About 2 months ago, I needed to test my engine cooling fan motor. Was using two lead test wires with alligator clips and stupidly connected the battery ends first and then the wind blew the other connection ends together creating a spark. That may or may not have anything to do with what has happened since then. About 3 days after that, I started to notice that my lights were dimming and ac fan motor was slowing when the engine was under load as in 1st or 2nd gear or going uphill. After about a week, the ALT/BAT light came on and I replaced my alternator. The problem with that was that getting the belt back on the new ALT nearly killed me and I got several pretty bad injuries and ended up having to get help to get it done. The old alternator tested bad so I figured my problem was solved. But ever since then, I am still getting the same problem - a voltage drop or loss of electrical power causing the lights to dim and the fan to slow - except this time is doesn’t happen under load - it happens while driving and just after I have come to a stop at a light or stop sign. I have had the battery and alternator tested several times and they always show up as being fine. I checked all the cables and connections and changed the positive battery cable bolt. I get no codes thrown. I cant just take the alternator out and change it because of what I went through with the last one - not to mention it tests OK so I can’t use the warranty as long as that is the case anyway. So I am left with this troubling, annoying problem and don’t know what the cause or solution is. I am guessing the list of options has to have the alternator, battery, connections, belt (though I have no sounds or evidence there is a problem with the belt - I suppose if the tensioner was damaged with the belt fiasco that it might cause such a problem). Anybody have any ideas or suggestions about what to do? It is the biggest problem at night because my headlights are affected.
Get a voltmeter that plugs into the cigarette lighter so you can see what the charging system is doing in real time. (Remember to take it out if it’s not turned off with the ignition switch.)
To me, it sounds like your battery has developed an internal short. If so, you may start finding the battery dead when you go to start the vehicle after it has sat for awhile. It is probably not related to the alligator clips shorting.
As you can see, the PCM regulates the voltage out of the generator.
So when those alligator clips got together and sparked, it may have created a voltage spike which damaged the ECU. So now it’s no longer regulating the voltage correctly.
It’s possible the spark damaged something, more likely it would be an alternator diode. But could be the computer too. What happens, when the wires get shorted like that a large magnetic field is created by the current flow, then when the current stops, that magnetic field collapses and can create a brief but sizable negative voltage spike, which the car’s electronics isn’t always able to handle. That wouldn’t be my first guess for the cause of your problem, but it is certainly possible.
I guess the first thing I’d do in this situation is use a portable volt meter to measure the voltage at the battery terminals, both before the engine is started, and after the engine is running. Do this the first start of the day, so the battery has sit unused overnight… Before starting the engine the battery should measure from 12 to 12.8 volts. Immediately after starting the engine the battery should read between 14 to 16 volts, and that voltage should slowly drop as the engine warms up, to around 13.5 to 14 volts. ’
Then once the engine warms up and the voltage has stabilized you can turn on the head lights and measure how that affects the battery voltage. It should have little effect. Try it both ways, with the lights on dim and bright.
I should mention that if you’ve never done this before, suggest to ask someone car-repair knowledge to show you how to do it the first time. It’s possible to damage components in the car, your meter, or yourself if this is done incorrectly. Whenever working near the battery, I always wear eye protection, and usually a full face shield. Batteries can explode and spew acid everywhere if they get shorted out. Likewise with the mechanical stuff, find someone who knows how, has done it before, to show you how it is done and what tools are required the first time.
