|
|
You need to do a 4 wire "Kelvin" style resistance measurement.
http://www.allaboutcircuits.com/vol_1/chpt_8/9.html
The tricky part for the typical home builder is the current source. You can make a current source that is good enough with a battery charger and a headlight.
Put the headlight in series with the positive lead of the battery charger.
Using a jumper wire, connect the other terminal of the headlight to the negative post of the battery.
Connect the negative lead of the battery charger to the engine block.
Read the current on the charger ammeter. It should read about 6 amps for the high beam and about 4 amps if you connected to the low beam.
Using a good quality voltmeter, set on the 10 millivolt scale, check the voltage between:
1) Negative post and the airframe. (0.001 Ohm or less. High beam = 6 mV or less)
2) Negative post and the engine block. (0.001 Ohm or less. High beam = 6 mV or less)
3) Engine block and the airframe. (less than 0.001 Ohm High beam = 6 mV or less)
4) Across the ground strap (terminal to terminal, NOT bolt head to bolt head.) (About 0.0005 Ohm. High beam = 1 mV to 3 mV)
5) Bolt heads and the terminals they are holding. (less than 1 mV)
6) Across a large section of the fuel line. (much less than 1 mV)
Calculate resistance by dividing the voltage by the current. Resistances should be less than 0.001 Ohm.
Take notes as you measure voltages. Anything that is carrying current will have a voltage drop. The fuel line should not carry significant current, so it should have a very tiny voltage drop.
The ground strap should be carrying all of the current, so it should show some voltage drop. If it has no voltage drop, it is carrying no current and something is wrong. It should show a voltage drop similar to the drop on the negative battery cable (per foot.)
Number 4 cable has 0.000253 Ohms per foot.
Number 2 cable has 0.000159 Ohms per foot
If your battery charger shows 6 amps, you should measure about 1.5 millivolts across a 1 foot #4 ground strap.
Bill Dube'
|
|