Bricoleur

The problem: I replaced my worn original Honda (Civic DX 1.5L) alternator (which was badly overcharging) with a rebuilt one. However, after only 4 days, the new one has started overcharging, but not to the same extent as the old one. The battery itself is newish (4 months old).

The question: Could this be due to either a bad rebuilt alternator, a bad ground, or a faulty engine computer (is there not a line that connects the engine computer to the alternator)?

The backstory: About 3 weeks ago, my Honda's original alternator started putting out too much voltage. It began suddenly while driving on the freeway; the Battery Light on the instrument cluster turned on and stayed on after having driven for a while on the highway. If I reduced my speed to say below 20 mph, the battery light would go off. If I turned on the headlights or the air conditioner, the battery light would also turn off. Both of these incidents made me suspect that the battery was putting out too much voltage. I verified this by metering out the battery and the alternator with a DVM. The alternator was putting out 17 volts or more at high rpm. The battery itself had an initial charge of around 13 V.

This made it seem like the voltage regulator inside the alternator was bad, and so I decided to replace the entire unit as what else could be going bad was a thought. However, after just 4 days on the new alternator, the dreaded battery light came on. Unlike the times with the old alternator, the battery light would only come on for a split second and then turn off. Again, the higher the speed, the more likely the battery light would flash.

I bought a voltage tester that plugs into the cigarette lighter and so it allows me to watch the voltage I drive around. At idle, the voltage ranges rom 13.8 to 14.2V. However, once the car warms up a bit or I drive it for a few minutes, the voltage will start being erratic, ranging from 13.8 to 16.8 volts, constantly changing like: 14.5 V, then 15.2 V, then 14.8 V, then 15.6 V, then 16.0 V, then 14.8 V, etc. Each the time the voltage would spike, the battery light would come on. The only way to reduce the output voltage was to turn on the headlights and the air conditioner; the the voltage would drop to 13.8-14.3 V. If I turned off one of these accessories (but left the other one on: that is using only headlights or only air conditioner on cold), the voltage would rise to, say, 15.5 Volts, and fluctuate .

Any ideas? Could this be another bad alternator, a bad ground, or is there a defective line coming from the engine computer?

ezone

I'd guess another junk aftermarket alternator.

Check the large ground cables from battery to chassis to engine and trans.
Make sure the alternator mounting hardware is tight.

drhawkinz

Definitely do this!

Maybe scrub it with a wire brush so you get some fresh metal on metal contact between the alternator and the motor so you're getting a good grounding connection.

Bricoleur

I took my car back to the automotive electrical shop from which I bought the rebuilt alternator. Instead of getting another one and putting it in myself as I had done, I had the shop scope out the problem as I felt this was too strange behavior.

They replaced the alternator with another one and the voltage problem has changed. No longer does the voltage swing wildy after the engine warms up; rather, it stays more or less constant, however, the battery voltage tends to fall to below 13 V (e.g. 12.3, 11.8, etc.) when all the accessories and headlights are on and the engine is idling. I am not sure if this is a problem.

But a new problem has arisen: whenever the car is in Park or Neutral (it is automatic transmission) and with all accessories off, the engine revs high, then low, then high, then low, continually without stopping. If I put the car into Reverse or Drive, this problem goes away. Turning on the accessories, such as the headlights and the air conditioner (see my original post), lessens this high revving and low revving cycle, but it does not go away completely.

Note: the mechanics clearned out the throttle body for me, and I wonder if this is causing the problem?

I might post this in a new thread, under the appropriate category.

ezone

That's a problem IMO, voltage should not drop that low even with all accessories operating. Return to installer again with the specific symptoms noted.

Related. Is the IAC now stuck? Again, return to the shop because it wasn't doing this before they touched it (I assume).