current used by light in a DC circuit tester

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Cleveland Apprentice

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Cleveland, Oh
Hello,

I have a 12 volt DC circuit tester. Per the instructions, it is not recommended that it is used to test for power on computer circuits due to the risk of damaging the computer circuitry as a car's ECM and BCM, which I understand. For the heck of it, I used a INNOVA 3320 multi-meter and measured 6.3 ohms through the tester. At 12 volts this equates to (don't have exact figure) something like 1.9 amps. This seems too high for the bulb in the tester to draw that much current. Does anyone agree? Do I have to make adjustments for the resistance in the multi-meter test leads? How would I do this if required? Also, the bulb is not LED.

The INNOVA 3320 multi-meter is advertised at 10 MegaOhm input on DC circuits for safety. I don't understand this, can anyone explain this to me? Unfortunately, I am not well versed in this.

Thanks in advance!
 
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Test light resistance

Test light resistance

If your test light is incandescent, and most probably it is, then its cold (unlit) resistance will be much lower than its hot (or lit) resistance.

You can read its hot resistance by using a multimeter with a DC current range of say 1 ampere. Put one end of the test light on ground, the other end to the current test lead and finally connect the common test lead to the hot on a car battery.

Then put the multimeter on to DC volts (20V or more) and read the voltage of the battery.

Now you can use Ohm's law to calculate the hot resistance of the test light.
 
Computers

Computers

I am not directly familiar with automotive computers. I am quite familiar with computers in general. The rated load of many computer signal lines (to sensor for instance) is on the order of 20 milliAmps (0.020 amperes).

Your test lamp connected to a computer signal line that expects currents like that would look like a bolted fault and could easily destroy a component of the computer (and perhaps the whole computer). Pretend that the signal voltage is 6.3 volts, then the cold resistance of your test light would draw 1000 milliamps (1 amp). I would suspect that a computer in such an electrically hostile environment as a car to be pretty well protected, however.

Your meter, on the other hand, has a resistance of 10MOhms. For a voltage of 6.3 volts, the current would be .0000006 amps (or 0.6 microAmps).
 
The other problem with using a low impedance test light on a signal lead is that it will look like a closed circuit to ground and the computer will react accordingly, possibly causing damage elsewhere. You do not want to lie to the engine control computer unless you know exactly what the results will be. :)
Putting a high current onto a signal lead (for instance connecting the test lamp from +12 to the lead) is far more likely to burn something out.

Tapatalk!
 
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