JHZR2
Staff member
Originally Posted By: meep
MUCH EASIER WAY TO CHECK GROUNDS.
engine off, key on.
put extension leads (scrap wire) on the multimeter. Set it to DC volts. Probe between battery post (-) and different points on the chassis. Try to run a healthy load, headlights and brights. Maybe the heater fan. Work out the draw of these devices. Say 8 amps for fan, 10 amps for lights, and 5 for engine electronics. That's 23 amps coming from battery (-).
now, V=IR ... volts = amps x resistance. volts displayed/23 amps = resistance. but you don't need to even worry about the math. You shouldn't see more than .5V lost over ground, preferably much less.
Now start the car and let it run for 10 minutes. Do the same test between ALT frame and battery (-) and ALT and chassis. anywhere you get a higher V reading, there is resistance in that path.
in the bimmer, your 11.9-12.3 V battery may be toast.
13.8 and 14.1V is very healthy for a vehicle, unless the battery temp is cold, like 40F or less-- when it needs a higher voltage to charge quickly.... BUT this only works if the voltage is lowered as the battery warms up... otherwise you overcharge and burn off fluid.
So--- you may not have a problem with the regulator, in other words.
The higher the voltage displayed, the more resistance that exists between those two points.
If you are seeing 12.3 V at battery after a long drive, you may have a shorted cell. 12.6 V = full charge. 13.2V is possible with full charge + surface charge.
Telltale will be the volt drop test between alt frame and batt (-). if you see anything approaching .5V, you need to clean grounds, replace grounds, or run supplemental ground.
Good stuff except that it is a long run to the battery, which is in the trunk.
means I have to account for the resistance in the lead. No big deal, and this may be the path I need to take. Thanks!
MUCH EASIER WAY TO CHECK GROUNDS.
engine off, key on.
put extension leads (scrap wire) on the multimeter. Set it to DC volts. Probe between battery post (-) and different points on the chassis. Try to run a healthy load, headlights and brights. Maybe the heater fan. Work out the draw of these devices. Say 8 amps for fan, 10 amps for lights, and 5 for engine electronics. That's 23 amps coming from battery (-).
now, V=IR ... volts = amps x resistance. volts displayed/23 amps = resistance. but you don't need to even worry about the math. You shouldn't see more than .5V lost over ground, preferably much less.
Now start the car and let it run for 10 minutes. Do the same test between ALT frame and battery (-) and ALT and chassis. anywhere you get a higher V reading, there is resistance in that path.
in the bimmer, your 11.9-12.3 V battery may be toast.
13.8 and 14.1V is very healthy for a vehicle, unless the battery temp is cold, like 40F or less-- when it needs a higher voltage to charge quickly.... BUT this only works if the voltage is lowered as the battery warms up... otherwise you overcharge and burn off fluid.
So--- you may not have a problem with the regulator, in other words.
The higher the voltage displayed, the more resistance that exists between those two points.
If you are seeing 12.3 V at battery after a long drive, you may have a shorted cell. 12.6 V = full charge. 13.2V is possible with full charge + surface charge.
Telltale will be the volt drop test between alt frame and batt (-). if you see anything approaching .5V, you need to clean grounds, replace grounds, or run supplemental ground.
Good stuff except that it is a long run to the battery, which is in the trunk.
means I have to account for the resistance in the lead. No big deal, and this may be the path I need to take. Thanks!
