I'm usually pretty good with cars, but this one has me scratching my head.
I start my 1990 accord (245K miles - yeah baby!), and the alternator doesn't charge the battery. Voltage is around 12v, then drops as I drive to work (~10 minutes). By the time I get to work the battery voltage is approaching 9.5 volts, and that's when the tach and odometer begin to jump - evidently they don't work below 9.5 volts. The car is now sputtering if the accelerator is pushed hard. I make it to my parking space and let the car idle a few minutes. At about the 15 minute interval, the alternator kicks in, the voltage jumps from 9.0 to 13.8! I allow the alternator to charge the battery a few minutes, shut the car down, then suffer the same experience on my way back home in the evening.
None of the idiot lights illuminate (the dashboard battery light is working and good - I checked that following my Haynes manual instructions). The ECU has no codes stored. The alternator connector appears to be good. Battery cables are tight. Ground connection to body is good.
I suspect it is a bad alternator (replaced OEM alternator 11 months ago along with battery), but because the ECU controls the alternator, I'm not so sure. The intermittent nature of the problem has me baffled. Any ideas, diagnostics, and troubleshooting hints are appreciated!
Thanks!

Have you thought about changing out the alternator for a new one? That should solve your problems in a heartbeat even though you replaced the alternator 11 months ago. Alternators can be "bad" right out of the box. Nothing unusual about that.

Have you thought about changing out the alternator for a new one? That should solve your problems in a heartbeat even though you replaced the alternator 11 months ago. Alternators can be "bad" right out of the box. Nothing unusual about that.

Twinkie,
Thanks for your response. Yes, replacing the alternator is an option I have considered, but hesitate to do so. What crosses my mind is this: if the alternator is controlled by the ECU, then replacing the alt won't solve the problem and I'll be out $$ + time. Is there an easy way for me to (a) check the alternator or (b) check the ECU? I'm not certain. It just seems odd to me that it ran fine for 11 months then failed. But then again, when on begins to troubleshoot one should go back to the most recently replaced part... so pulling the alt is on the agenda for this weekend if no other suggestions are made.
Thanks again for your quick response.

. Remove and fully charge the battery. This variable needs to be removed from the equation immediately. Most auto parts stores will test and charge batteries and alternators for free. I recommend that you do that. It's not a good idea to try and charge a seriously discharge battery with the alternator--it can burn them out. The problem may be the battery, the alternator, or both. Which is the cause and which is the effect? Even new batteries should be fully charged, before installation.

. Check battery voltage with the engine off. It should be approximately 12 volts.

. Check regulated battery voltage with the engine running. It should now be approximately 14 to 15 volts. If the voltage reading is less than specified, test all under hood and under dash fuses with a test light or multimeter:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post252145

If the battery and fuses are good, the voltage regulator is suspect. Here's your Saturday morning project, if your alternator is not under warranty:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-6.html#post666293

Cleaning the slip rings and replacing the brushes may even solve the problem. The erratic symptoms can mimic a bad voltage regulator.

Voltage regulator issues normally are detected with the "idiot light". What isn't detected is a broken/intermittant connection to the rotor.

If you can I would examine the brushes and rotor connections.

. Remove and fully charge the battery. This variable needs to be removed from the equation immediately. Most auto parts stores will test... .

TxGreaseMonkey,
Thanks so much for your post and your troubleshooting tips... I think they are a blessing to us all. Looks like I'll have something to keep me busy this evening as well.:)
I also appreciate your witness and kingdom perspective.
I'll make a post once I determine the final problem.

Thank you. Great.

Voltage regulator issues normally are detected with the "idiot light". What isn't detected is a broken/intermittant connection to the rotor.

If you can I would examine the brushes and rotor connections.
Thanks! I'll make a close inspection of these if the fuses don't reveal the problem first.

Another hypothesis, to keep in the back of your mind, is a failing Ignition Control Module (ICM). It's common for the tachometer to become erratic and the engine to sputter, when the ICM is bad. The tachometer wire (BLU) goes directly to the ICM inside the distributor. I normally recommend replacing ICMs and coils every 120,000 miles or 10 years, whichever comes first.

Here's what's involved:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-4.html#post265896

It's fixed! This morning I took both the battery and the alternator to the auto parts store. I left the battery on the charger all night - last night it was taking 2.5 amps and 1.5 amps this morning (which seemed hi to me).
The auto parts store said the battery was good and the alternator was bad - but just barely. The alt was delivering 13.5 volts, just under the expected 13.8 volts. The alt was still under warranty, so they handed me a new one with a smile. I replaced it in time to make my 10:30 appointment. That was a GOOD morning.
Here are the current stats on the Accord:

Battery Voltage w/ engine off 12.7 V
Voltages when car running:
_Idle_ _2000 RPM_
Engine Running 14.3 14.4
Bright headlights on 14.2 14.1
" +Lights+hi fan+a/c 12.8 14.1
" " " " + defrost 12.4 13.1

I've had the DVM on it all day - 14.4 volts is normal, stopping at an intersection the voltage drops to ~12.6, then recovers. I think we're good to go.

Thanks for all the suggestions and assistance. You folks are great.

I knew you would get it fixed today--nice job. As long as the battery charger cuts back to 2 amps or less, the battery is fine.

What do you teach?

I knew you would get it fixed today--nice job. As long as the battery charger cuts back to 2 amps or less, the battery is fine.

What do you teach?

Leadership and communication - doing what I can to stomp out ignorance :)
And develop the minds of our future leaders

College professors, who keep their cars a long time and repair them themselves, are demonstrating excellent leadership to their students. The skills you develop working on cars carry over into many other aspects of life.

College professors, who keep their cars a long time and repair them themselves, are demonstrating excellent leadership to their students. The skills you develop working on cars carry over into many other aspects of life.
Thank you for your kind words. I think the primary skills illustrated are tenacity and frugality. I have an excellent mechanic nearby, but I hate to part with my money, especially on a simple job like this. Also, I think it sends a good message to my children: you can fix it yourself with less time than would be required to drop it at a mechanic and pick it up the next day.
To reflect the compliment, I appreciate your willingness to share your wisdom about automobiles with the world. Your self-help trouble shooting guide is excellent! You invest a substantial amount of time into replying to postings each day, from which I, as well as many others, have benefited. Thank you for your patience with less-than-mechanically-inclined "shade tree" mechanics. You would make a great teacher.

Is your Accord a 5-speed?

Biloxi! That's a long way from TX (I've made the drive).
How do I set up private messaging? I don't see it on my profile settings.

Is your Accord a 5-speed?

Yes, it is.

---10 day update---
The Accord is purring along nicely ;-)
Replacing the alternator solved all the problems
In fact, it topped 245,000 miles last week!
Thanks to all who provided input and advice - it was all very helpful!
Now I can focus on repairing the passenger side shoulder harness - it's stuck in the up position. Oh, the joys of older cars!

Try cleaning the track with WD-40, a small screwdriver, and a rag. 250,000 miles should be the minimum objective for all of us.

Problems on this site often go in cycles--now, we are experiencing a lot of charging system problems.