of the special measuring device – an oscilloscope. First of all by means of the selector switch the range of tension of a direct current of DCV, contrary to ACV tension of alternating current is established. Then measurement range is chosen. As the car except system of ignition has no voltage over 14 Volts, it is necessary to set the top limit of the established range of measurement it is slightly above (about 15-20 V). If the measured tension obviously much below, for example, it is in range of two Volts, then it is possible to switch to narrower range to reach the bigger accuracy of counting. If higher tension is enclosed, than it is defined by the range of measurement of the device, then the last can be spoiled.

According to the drawing, connect wires of the measuring device parallel to the consumer. At the same time the red measuring wire is connected to the wire going from a positive pole of the rechargeable battery, and a black measuring wire – to the wire connecting the rechargeable battery and the case of the car, or directly to the mass of the car, for example, to the block of cylinders of the engine.

Example of control: if the engine is not started properly because the starter too slowly rotates, it is expedient to check tension of the rechargeable battery during pressing of a starter. For this purpose (+) the voltmeter attach a red wire to the red plug of a positive pole of the rechargeable battery, and a black wire – to the mass of the car (–). Then charge to the assistant to press a starter and consider tension size. If tension is lower than about 10 V (at a temperature of rechargeable battery of +20 °C), then it is necessary to check the rechargeable battery and, perhaps, before the following attempts of start of the engine to charge it.

Measurement of current

In the car rather seldom it is required to check current. For this purpose the ampermeter which is built also in in the combined measuring device is required.

Before measurement of current the measuring device is installed on the range in which approximately there is a measured current. If it is not known, then establish the highest measuring range and if there is no indication, consistently switch the device to lower ranges.

For measurement of current it is necessary to separate an electric chain and in a rupture of a chain to connect the measuring device (ampermeter). The plug, and a red wire is for this purpose pulled together (+) the ampermeter it is connected to a conducting wire. The black wire (–) is connected to contact to which the broken-off line is connected. In this case the grounding contacts of the consumer and the plug have to be connected by means of an auxiliary wire.


At all do not measure current by the ordinary ampermeter in the wire going to a starter (about 150 And), or in glow plugs at diesel engines (to 60 A). Because of the high currents arising at the same time it is possible to burn the measuring device. In auto repair shops for such measurements the ampermeter equipped with pincers of a direct current is used. At the same time current-measuring pincers are clamped over the isolated electrical wire, and the size of current is measured by means of induction.

Measurement of resistance

Before measurement of resistance it is necessary to be sure that the design element to which the ohmmeter is connected is not energized. Therefore every time pull down the plug in advance, switch off ignition, for example, disconnect a wire from the unit or disconnect the rechargeable battery. Otherwise the measuring device can be damaged.

The ohmmeter is connected to two plugs of the consumer (loading) or by two ends of the electric line. At the same time does not play any role what wire of the measuring device joins plugs.

Measurement of resistance in the car is subdivided into two main areas:

Check built in in an electric chain of the resistor or an element of a design.

"Test for passing of current" in the electric line, in the switch or a heating spiral of a heater. At the same time it is checked whether the electric chain in the car is broken off and therefore the connected electric device cannot function. For measurement the ohmmeter is connected to both ends of the respective electric line. If resistance is equal Oh, that in this case electric current, i.e. the electric line passes as it should be. In case of the broken-off line the measuring device shows infinity (in Ohms).

Installation of additional electric accessories


Wires which at installation of accessories have to be laid to the built-in set of wires in the car, whenever possible, should be stacked always along separate plaits with use of collars for their fastening and rubber plugs. To prevent emergence of noise during driving and to avoid attrition of wires, again laid wires should be strengthened by means of an insulating tape, plastic weight, tape collars for their fastening, etc. At the same time especially it is necessary to pay attention to that between pipelines of the brake drive and rigidly laid wires the minimum distance in 10 mm was maintained, and also between the pipeline of the brake drive and wires which vibrate together with the engine and other details of the car, there was the minimum distance in 25 mm.

When drilling openings in a body it is necessary then to smooth out, otgruntovat and varnish edges of openings. The shavings which are inevitably formed when drilling should be removed from a body completely.

When carrying out all installation works which concern electric conducting to prevent short circuits in system, it is necessary to disconnect the wire of mass of the rechargeable battery (–) connecting it a pole to the car case and to hang it aside.


If wires are disconnected from plugs of the rechargeable battery, then under the known conditions the device of storing of malfunctions in a control system of the engine and transmission and in anti-blocking system, and also other electric devices as, for example, the radio receiver and hours, and the saved-up sizes are erased are taken out of service.

If additional electric consumers are established, then in each case it is necessary to check whether it is possible to load still in addition available three-phase generator of alternating current. If it is required, it is necessary to provide installation of the new generator having rather big power.