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Low Alternator output at idle

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ddfarm View Drop Down
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    Posted: May/23/2011 at 6:16am
I checked out the Alternator shop and they are telling me that it is NORMAL for these alternators to "cut out" around 700-600rpms considering the size of crank shaft pully used on AMCs. Can anyone tell me if this is true? If it is true, then my problem is not the alternator nor the regulator.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ddfarm Quote  Post ReplyReply Direct Link To This Post Posted: May/22/2011 at 3:46pm
Thanks, the specs would be great. I have already tried 3 regulators and all produce the same thing. And not all these regulators were bought from the same place.

I might add to my observations. 

@900 rpms with everything OFF the volage from the back of the alternator is 14.1
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: May/22/2011 at 2:18pm

There is a thermistor in the regulator. When it's cold, it charges about a half volt or so higher (I'd have to refer to my Motorola references to get the temp and voltage charts.)

As it warms up (ambient temp warms, as in under-hood temp) it drops the charging rate (voltage)

The reason is that cold batteries take more to charge. That's one reason the regulator is near the battery.

In any case, 13.6 or whatever you stated is low. It should be up higher than that.

I'll get the exact specs for your alternator later tonight if I remember.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 69 ambassador 390 Quote  Post ReplyReply Direct Link To This Post Posted: May/22/2011 at 2:04pm
Try a different regulator.  They are temperature regulated and there may be a malfunction.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ddfarm Quote  Post ReplyReply Direct Link To This Post Posted: May/22/2011 at 1:35pm
Not real sure but I think they had a chart for this alternator. I will check tomorrow. Can you answer the questions in my last post without this info from the shop/
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ddfarm Quote  Post ReplyReply Direct Link To This Post Posted: May/22/2011 at 1:31pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote poormansMACHINE Quote  Post ReplyReply Direct Link To This Post Posted: May/22/2011 at 7:44am
Smaller pulleys work nice on paper but now you have less less area for the belt to actually drive a greater load that you're introducing so it will slip.
What rpm and load is the shop testing the alternator at that your home made test isn't?
 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote ddfarm Quote  Post ReplyReply Direct Link To This Post Posted: May/22/2011 at 7:15am
I still have the problem!!!!! Here are some other facts that I have done/found during my ordeal.

 Cleaned and tested every ground under the hood.
Had alternator and regulator checked at the alternator shop, which checked out OK.
Tried a smaller pulley from 2 5/8 inch(orginial) to 2 inch with no noticeable difference. 2" is smallest available.

Here are the results with the orginal pulley with car in gear. 

Test 1=@600 rpms alterantor works great with Lights-off, fan-off......13.9 voltage at back of allt
Test 2=@600 rpms alternators works great with lights-off, fan-on......13.8
Test 3+@600 rpms alternator stops putting out with lights-on, fan-on......12.1 at back of alternator and working strickly off the battery. No help from alter

With car in neutral and rpms @900 I get 13.7 volts at back of alternator even with everything on.

What other test should I do? Are the lights pulling too much Voltage? What should the voltage be at the yellow wire going into the regulator even when the lights are on? If this voltage is below 12 volts would the alternator stop output. If so at what point would it drop out?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: May/10/2011 at 10:55am
Originally posted by BDCVG BDCVG wrote:

Wouldn't a larger pulley actually spin faster because the belt has to cover more circumference in the same amount of time, A.K.A.  buy underdriven pulleys to gain HP but sacrifice at the alternator. This would explain the #'s with the pulley swap above.


No, the alternator is the "driven" pulley...... look at the cam to crank ratio, or a multi-speed bicycle... you derail the chain to a larger diameter sprocket to slow it down, and gain power.
To make the alternator spin faster, you decrease pulley size, like you'd decrease rear sprocket size to make the bike move further for every push of the pedal crank.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BDCVG Quote  Post ReplyReply Direct Link To This Post Posted: May/10/2011 at 9:52am
Originally posted by 69 ambassador 390 69 ambassador 390 wrote:

Something is very wrong.  You actually are saying that you have no charging at idle and the voltage is actually less with the small pulley and faster spinning Alt. with the lights on.  Does your dash warning light work?
Wouldn't a larger pulley actually spin faster because the belt has to cover more circumference in the same amount of time, A.K.A.  buy underdriven pulleys to gain HP but sacrifice at the alternator. This would explain the #'s with the pulley swap above.
1970 AMX 390 5 spd full Control Freaks front and rear suspension
2014 E63 AMG-S wagon
1965 Austin-Healey 3000 MK III              
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