Charging / Alternator / Exciter Wire Questions
Thread Starter
Teching In
Joined: May 2007
Posts: 12
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From: Gastonia, NC
Charging / Alternator / Exciter Wire Questions For the last few months I have been having a sporadic no-start on my 99 Z28. The motor will turn over but never fires. It has to be jumped off when this happens. I suspected the battery was bad so I replaced it and the cables since they were looking rough at the terminals. This has not solved the problem.
When the no start occurs I can hook up a battery charger and it shows 25% charged and takes a couple hours to reach full charge. I have the original alternator with 144K miles so I had it tested on the car and it and the voltage regulator tested OK.
Once the car starts it runs fine and the gauge shows plenty of voltage and is steady.
I read about the exciter wire needing 5 volts to activate the alternator charging so I pulled the connector loose to test it. It only shows 0.02 volts with the motor running. I traced the wire back 10 to 12 inches and stripped off a small spot to test and it shows the same. What prompts the PCM to send voltage to the exciter wire? Is there a sensor or does it use the voltage that is powering it? I even tried turning on the lights and fans to add extra load but that did not change anything.
Can the exciter wire be replaced all the way to the PCM or should I not trust the alternator test and replace it? Any help getting the charging system back in order is appreciated.
When the no start occurs I can hook up a battery charger and it shows 25% charged and takes a couple hours to reach full charge. I have the original alternator with 144K miles so I had it tested on the car and it and the voltage regulator tested OK.
Once the car starts it runs fine and the gauge shows plenty of voltage and is steady.
I read about the exciter wire needing 5 volts to activate the alternator charging so I pulled the connector loose to test it. It only shows 0.02 volts with the motor running. I traced the wire back 10 to 12 inches and stripped off a small spot to test and it shows the same. What prompts the PCM to send voltage to the exciter wire? Is there a sensor or does it use the voltage that is powering it? I even tried turning on the lights and fans to add extra load but that did not change anything.
Can the exciter wire be replaced all the way to the PCM or should I not trust the alternator test and replace it? Any help getting the charging system back in order is appreciated.
When the car doesn't start, have you verified the battery voltage with a VOM, at the battery terminals?
When the engine can be started, do you see at least 13.5 charging volts across the battery terminals. If you have proper charging voltage, there may be a parasitic draw off the battery.
When the engine can be started, do you see at least 13.5 charging volts across the battery terminals. If you have proper charging voltage, there may be a parasitic draw off the battery.
Last edited by gMAG; May 20, 2014 at 10:50 PM.
Joined: Aug 2007
Posts: 24,241
Likes: 89
From: Turnin' Wrenches Infractions: 005
If the exciter isnt showing voltage, the alternator will not "turn on". It must be showing + voltage on that line while the engine is running. You can test directly at the pin at the PCM to verify the wire to the alternator isnt broken or compromised.
Thread Starter
Teching In
Joined: May 2007
Posts: 12
Likes: 1
From: Gastonia, NC
The voltage across the battery is 12.4 volts when the engine is off and 14.6 volts when it is running. Does that mean that it should be charging?
How can I test the voltage at the PCM pin, can you get to it without taking the connector loose?
How can I test the voltage at the PCM pin, can you get to it without taking the connector loose?
Joined: Aug 2007
Posts: 24,241
Likes: 89
From: Turnin' Wrenches Infractions: 005
12.6 is battery voltage, 2.1v per cell, 6 cells total. Anything above 12.6v is a charging state. At 14.6 it is most definitely charging its *** off.
helping a friend with a similar problem....does anyone know if these cars will run off the alternator?
in other words, the car is running and the I disconnect the battery. Will it continue to run?
in other words, the car is running and the I disconnect the battery. Will it continue to run?
Joined: Aug 2007
Posts: 24,241
Likes: 89
From: Turnin' Wrenches Infractions: 005
Probably so, and it doesn't mean you should do it. I can't think of one reason in my almost 20 years of working on cars someone would need to make it "run off the alternator". Stop being a bonehead.
