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Discussion Starter · #1 ·
In daytime, while driving, it reads about 13-14v on the voltmeter. At idle (800 rpm) sometimes it's at 13-14v and sometimes drops to 12v or less. While driving last night at dusk, as soon as I turned the lights on, the volts dropped to barely 12v and when I pulled up to a stop sign the voltmeter dropped dramatically to 9-10v (red to yellow bands on the voltmeter) and the headlights dimmed correspondingly. If I reved up a bit it at least went up to about 11-12v and the lights got brighter. I have a Wilson alternator and underdrive pullys. Any idea what's wrong?
:(
 

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Not sure anything is wrong. I had the same issue with the old style Ford Alternator. Not sure about the Wilson. On the old style one wires the excitor is activated by hitting a certain rpm level which is raised due to the underdrive pulleys, before that the alternator is not putting out. You might notice when first starting it up that the volts are low until you rev it up. With the lights on at idle and underdrive pulleys it is going to show lower voltage. It sounds like the alternator is putting out. I never had any problems with mine. You don't idle that much anyway.
 

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When you went to the underdrive pulley set , did you also replace the alternator pulley , to correspond with the new pulley ratio?...If not , you need to go to a slightly smaller pulley , so the alternator turns at the right RPMs at idle...
Also , when the car is cold , the fan does not go on....when hot, the fan comes on (or is turned on), and the fan pulls enough juice to be noticable on the battery gauge....Especially at night , when all your lights are on , is when it is most noticable.
If you have the stock Alt., it puts out about 75amps , not a lot when you start turning on the radio, the lights , the fan , the turn signals, the brake lights etc.
 

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Discussion Starter · #4 ·
Mr. Barry, I have the small pully on the alternator, my fan is manual and I rarely have to turn it on, I have a radio, amplifier and 6 disk CD changer but rarely use them at all - can't hear em much over exhaust. If this alternator is too small for this car, I can't imagine what it was built for. I can't help but think something might be wrong with the alternator. Can something be burned out in the alt to where it only puts out half power or something?
 

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I had a Pontiac many years ago that would eat a diode regularly. It would keep the battery charged during the day. But at night with the lights on, I could adjust the brightness of the alternator light by turning up the volume on the radio. I think one bad diode will cut the output amps by 2/3.
 

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Discussion Starter · #8 ·
Thanks guys. I guess we start with the simplest thing first. I'll yank it out and have it tested. A bad diode seems to be the concensus of opinion.
 

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Discussion Starter · #9 ·
Just had alternator tested off the car. Tested perfectly in all areas. Put out 14v. Also had battery tested under load in car and it tested fine. Do you think it is the "ground" grimlim?
 

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I still have my money on the underdrive pulleys not turning the alt. fast enough at idle.
 

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Get hold of Tom Coker at Mr Alternator. He is listed in the Vendor section. His phone number is 864-591-1066 in SC. His website is: http://www.mralternator.com/
Geat guy and knows his stuff. He did tell me specificaly not to buy the underdrive pulley.
 

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Discussion Starter · #12 ·
If it was the pully and I needed a smaller one, how can I get 13-14v (sometimes) showing on my volt guage at 800 rpm idle? Seems I could never get that high of voltage unless I revved it up a bit? And why would my voltage drop by 2+v when I'm driving down the highway at 2000 rpm and turn on the lights? BTW, I will investigate the pully issue. The more I have learned over the years the more I realize I don't know. Seriously... I have been so sure of some fixes only to find out it was something else.
 

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AFO,

Here are some thoughts about the electrics in your car…

Measuring the output of your alternator in volts, at anything but the output terminal(s) is just about useless. If you are using a dash mounted voltmeter you would have to run a dedicated wire from the alternator output terminal and a dedicated ground for just the gauge. If you did that and you calibrated the gauge against a digital volt meter, you would be reading true alternator voltage..! If you expect to read true alt voltage through the harness you will always get the wrong reading unless there was absolutely no other load except for the gauge.

The alt is subject to all of the bad stuff in your overall electrical system… like bad grounds, high resistance connections and dirty (oxide) connections to the battery. I bet that it is not the alt, but wire size or dirty (oxide or corrosion) at the battery or some other high amperage connection or a bad primary ground to the frame or motor…

Check those things first, now that you know that your alt is OK. Start by measuring the resistance between the battery terminals and the frame with everything off and with the car running. You should have .1 ohms or less under both conditions. If that is not the case you should start by re-grounding the battery and motor (alt works through motor ground) and being totally sure that there is not PC or paint under the connections. A copper washer and a smooth clean attachment point is a great place to start…

I use to work for a semiconductor company as an EE. We did a huge study and came to realize that most electrical problems are related to electromechanical failures and not semiconductor failure… Food for thought… Good luck

Chris
:D
 

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I tend to agree with the marginal connection somewhere camp. I was curious in reading this thread why one would put on underdrive pulleys and then turn around and put on a smaller alternator pulley to, effectively, eliminate the underdrive. You'd be right back where you started in terms of engine load and the belt life would be shorter due to the more extreme bend at the smaller pulley. Just rambling.
 

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Discussion Starter · #15 ·
Chris L. Thanks for your expertise, I totally agree. I will start going over all my connections this weekend. Ed T. I agree with you also. It had underdrive pulleys on when I bought it (I'm not the orig builder). I can go a smaller alt pulley but then it does negate the whole effort. I've been fiddling around long enough with cars to know how bad electrical connections can show up as all sorts of weird anomolies. Good food for thought from all - thanks!
 
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