procedure for testing the altenator?
Drifting
Joined: Jul 2002
Posts: 1,329
Likes: 8
From: Corpus Christi Tx.
The easiest way is take the car to Autozone or O'Rilley's they will normally test it for free. What do you want to test ? If you want to see if it is working or not just remove the positive lead from the battery with the engine running, if the car dies the alternator is NOT working. If you want to measure the voltage output under load connect a multimeter set to 20 volts DC between the red lead on the back of the alternator and ground. if you want to read the output without load remove the connector from the rear of the Alternator (leading to the battery) and read the voltage on the alternator terminal where you removed the cable from. When the cable is removed from the Alternator DO NOT let it come in contact with any METAL !!!
Instructor
Joined: Feb 2003
Posts: 150
Likes: 0
From: Staten Island NY
Just put a voltmeter on it while its running. It should be putting out about 14.8 Volts. Pulling off the positive lead only tells u if the alternator is completely dead, what if the output is only diminished?
Tech Contributor
Joined: Dec 2006
Posts: 10,962
Likes: 29
From: Van Buren Arkansas
Wounded Warrior Escort '11
Another backyard mechanic simple test is see if a tool will stick to it magnetically. If the alternator is charging, it should be magnetized. If nothing sticks, its not charging.
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
DO NOT!!!!!!! I REPEAT<<DO NOT remove the battery lead to test the alternator!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!
Thats an easy way to FRY the alternator and damage your IPC, BCM and PCM.
Your alternator puts out a square wave DC voltage. Its NOT pure DC and your electronics will not like the experience!!!
The battery acts as filter for the alternator and supplies a PURE DC voltage to the electronics!
You need a Digital Voltmeter to troubleshoot!
DO you have one that you can use and READ??
BC
Thats an easy way to FRY the alternator and damage your IPC, BCM and PCM.
Your alternator puts out a square wave DC voltage. Its NOT pure DC and your electronics will not like the experience!!!
The battery acts as filter for the alternator and supplies a PURE DC voltage to the electronics!
You need a Digital Voltmeter to troubleshoot!
DO you have one that you can use and READ??
BC
Drifting
Joined: Mar 2010
Posts: 1,628
Likes: 15
From: Seattle WA
In addition to Bill's information, if you are possibly having problems with your alternator, the battery serves as an "accumulator" (thinking hydraulic terms here), so if your voltage regulator is not working properly, removing the battery from the loop could cause a voltage spike well above the 14v your car is used to seeing.
Get a multimeter if you don't have one and do some voltage checks, and don't EVER disconnect the battery from a running vehicle if you suspect alternator problems (even more so if its modern and has sensitive electronics).
Get a multimeter if you don't have one and do some voltage checks, and don't EVER disconnect the battery from a running vehicle if you suspect alternator problems (even more so if its modern and has sensitive electronics).
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Le Mans Master
Joined: Nov 2005
Posts: 6,993
Likes: 25
The easiest way is take the car to Autozone or O'Rilley's they will normally test it for free. What do you want to test ? If you want to see if it is working or not just remove the positive lead from the battery with the engine running, if the car dies the alternator is NOT working. If you want to measure the voltage output under load connect a multimeter set to 20 volts DC between the red lead on the back of the alternator and ground. if you want to read the output without load remove the connector from the rear of the Alternator (leading to the battery) and read the voltage on the alternator terminal where you removed the cable from. When the cable is removed from the Alternator DO NOT let it come in contact with any METAL !!!
1 had and internal short that drained the battery over a day or 2 but when it ran, it put out the correct voltage/current.
The other checked out on the machine but set the "Charge Fault" indicator on my wife's Grand Prix-most likely the regulator was getting ill.
The only REAL way to tell if an alternator is good is by measuring the output voltage and current AND getting it on an oscilloscope to make sure of the output shape (none of the parts stores do this).
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
NOTE!!!!!
(see,, I did it again
)
The VOLTAGE on the DIC and IPC volt meter is NOT the true BATTERY VOLTAGE. Its the voltage supplied to the B+ (Hot in RUN & START and or HOT in RUN & ACC)
That supply voltage goes THRU the "IGNITION SWITCH" and can be less than the actual battery voltage measured at the battery terminals or on the HOT at ALL TIMES battery buss. If the ignition switch contacts are compromised by carbon or burnt contacts, the voltage form the ignition switch will be LESS than actual battery voltage.
SEE THIS POST:
-C5 ignition Switch repair - http://forums.corvetteforum.com/c5-t...ch-repair.html
dougbfresh
YES,, thats a true statement!
Most AUTOMOTIVE STORES do not check the alternator for reverse current draw. When the car is OFF, the alternator should NOT draw any current from the battery on the BATT terminal on the back of the alternator.
