fairlooki.blogg.se

The added resistor will limit current flow and prevent circuit damage during testing. This tool is available from Schaffer Test Products. Photo 4 shows a test light that has been modified by replacing the bulb with a common 10-ohm by 10-watt resistor. Six amps is enough to power the headlights and could damage the wiring and supply circuits.Ī safe way to load-test low-resistance circuits is to limit the amount of current during the load test by adding extra resistance. Example 2ġ2-volt circuit with a 2-ohm solenoid will draw 6 amps of current Resistance in these solenoids is as little as 2 ohms Connecting 12 volts directly to a low-resistance solenoid can draw enough current to cause circuit or component damage. When load-testing this way you must be careful when energizing low-resistance solenoids such as PWM, EPC or fuel injectors. Again apply Ohm’s law to determine exact circuit resistance and compare this with the solenoid resistance specifications. Although this does not check the computer or its grounds directly, it is a versatile way to load-test the solenoids and wiring. Remove the connector from the computer and supply power or ground through your ammeter to the individual solenoid wires at the computer connector (see Photo 3). You can perform the job of the computer and turn the solenoids on and off with your ammeter. Use a schematic to identify wire color and terminal location and to determine whether the computer supplies power or ground to the solenoids. If your scan tool cannot control individual solenoids, a more-universal method to load-test circuits is to use your ammeter to energize solenoid circuits directly by supplying power or ground as needed. Not bad for five minutes’ work with a scan tool, ammeter and calculator.īi-directional scan-tool controls apply to only some vehicles.

This will quickly verify good circuits and isolate bad ones. Testing this way checks everything – power supply, ignition switch, alarm relays, wiring, solenoids, computer and grounds. Example 1ġ3.88 volts divided by 1.13 amps equals 12.2 ohms circuit resistance. To determine your exact circuit resistance, check your exact voltage and apply Ohm’s law. Generally a 24-ohm solenoid will draw about 1⁄2 amp. Use a scan tool with bi-directional controls to command the individual solenoids on (see Photo 2) and measure individual solenoid current draw. If the shift indicator or some other device is drawing current from that fuse location, then zero out your ammeter before proceeding. Now any current (amps) that feeds the transmission will go through the ammeter. You can connect your meter leads to the fuse receptacle with the blades from an old fuse. Put the leads of your ammeter in place of the fuse that was removed (see Photo 1). If you have a GM vehicle with solenoid or quad-driver codes, or you need to verify system integrity, try this method: Here are three approaches to load-testing circuits and performing amperage tests that can make life in a transmission shop easier. Testing current flow through a circuit can be done at the beginning, middle or end of the circuit, since the amperage is the same in all parts of the circuit.

If you know the amperage a solenoid draws and the circuit voltage, then circuit resistance is easy to find by applying Ohm’s Law. The way I like to remember Ohm’s Law when in the shop is with the acronym VCR, same as the now-outdated videocassette recorder, only here it refers to V= voltage, C= current, R= resistance. Load-testing stresses the whole circuit the same way it works during normal operation and is the fastest and most-effective way to diagnose electrical-circuit problems. Although this works well in many instances, there will always be circuits that check OK by resistance but will fail when a load is applied. We have all been taught to test circuits by measuring circuit resistance with an ohmmeter.

The volt (V) is the unit that indicates electrical potential.The ohm (Ω) is the unit that indicates resistance to current flow.The ampere (amp) is the unit that indicates the rate of electric current flow.