Alternator AC Voltage

[GSUjinih]

I read that if one of the diodes go bad then you will have AC instead of DC voltage coming from your alternator. What is the maximum amount of AC voltage allowable? Also what is the trigger for the low battery message?

[Q451990]

In my experience you'll see DC voltage, just less (or none). The threshold for the message is in the FSM - www.nicoclub.com/FSM/

Heath

[Skibane]

If one or two of the alternator diodes goes bad, you'll still usually get DC voltage, but it'll have a lot more AC "ripple" in it than normal.

With all the diodes working normally, the battery doesn't have much trouble filling in the short time gaps between when each diode is contributing some voltage. However, if a diode fails, it creates a much longer time gap before the next working diode starts to take up the slack - and the DC voltage momentarily sags until then. The DC voltage doesn't ever fall entirely to zero or go negative (which is the definition of AC), but it wavers some - and this wavering is commonly called "ripple". The frequency of the ripple varies with engine speed - it gets higher at higher engine speeds.

Some digital multimeters will allow you to measure the amount of AC ripple at the battery posts, just by using the meter's AC voltage measurement function. The actual amount of ripple varies quite a bit depending on the size of electrical load the alternator is powering, battery age and condition, engine speed, etc, but it's normally not more than a small fraction of a volt. (For grins, I just went out and measured it on my new Frontier - it was right around 0.03 volts AC, with or without the headlamps on.)

[GSUjinih]

In my experience you'll see DC voltage, just less (or none). The threshold for the message is in the FSM - http://www.nicoclub.com/FSM/

Heath

Which part of the FSM? Looked in several sections already.

[Q451990]

EL-59. It's looking for 8VDC or more..

Heath

[qship96]

8 volts??? Thats WAY too late to wait to trigger any kind of warning! s*** in the toilet way before then! I am shocked they did not use a much higher voltage as when to trigger a warning that alternator is not providing juice to the battery.

[Q451990]

I agree... remember that it's monitoring the voltage output at the alterntor, not at the battery.

Heath

[GSUjinih]

Hmm, that is strange as I get this message before I crank the car. I let the fuel pump cycle off before I start the car and during that time it gives the low battery message. Once the car is cranked it then says OK. So obviously the alternator is not outputting any voltage with the car not running. Weird that I get this message.

[Skibane]

8 volts??? Thats WAY too late to wait to trigger any kind of warning! s*** in the toilet way before then! I am shocked they did not use a much higher voltage as when to trigger a warning that alternator is not providing juice to the battery.

Apparently, it doesn't actually provide any warning about low voltage output from the alternator - Just a warning if the output drops to zero.

MALFUNCTION INDICATOR
The IC regulator warning function activates to illuminate “CHARGE” warning lamp, if any of the following symptoms occur while alternator is operating:
- Excessive voltage is produced.
- No voltage is produced.

[djwarner]

....I get this message before I crank the car. I let the fuel pump cycle off before I start the car and during that time it gives the low battery message. Once the car is cranked it then says OK.....

This would be normal operation with a dying battery. As the battery ages, its internal resistance builds. Any current passing through this resistance, causes the apparent voltage outside the battery to appear low. Once the alternator starts putting out power, the voltage climbs again.