The bridge rectifiers prevent it from drawing current when the engine is OFF. If one or more of the rectifiers are bad, the alternator will draw power in the field windings. The alternator will be WARM to the touch if the current draw is bad enough. Ive see a 5 AMP draw on a BAD alternator and that will make a battery die really fast!
BC
(see,, I did it again
)The VOLTAGE on the DIC and IPC volt meter is NOT the true BATTERY VOLTAGE. Its the voltage supplied to the B+ (Hot in RUN & START and or HOT in RUN & ACC)
That supply voltage goes THRU the "IGNITION SWITCH" and can be less than the actual battery voltage measured at the battery terminals or on the HOT at ALL TIMES battery buss. If the ignition switch contacts are compromised by carbon or burnt contacts, the voltage form the ignition switch will be LESS than actual battery voltage.
SEE THIS POST:
-C5 ignition Switch repair - http://forums.corvetteforum.com/c5-t...ch-repair.html
dougbfresh
YES,, thats a true statement!
Most AUTOMOTIVE STORES do not check the alternator for reverse current draw. When the car is OFF, the alternator should NOT draw any current from the battery on the BATT terminal on the back of the alternator. The bridge rectifiers prevent it from drawing current when the engine is OFF. If one or more of the rectifiers are bad, the alternator will draw power in the field windings. The alternator will be WARM to the touch if the current draw is bad enough.
Ive see a 5 AMP draw on a BAD alternator and that will make a battery die really fast! BC
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
NO!
Voltage measured directly on the battery terminals should be
CAR OFF. 12.00 - 12.5
Engine ON: 14.3 - 14.7
Depending on battery charge and battery condition, battery voltage can be as low as 13.5 and still be fine.
Measure the BATT terminal on the back of the alternator (under the rubber boot) to chassis ground. It should be the ame as the reading on the battery POSITIVE terminal to ground.
BC
Voltage measured directly on the battery terminals should be
CAR OFF. 12.00 - 12.5
Engine ON: 14.3 - 14.7
Depending on battery charge and battery condition, battery voltage can be as low as 13.5 and still be fine.
Measure the BATT terminal on the back of the alternator (under the rubber boot) to chassis ground. It should be the ame as the reading on the battery POSITIVE terminal to ground.
BC
Drifting
Joined: Mar 2010
Posts: 1,628
Likes: 15
From: Seattle WA
Those readings are close enough it doesn't sound like you have significant resistance between the battery and alternator (which is what BC was trying to get you to measure there). If that had been the case, you'd need to check that wiring to see what's up.
Now that we have the "engine off" measurements, go start the car and get the same measurements again. Hopefully you're well above 13v idling, if not you likely have a faulty alternator.
Also, your battery might be a bit weak....I like to see above 12v across the terminals.
Now that we have the "engine off" measurements, go start the car and get the same measurements again. Hopefully you're well above 13v idling, if not you likely have a faulty alternator.
Also, your battery might be a bit weak....I like to see above 12v across the terminals.
Thread Starter
Melting Slicks
Joined: Jun 2007
Posts: 2,704
Likes: 4
From: Lone Star State
Battery was 12v idling and 13.5 v with accessories on.
Those readings are close enough it doesn't sound like you have significant resistance between the battery and alternator (which is what BC was trying to get you to measure there). If that had been the case, you'd need to check that wiring to see what's up.
Now that we have the "engine off" measurements, go start the car and get the same measurements again. Hopefully you're well above 13v idling, if not you likely have a faulty alternator.
Also, your battery might be a bit weak....I like to see above 12v across the terminals.
Now that we have the "engine off" measurements, go start the car and get the same measurements again. Hopefully you're well above 13v idling, if not you likely have a faulty alternator.
Also, your battery might be a bit weak....I like to see above 12v across the terminals.
Tech Contributor
Joined: Dec 2006
Posts: 10,962
Likes: 29
From: Van Buren Arkansas
Wounded Warrior Escort '11
12 volts with the car off is a bit too low. 12 volts with the car idling is way too low. I would be looking at two possibilities. Look at the connections of the starter solenoid as this is where the battery, alternator and fuse boxes all come together. Do not forget that its electrically hot down there unless you disconnect the negative battery cable, at the battery, first. You might consider pulling the alternator and having it tested. Before I forget, there is a fuseable link that feeds the connection to the starter solenoid just back up inside the cable going down there but if you are reading 12 volts on the field wire of the alternator with the engine off, most likely the link is good.
Drifting
Joined: Mar 2010
Posts: 1,628
Likes: 15
From: Seattle WA
Do you have an underdrive pulley perchance? If so, perhaps your alternator isn't quite spinning fast enough at idle.
When you say "with accessories on," which do you mean? The A/C might raise idle speed a bit to kick on the alternator.
If you don't have an underdrive pulley, then 12v at idle = a bad alternator.
When you say "with accessories on," which do you mean? The A/C might raise idle speed a bit to kick on the alternator.
If you don't have an underdrive pulley, then 12v at idle = a bad alternator.
