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I own a 1994 Mazda Protege. On Sundays, I measure the voltage at the battery terminals, and register it. First with the car turned off, and then with the car idling. The voltage with the car turned off is usually between 12.76 V to 12.8 V. With the car idling and the alternator running, it usually goes up to 14.46 V to 14.5 V. But yesterday something weird happened: the battery is ok, but when I start the car, and measure the voltage at the battery terminals, it does go up to 14.5 V, but it drops suddenly for about half a second to very low voltages, ranging from 10 V to as low as 5 V! Then it goes up again to over 14 V, holds there for 6 to 15 seconds, and then dips again. The multimeter is fine, because I tested it on my other car, and there is no problem, and I tested it on another person's car just to be sure and it also worked well. So the problem must be on the Mazda's side. My questions are the following:

  1. What can cause these fluctuations in voltage?
  2. This second question is just out of "scientific" curiosity: even if the alternator was dead, shouldn't there be a reading of at least 12 V? How is it electrically possible for the voltage to drop so low, if I'm measuring the voltage at the battery terminals and the battery is ok, meaning it's charged and has a voltage over 12 V?

Thanks in advance for your help!

  • Hi Cesar. Are the low voltages being witnessed while the car engine/starter is cranking? – mike65535 Nov 12 '18 at 19:27
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    One possible explanation is a poor connection between the probes and the battery terminals... – Solar Mike Nov 12 '18 at 19:29
  • @mike65535 No, the low voltages were witnessed while the engine is idling. I did, out of curiosity though, measure the voltage at the moment of cranking, and it went more or less normal, the battery right before the starter kicks in is upwards of 12 V, then when the cranking is going on it dips a little bit to 11 V, and then it goes up to the 14 V once the engine starts. – Cesar Nov 12 '18 at 21:35
  • @SolarMike Well, I took the clamps off, sanded the battery terminals well, as well as the clamps, and then I connected everything back tightly. I don't think it's a poor connection between the probes and the terminals, here's why: when I measure the voltage with the car stopped, the multimeter doesn't fluctuate at all, I held it there for 45 seconds and the voltage didn't dip (other than the normal 0.01 V variation that happens sometimes). But when I turned on the car, after cleaning and sanding everything, the fluctuations came back for the first two minutes, went away as the car warmed up. – Cesar Nov 12 '18 at 21:39
  • I will let the car cool down, and do the process once again, and will report what happens. I also made sure in another car that the multimeter is fine, I applied different pressures to the terminals of the other car, I jiggled the cables of the probes, and nothing happened, in the other car, the voltages remained steady. – Cesar Nov 12 '18 at 21:40
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There are three parts to the charging system on newer cars. 1) The alternator (built-in regulator) 2) The wiring 3) The battery

We can eliminate three immediately. the car starts on its own and the battery is holding voltage. The alternator is a little more tricky. You are most likely to see problems in the alternator when it's cold and idling, but unfortunately that applies to the wiring as well. The fact that you see voltage dropping BELOW the battery voltage makes me think that it's not the alternator. This leaves the wiring. Here's what I deduce: I believe you have a loose connection that firms up as the engine heats up. I am making assumptions here but my reasoning is sound. Assumption 1 is that when you were checking voltage, you were putting your meter on the wires and not directly on the battery.

However, i have a diagnostic for you to do that will narrow it down. Turn on your headlights while it is doing this weird voltage jiggling. If the lights stay a constand brightness while the voltage wobbles, the alternator is definitely not the culprit, and it must be the wiring, as when a car is running, it gets all of its power normally from the alternator, so it would behave normally. Conversely, if you see the lights wavering along with the voltage, then the alternator has catapaulted itself into prime-suspect and it's most likely the voltage regulator. The regulator may be replacable. It depends on the alternator.

  • Thanks a lot for the idea, John! I will definitely give that a try. Now, since you mentioned the Assumption 1, I have to be honest: I have always put the probes on the top of the battery terminal, not the clamps. Having said that, when I measure a car and something is a bit off, I put the probes of the multimeter on different places, on the top of the battery, and I also measure on the ring of the clamp that's attached to the cables of the car. I did that, curiously, today, but the results were the same. And it's the fact that I'm touching the battery that really made me wonder. – Cesar Nov 13 '18 at 6:31
  • I will, nevertheless, do the lights test and report back. Thanks again! – Cesar Nov 13 '18 at 6:32
  • Just a correction, I was using the wrong word to refer to the place I usually measure voltage. I usually measure voltage at the top of the battery post. I was using terminal wrong. But as I explained in the last comment, today I measured voltage on both places, on the battery post, and on the battery terminals (the clamps that connect to the battery posts) and the results were the same. – Cesar Nov 13 '18 at 6:56

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