Electrical system

94 Honda Accord wagon. Battery and charging system show they are charged and working, but when headlights are turned on battery light shows on dash. Looking for ideas.

How are you checking the battery and charging system? And can you check with the same method with the lights on?

This is actually for my kid, who is in Colorado. He had the system checked at an autoparts store. i don’t think they checked with the lights on.

A proper load test will check the overall condition of the alternator, and determine if the capacity is compromised. The alternator for this car should be able to output 65 amps, which is enough to keep the battery charged and the lights burning strong. However, with age, the alternator capacity could drop, and not be able to push the amperage necessary to keep the system in good shape with the lights on. It takes less than 10 amps to power the electrical needs of the engine, but the Halogen headlights require more than 30 amps of power. If the alternator is weak, the problem will show up with the lights on.

I’d guess that one of the rectifier diodes in the alternator has failed. 2 out of the 3 phases are still charging, which is why the alternator checks ok, but under load the 2 phases cannot supply all the current required. This is precisely why alternators are supposed to be tested under load.

Have the alternator checked again, by someone more qualified.

Thanks. That seems consistent with what he is experiencing. Any thoughts on how far or long this could be pushed until repair? Using lights or driving daylight without? He is about 200 miles from his current base, with few services readily available.

JayWB is right. Have a real shop do a Vat test- voltage/amperage test, goes by other names. Headlights don’t have to be on for this. Not free. They should check battery first: 1. Test as to how charged up battery is. 2. Test battery w/load tester- this a real performance test. Next- charging system output test- using same load tester on battery w/engine running @ about 2000 rpm’s mechanic will see how much current

alternator puts out. Since battery is being “dragged down” @ this point, alt. should “full field” (put out it’s max current). Technically, alt. should put out 90% of it’s rated output listed on side of alt, but you can never find that damn stamp, so Busted Knuckles’ spec of 65 amps sounds good to me. If alt output is low, it’s bad alt, bad voltage regulator, or the wiring that connects these components- this is

getting technical- the mechanic who does this test will know this stuff. Ask for the following values to be written on receipt, and your son shouldn’t square up till they are: 1. Battery passes load test? Y/N. 2. Charging system max output @ what engine rpm’s. (about 65 amps) 3. Charging volts @ idle- should at least be in low 13’s- 13.2, 13.3. They may not check for parasitic draw, but it should be under .150 amps.

They might charge a tiny bit xtra for this. Guys at the auto parts stores are real nice guys and they’ll do stuff for free, but they’re not really mechanics and I see sometimes they don’t really have a handle on what they’re doing. Tell your son before going to garage to eyeball stuff like corroded battery connections, etc; loose alt. belt. Drive as much as pos. w/lights off but be safe; keeep all unnecessary

electrical things turned off, etc. Larry- I wish I could tell you over the internet how to diagnose this stuff w/o proper equipment, but that’s impossible. Good luck; post back!

A charged battery and a working alternator isn’t enough. The wiring, and terminals, have to be in good condition, too. Take care of the usual culprits: battery cables connections, on the battery and their attachments on the starter (power), alternator (power), body and engine block attachments (grounds). A wiping action occurs when the nuts or bolts are loosened (two turns), and the terminals are wiggled side to side, and the nuts/bolts re-tightened. The battery cables should be disconnected, inside the loops scraped/sanded, and the battery posts sanded. With the battery cables disconnected, loosen and re-tighten the battery cable terminal on the STARTER. On the alternator, the power terminal is under a black rubber boot, two inches from the radiator. With a 10 mm socket, loosen and snug (SNUG, only) that terminal nut. NOW, reattach the battery cables. Try the lights.

It sounds like he will be OK if driving in daylight without the lights on to get “home.” I’d take it someplace more competent than the auto parts store for a battery and alternator check. It sounds like the alternator is on its way out, but should get him 200 miles if he doesn’t need the headlights.

Besides leaving the headlights off, to lessen the load on the electrical power supply, he should turn the heater blower off, also. Heat will still come through with the vents open and the car moving.