Does an alternator draw when car off???
Thread Starter
Instructor
Joined: May 2006
Posts: 122
Likes: 1
From: hazlet township nj
Does an alternator draw when car off???
I know I have an electrical drain due to my door pins (courtesy light) but I suspect my alternator is also defective. With a test light I determined that the alternator is drawing a little bit of power. Is this normal?
Team Owner
Joined: Aug 2006
Posts: 24,125
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From: Columbia Missouri
If an alternator is functioning properly it won't drain a battery, but a bad diode pack can certainly cause a battery to drain.
Thread Starter
Instructor
Joined: May 2006
Posts: 122
Likes: 1
From: hazlet township nj
I did unplug it and the light did go out. Mind you the door pins were also pressed shut at the time. How do I know the problem is the alternator and not the wiring harness from the alternator? Any help would be great.
Race Director
Joined: Sep 2004
Posts: 11,874
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
i dont see how a test light could help you with this....the alternator will always have unswitched power going to it so the test light will always illuminate......put an ammeter in the circuit and perhaps you could measure the drain if there was one.
Melting Slicks
Joined: Mar 2008
Posts: 2,336
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From: North Bellmore Ny
St. Jude Donor '09
Being in the electrical industry a test light will only let you know if power is present which power should be at the alternator even with the car off. you need a dc amp meter to test for a draw.
Le Mans Master
Joined: Oct 2006
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
A test light will show a completed circuit when put in series. If you disconnect a battery cable, put the test light in between the battery post and the cable, open a door, the light will come on. No meter needed. Quick and dirty, "never fails" test. If you unplug the alternator connector, key off, and the test light glows on either terminal, the alternator has a bad diode, and is putting a small drain (load) on he battery.
One step more- if your test light has an 1894 bulb in it, you can test gauges with it- the resistance of that bulb is 1/2 scale on the fuel, temp, and oil pressure gauges.
One step more- if your test light has an 1894 bulb in it, you can test gauges with it- the resistance of that bulb is 1/2 scale on the fuel, temp, and oil pressure gauges.
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Le Mans Master
Joined: Oct 2006
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
On the connector side, yes, there should always be power. And, of course, on the BAT wire on the back. But if you check from the connector to the corresponding terminal on the alternator, the light should not come on at all.
Pop the battery cable off the negative terminal, put the test light in there, and then pull the plug on the alt. If the light is on with it plugged in and goes out when you unplug it, you have a bad diode.
Pop the battery cable off the negative terminal, put the test light in there, and then pull the plug on the alt. If the light is on with it plugged in and goes out when you unplug it, you have a bad diode.
Thread Starter
Instructor
Joined: May 2006
Posts: 122
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From: hazlet township nj
That is exactly what I did. My test light was on the battery. When I unplugged the alternator my now illuminated test light shut off. I guess I need a diode. Sounds like it is difficult to fix, so I'll just buy a new one. Thank you all
Melting Slicks
Joined: Mar 2008
Posts: 2,336
Likes: 1
From: North Bellmore Ny
St. Jude Donor '09
On the connector side, yes, there should always be power. And, of course, on the BAT wire on the back. But if you check from the connector to the corresponding terminal on the alternator, the light should not come on at all.
Pop the battery cable off the negative terminal, put the test light in there, and then pull the plug on the alt. If the light is on with it plugged in and goes out when you unplug it, you have a bad diode.
Pop the battery cable off the negative terminal, put the test light in there, and then pull the plug on the alt. If the light is on with it plugged in and goes out when you unplug it, you have a bad diode.
100 percent correct. Thanks because this is not something I would think to check.
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
If your alternator is 'original' and you want to keep it for that reason, replacing a diode set is not all that difficult. You need a vise and some basic tools....plus a good set of instructions for the repair. Otherwise, get a replacement unit. Also, consider that the diode pack went bad for a reason.... Have you installed any additional electrical equipment (elect. fans, stereo amp, etc.) that would cause excessive draw on a stock alternator? If so, you may want to upgrade to a higher amp alternator.
Race Director
Joined: May 2006
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From: Dayton, Ohio
There is still the possibility that removing the alt plug test means a bad alt is not true.The plug being connected still supplys a "path" and then interups that path when unpluggged.Buying an alt. is a cheap way out but just be aware it might not fix the problem.
Here is a link to the same issue.I wish the guys working on this would have gone the extra mile to find the actual problem as there would have been a benefit from knowing.I believe they chose to wire around rather than pinpoint the problem which met their needs at the time.
http://forums.corvetteforum.com/show....php?t=1749729
Here is a link to the same issue.I wish the guys working on this would have gone the extra mile to find the actual problem as there would have been a benefit from knowing.I believe they chose to wire around rather than pinpoint the problem which met their needs at the time.
http://forums.corvetteforum.com/show....php?t=1749729
Racer
Joined: Apr 2008
Posts: 254
Likes: 5
From: ingleside illinois
A test light will show a completed circuit when put in series. If you disconnect a battery cable, put the test light in between the battery post and the cable, open a door, the light will come on. No meter needed. Quick and dirty, "never fails" test. If you unplug the alternator connector, key off, and the test light glows on either terminal, the alternator has a bad diode, and is putting a small drain (load) on he battery.
One step more- if your test light has an 1894 bulb in it, you can test gauges with it- the resistance of that bulb is 1/2 scale on the fuel, temp, and oil pressure gauges.
One step more- if your test light has an 1894 bulb in it, you can test gauges with it- the resistance of that bulb is 1/2 scale on the fuel, temp, and oil pressure gauges.
Last edited by corvette744; May 28, 2008 at 09:47 AM.
