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Low Alternator output at idle

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ddfarm View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ddfarm Quote  Post ReplyReply Direct Link To This Post Topic: Low Alternator output at idle
    Posted: Apr/30/2011 at 8:10am
I have searched this topic and found several good threads. But none seem to answer my question. I have a 73 AMX 304 (AT)with a Edlebrock performance package( including cam). My car idles at 1200 rpm out of gear and 700 in gear. This car is a off frame restoration and all connections are clean. The alternator is a 55 amp stock(new). The lights dim at idle(700) and if the rpms remains there for a extended period the lights just cut off. If you put it back in neutral(1200 rpm) the lights come back on. I do have a electronic ign. So I have 2 questions.

1.Is there another alternator that I can get that will put out more at 700 rpms?

2.Why would the light just cut off completely? Then cut back on with more rpms.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FuzzFace2 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/30/2011 at 8:42am
1. I dont know but if the ALT puts out the right volts & amps at higher rpm then I would say the ALT side is good but the REG side on the weak side. Recheck ALL grounds and if everything has fresh paint it may not be getting a good ground. You have the ground cable from motor mount bracket to cross member?
2. If I under stand right as volts go down the amps go up. Running at low volts as you say may be over loading the breaker in the head light switch and why the lights go out.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ddfarm Quote  Post ReplyReply Direct Link To This Post Posted: Apr/30/2011 at 10:23am
Thanks Dave. Yes I do have the ground cable  from mount bracket to cross member. And it is a good clean connection. The voltage reg is new but I will recheck to see if I have hooked it up correctly. I did not know that the the head light switch had a breaker in it. Now I understand the lights cutting off.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Apr/30/2011 at 11:05am
First, we can't diagnose anything until you measure the actual voltage at the battery. Until then we are just guessing about grounds, connections, etc.
55 amp is MORE than enough, even with electronic ignition, which really doesn't take much more than stock, some less (more effecient)
Measure voltage - if it's in the 14v range, then the alternator is fine. If not, then you may have:
bad/worn brushes
worn/burned slip rings
bad diode
bad regulator
bad stator
 
Your problem is not a 55 amp alternator, you problem is a connection, ground, switch, or problem with the alternator or regulator.
A light switch will cut out much easier once the breaker has tripped a time or two. They are weakened by tripping - IF that's what is happening.
Take voltage measurements at the battery, at the back of the alternator, at the headlights themselves.
Check grounds - good looks doesn't mean good connectivity, and cables can be bad where the wire connects to the cable end - down inside where you can't see.
Measure for voltage drop across connections, too.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ddfarm Quote  Post ReplyReply Direct Link To This Post Posted: May/01/2011 at 6:00am
Okay, I have checked my grounds at the wiring harness and cross member. There is hardly any resistance at both(voltage is same). I also jumped a ground wire from the battery to the harness. Also jumped a ground wire from the battery to the lights. Also jumped a ground wire from battery to regulator ground. All results were no difference. The lights still dim at idle in gear. Voltage across the battery 13.9 at idle out of gear(1200). 12.8 at idle in gear(800prm). Voltage at back of alternator is same(12.8/800, 13.9/1200). Voltage at headlights is same as other test. Headlights are diffiently dimming due to voltage drop.

Is there another test I can run? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ddfarm Quote  Post ReplyReply Direct Link To This Post Posted: May/01/2011 at 8:08pm
One additional thing. My AMX has a amp meter in the dash gauges and the meter falls below the middle point when the rpms are dropped below 1200.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote offroaddodge Quote  Post ReplyReply Direct Link To This Post Posted: May/01/2011 at 8:27pm
the diode is bad that will stop output at low rpms
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Post Options Post Options   Thanks (0) Thanks(0)   Quote offroaddodge Quote  Post ReplyReply Direct Link To This Post Posted: May/01/2011 at 8:29pm
is that a motorola alt does it have a red bridge on the back that needs to replaced
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Post Options Post Options   Thanks (0) Thanks(0)   Quote offroaddodge Quote  Post ReplyReply Direct Link To This Post Posted: May/01/2011 at 8:30pm
bad regulator maybe or wrong one
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Post Options Post Options   Thanks (0) Thanks(0)   Quote poormansMACHINE Quote  Post ReplyReply Direct Link To This Post Posted: May/01/2011 at 8:34pm
With a 9% change in voltage, lamps will dim and brighten accordingly.
Pretty normal for the era.
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