Wadoka

This isn't for me, or my Corvette, but -

A neighbor has a late-model Olds Cutlass. He just replaced the battery, and it's been charged up, but he asked me, as the resident 'car guy', how to check to make sure his alternator is charging the battery.

His dashboard voltmeter is crap, about like the analog gauge in a late C4. Can't really tell if it's reading 14-15 volts like it should, or 11-12 volts like it was running off battery only.

So, given that I have a working multitester, what would be the quickest way to prove that the alternator is charging the battery up to spec? I told the neighbor that the Forum Collective knows everything...:D

~Matt

robblk90

I would set the tester to volts put the leads on the correct posts and check the voltage with the car off then with the car running. You can see what the volts are, whether or not they are in the correct range.

6 speed Sid

With the car running you should read about 14 to 14.8 volts or so on your voltmeter. Sid.

GlockLT4

13.6-14.8

not all get up to 14

65Z01

When my alternator failed even with the known good battery from the IROC the voltage only read around 12Vdc with the car running. It read higher with the engine off.

GlockLT4

Same here. :yesnod:

0TomLzzz

While the car is running unhook the battery. If the car dies alternator is not working. If it runs it is running on the charge from the alternator.

jfb

A fully charged battery will read 13.2 volts. The alternator must place more than 13.2 volts across the battery in able to charge it. A cold alternator will put out 14.3-14.7 volts and this voltage drops with increasing alternator temperature to about 13.3-13.6 volts hot. As long as the battery voltage is higher than 13.2 volts with the engine running, then the alternator is doing its job.
When the alternator is not working, then the battery voltage will read lower with the engine running because the spark module, ECM, dash lights, etc are drawing current and with the engine off, only leakage current (under 30 milliamps) is drawn and the battery voltage will read higher.

lite blue

Unhooking the battery with the engine running in a computer controlled car is a no-no! You risk damage to the ECM! :eek:

Ray Vargo

[QUOTE]This isn't for me, or my Corvette, but -

A neighbor has a late-model Olds Cutlass. He just replaced the battery, and it's been charged up, but he asked me, as the resident 'car guy', how to check to make sure his alternator is charging the battery.

His dashboard voltmeter is crap, about like the analog gauge in a late C4. Can't really tell if it's reading 14-15 volts like it should, or 11-12 volts like it was running off battery only.

So, given that I have a working multitester, what would be the quickest way to prove that the alternator is charging the battery up to spec? I told the neighbor that the Forum Collective knows everything...:D

Lay a screw drive against the back of the alt if it's magntized it's working if it's
now it's not working.

Ray Vargo

GlockLT4

:yesnod: :yesnod:

R23HTC

Unhooking the battery only works on the REALLY old cars. This won't always work on a computer controlled car even IF the altenator is working properly.

65Z01

An alternator generates AC voltage that passes through a diode bridge to make a full wave rectifier. This means it's output is pulses of voltage.

The battery acts like a huge capacitor that smooths out these voltage spikes. Unhooking the battery with the engine running lets these voltage spikes enter the car's electrical system, not a good thing. :eek:

JasonL

Disconnecting the battery on a running engine is a very bad idea...the alternator generates a lot of current to charge a battery. When you disconnect the batt on a running engine, the built-in voltage regulator doesn't kick in fast enough to prevent a huge voltage spike of 60V or more. Good way to risk damaging a lot of electronics.

Learned this from my brother who was a engineer working with automotive circuit protection devices...

Use the multimeter.