Electrical Issue

[mbisett]

My recently completed MK4 battery is not holding a charge. The voltage gauge starts off (after I charge the battery) showing 12 volts then starts dropping down to about 4 volts after I have driven it for a short drive.. Would this indicate the alternator is bad or the battery (1 yr. old)?

[D Stand]

Sounds like an alternator. What voltage is displayed with the motor running? Does the battery hold a charge with the car just sitting for days?

[edwardb]

You should see your voltage gauge jump to approx 14 volts when the engine is running. If a one-wire, may need to bump the RPM's at first to trigger it. Some are better than others. But bottom line you should clearly see the voltage go up. If so, likely the alternator is OK. But first thing IMO is to get the battery tested. It's amazing to me how many threads on here start as "I have an electrical problem and the battery is fine..." and after a lot of exercise, turns out to be the battery. Alternator is possible, but start with the easy one.

[Jeff Kleiner]

Quick check that doesn’t require taking the battery or alternator out for testing. Start the car. Disconnect the negatiive battery terminal. Did the car die? If so the alternator isn’t charging; if it continued to run it is.

Jeff

[mikeinatlanta]

Question. You said recently completed. Has the car ever charged correctly, or been this way from the beginning? Component failure may be the most likely issue if it was working, but if it has never charged most likely cause is wiring.

[mbisett]

Thanks Jeff! I will give it a try.

[mbisett]

Well naturally, it died immediately when I disconnected the battery ground. I guess a new alternator is on my plate.

Thanks again for the guidance.

[Jeff Kleiner]

Well naturally, it died immediately when I disconnected the battery ground. I guess a new alternator is on my plate.

Thanks again for the guidance.

Well that solved part if the puzzle but as Mike said the other part is to determine whether it’s the alternator or wiring. Has it worked and now quit or are you not sure that it has ever charged?

Jeff

[mbisett]

The battery had seemed to have been holding a charge for the past several months but it has not been under much load on some minor road trips. Now that I got it road ready and taking some short road trips, I have had to charge the battery a couple of times to get it to crank. I found that the 1 wire to the alternator was pinched between the block and alternator, I could not tell if it was shorting there or not. I plan to pull the alternator and find out if it was actually shorting out there.

So the short answer is I am not sure the alternator has ever charges but the voltage gauge did show about 14 when started and now it only shows 12 when started after a full battery charge.

[edwardb]

...now it only shows 12 when started after a full battery charge.

Your voltage gage should show 14+ volts when running no matter the battery charge condition.

[mbisett]

That is good to know, I always assumed 12 volts was the target.

[mbisett]

I thought I would provide an update to help the next guy/gal on this. The original alternator was charging in the beginning but stopped charging my battery recently. So I bought a new alternator (1 wire) and it did the same thing - died upon removing the negative ground on the battery. I took both alternators and battery and had them checked. All 3 passed the test. The alternator shop I found was super knowledgeable and helpful - he told me that the new 1 wire alternators have a built in regulator and it requires a ground so when I removed the battery ground it shuts off the car. A quick fix was to add a direct hot wire from the battery to the alternator - bingo!

All good now however it did not answer the questions of what caused the original alternator to quit charging.

[runamuk]

I thought I would provide an update to help the next guy/gal on this. The original alternator was charging in the beginning but stopped charging my battery recently. So I bought a new alternator (1 wire) and it did the same thing - died upon removing the negative ground on the battery. I took both alternators and battery and had them checked. All 3 passed the test. The alternator shop I found was super knowledgeable and helpful - he told me that the new 1 wire alternators have a built in regulator and it requires a ground so when I removed the battery ground it shuts off the car. A quick fix was to add a direct hot wire from the battery to the alternator - bingo!

All good now however it did not answer the questions of what caused the original alternator to quit charging.

What do you mean "hot wire"?

Did you add another wire between the positive battery post and the alternator?

[mbisett]

Yes, sorry for the confusion

[Steve in Snohomish]

One other thing to look at. Does your system have a MIDI fuse? I had a similar issue after I rewired my 67 Firebird. Thought it was the alternator, bought new one. Same issue. Started looking at wiring and figured it "might" be a bad fuse. Looked at a replacement on line, at which the proverbial "Lightbulb" lit up. I forgot to put the fuse into the holder. Was amazing how it was now charging the battery:rolleyes:

[seagull81]

Mark, glad you have it figured out. I hate electrical ghosts.