Low charging voltage - New batt, new voltage regulator

hans747 · Post by **hans747** » 28 Jun 2022, 09:56

Hi Everyone!

Recently, my son's 2004 XC70 started exhibiting a weak battery. When I put a multimeter on the battery leads with the engine running, it was showing less than 12 volts. Last week, we replaced the voltage regulator with a new Bosch unit because the old brushes were a bit worn down. The voltage at the battery went up to 13.5v.

This week, the car is doing the same thing again. Dead battery after work, and when we start the car, the voltage at the battery is about 11 volts. Dangit! The battery is only 8 months old, but I took it to the shop today for a test, and it passed.

So to recap...
NEW VOLTAGE REGULATOR
BATTERY IS TESTED OK
THE CAR IS GETTING LESS THAN 12V AT THE BATTERY TERMINALS

Where should I look next? Are there any fuses or relays that could be causing my alternator to underperform?

Thanks!

Post by **volvolugnut** » 28 Jun 2022, 10:13

Check the terminals at the battery - they must be free of corrosion and tight on the battery posts. If this looks good, inspect the cables for swelling indicating internal corrosion. This has happened to others.
Check the voltage output at the alternator.
volvolugnut

hans747 · Post by **hans747** » 28 Jun 2022, 11:11

volvolugnut wrote: ↑28 Jun 2022, 10:13 Check the terminals at the battery - they must be free of corrosion and tight on the battery posts. If this looks good, inspect the cables for swelling indicating internal corrosion. This has happened to others.
Check the voltage output at the alternator.
volvolugnut

Thanks! The terminals all look good. I will go back and recheck the condition of the cables.

How can i check voltage at the alternator? Just put my multimeter on positive lead on the alternator and ground it to the chasis?

Post by **RickHaleParker** » 28 Jun 2022, 13:10

hans747 wrote: ↑28 Jun 2022, 11:11 How can i check voltage at the alternator? Just put my multimeter on positive lead on the alternator and ground it to the chasis?

Positive post of the alternator and the metal case of the alternator. Then move the ground lead to negative side of the battery and see if there is a difference. Move the negative lead back to the alternator then do the same with the positive lead.

With the engine running measure the voltage drops across the length of a cable and across a connector. That is one way to find bad cables and connectors. Cables can be corroded under the insulation.

For example if you put one lead on the negative battery post and the other lead on the negative cable clamp and get 0.2V you are not getting a good connection between the clamp and the post.

Post by **vtl** » 29 Jun 2022, 08:07

Alternator can be bad, too.

Post by **jonesg** » 30 Jun 2022, 03:36

hans747 wrote: ↑28 Jun 2022, 09:56 Hi Everyone!

Recently, my son's 2004 XC70 started exhibiting a weak battery. When I put a multimeter on the battery leads with the engine running, it was showing less than 12 volts. Last week, we replaced the voltage regulator with a new Bosch unit because the old brushes were a bit worn down. The voltage at the battery went up to 13.5v.

This week, the car is doing the same thing again. Dead battery after work, and when we start the car, the voltage at the battery is about 11 volts. Dangit! The battery is only 8 months old, but I took it to the shop today for a test, and it passed.

the alt isn't charging, the engine is just running off the battery.
The only way that can happen is if the alt is dead or fails to get a signal to energize.
Check the small signal wire for corrosion, its mostly exposed to the elements. Make sure it slides onto the connection firmly.

I believe if the alt was dead you would get the red warning light on the dash, same as if the serpentine belt snaps.
When the small signal wire is interrupted or broken the red warning light doesn't show and you don't get any charging.

Post by **jonesg** » 30 Jun 2022, 03:44

hans747 wrote: ↑28 Jun 2022, 09:56 Hi Everyone!

Recently, my son's 2004 XC70 started exhibiting a weak battery. When I put a multimeter on the battery leads with the engine running, it was showing less than 12 volts. Last week, we replaced the voltage regulator with a new Bosch unit because the old brushes were a bit worn down. The voltage at the battery went up to 13.5v.

This week, the car is doing the same thing again. Dead battery after work, and when we start the car, the voltage at the battery is about 11 volts. Dangit! The battery is only 8 months old, but I took it to the shop today for a test, and it passed.

the alt isn't charging, the engine is just running off the battery.
The only way that can happen is if the alt is dead or fails to get a signal to energize.
Check the small signal wire for corrosion, its mostly exposed to the elements. Make sure it slides onto the connection firmly.

I believe if the alt was dead you would get the red warning light on the dash, same as if the serpentine belt snaps.
When the small signal wire is interrupted or broken the red warning light doesn't show and you don't get any charging.

mine looked like this.

: corrosion.jpg (202.98 KiB) Viewed 4356 times

hans747 · Post by **hans747** » 30 Jun 2022, 07:03

Thanks for that tip. Mine looked pretty good. I pulled it off while the car was running, and stuck a multimeter to it. I was getting battery-level voltage at that wire. I am getting a $50 alternator from the local used parts shop. I'm going to see if that makes any difference.