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03 mazda protege

With key on engine off (KOEO) if I load the power window circuit, my voltage goes from 12.6 volts to 10 volts. This only occurs if I continue holding the window switch down after the window hits the bottom of its travel. When actually rolling down voltage only drops to 12.4v which is normal.

With key on engine running (KOER) if I turn on my headlights there is a noticeable rpm drop although it is slight. If I conduct the same window switch test with the headlights and engine on the headlights dim out and the alternator doesn’t seem to “pick up” lost voltage. Even accelerating from a stop the headlights will dim out and then brighten back up. All grounds are functioning properly with no resistance. Primary power cable is working properly with no resistance faults.

the only thing I can think of would be that the alternator had a large amount of water splashed on it and the main fuse (100A) blew. I replaced the failed alternator and the main fuse. Not sure if the fuse block was damaged or if any wiring was harmed although no other fuses were burnt.

battery tested fine when I conducted an internal resistance and load test with my professional testers.

DavidRecallsMonica
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lockheed
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1 Answers1

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There's no such thing as "no resistance" unless your car is in a tank of liquid helium. You're trying to measure resistance in the milliohm range with (I assume) a typical digital multimeter, and that's just not possible. Milliohms matter when you're dealing with heavy currents. As stated above, just a 100 milliohm resistance on each battery cable clamp will give you the voltage drop that you measured.

You're also trying to measure resistance under load, but that load is creating a voltage across your resistance that interferes with the meter's own applied voltage that's used to calculate resistance. That 2 ohm resistance reading will change (possibly to impossible negative resistance) if you reverse the meter's leads, which means that the 2 ohm measurement is invalid.

You can measure actual resistance in the milliohm range by running a known current (amps) through a connection and measuring the voltage in millivolts across the connection. But the current draw of your stalled window motor is unknown. If you follow the procedure in the comment above and assume 25 amps, you'll learn the battery terminal resistance +/- 50% or so.

In order to diagnose the possible alternator issue, you need something like a carbon pile battery tester to create a known and variable amp load for the alternator, a clamp-on DC ammeter and a digital voltmeter.

Before you even go there though, I suggest you measure battery voltage directly on the battery posts during window motor stall. If the battery post voltage is in the 10 volt range with this modest window motor current, you have a bad battery regardless of what the battery tester is saying.

For a proper battery test, fully charge the battery, then subject it to a load of 50% of the battery's CCA. (That's going to be 200-400 amps or so, requiring a carbon pile tester.) Maintain that current for 15 seconds, then measure battery voltage at the last moment of testing. If the battery voltage at that moment is less than about 10.5 volts, depending on temperature, the battery is past its useful life. Electronic battery testers that don't put a heavy amp load on the battery are worse than useless.

Once you know you have a good battery and you have a measured battery cable clamp and ground connection resistance in the low milliohm range, you can start troubleshooting the alternator by placing it under a known amp load at various engine RPM and watching how the voltage changes.

MTA
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