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Discussion Starter · #1 ·
In the middle of rewiring. Things are going well, pretty much have everything complete except the rear lights. While I had the gauge cluster out, I decided to install my new raingear wiper system. Again all went well and worked good(after adjusting the wiper park position, it was off 90 degrees).
I connect the battery and check things as I go, and noticed today a high pitched hum coming from the alternator when the battery is connected. Not running, key off.
Any ideas what to check???
 

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Trifive Automotive Electrical Wiring Expert
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27,024 Posts
In the middle of rewiring. Things are going well, pretty much have everything complete except the rear lights. While I had the gauge cluster out, I decided to install my new raingear wiper system. Again all went well and worked good(after adjusting the wiper park position, it was off 90 degrees).
I connect the battery and check things as I go, and noticed today a high pitched hum coming from the alternator when the battery is connected. Not running, key off.
Any ideas what to check???
Does the 2 small terminals have the wires reversed? The brown wire from the "gen"ight goes to the left terminal looking at the back of the alternator and the right terminal is connected to the battery or output post of the alternator.
 

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Cocke County, TN. 55 Bel Air Sport Coupe
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In the middle of rewiring. Things are going well, pretty much have everything complete except the rear lights. While I had the gauge cluster out, I decided to install my new raingear wiper system. Again all went well and worked good(after adjusting the wiper park position, it was off 90 degrees).
I connect the battery and check things as I go, and noticed today a high pitched hum coming from the alternator when the battery is connected. Not running, key off.
Any ideas what to check???
It's probably humming because it doesn't know the words. ;-)
 

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Discussion Starter · #4 ·
Don, using a GM late model alt that came with the serpentine kit. It has a 4 pin plug.
The pins are tagged as follows from top to bottom vertically... "P" (top / not used), "L" (second wire. alt excitor wire), "F" (not used) and bottom wire "S", the largest used as Battery.
This plug is locater on the upper LH side looking at the back.
On the upper RH side, is the lug with the 10 ga battery wire attached.
This was connected per the diagram that came with the alternator.
Any thoughts?
Thanks
 

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Trifive Automotive Electrical Wiring Expert
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27,024 Posts
Don, using a GM late model alt that came with the serpentine kit. It has a 4 pin plug.
The pins are tagged as follows from top to bottom vertically... "P" (top / not used), "L" (second wire. alt excitor wire), "F" (not used) and bottom wire "S", the largest used as Battery.
This plug is locater on the upper LH side looking at the back.
On the upper RH side, is the lug with the 10 ga battery wire attached.
This was connected per the diagram that came with the alternator.
Any thoughts?
Thanks
Does the "gen" light stay on with the key off? Disconnect the plug and see if it quits. If so, disconnect one wire at a time. Check that the gen light is connected to the L terminal. If you don't have a gen light, a resistance is needed in the excitor wire.
 

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Discussion Starter · #7 ·
disconnected the plug, still hums. gen light only comes on when key is on.
plugged plug back in and disconnected the "Bat" wire from the main lug on the alt, no hum.
Reconnect the "Bat" wire and hum starts again. Checked wiring diagram again and wires are connected per diagram. When you remove "Bat" wire, you can see a small arc, so something in the alt is drawing power from the battery.
When unhooked, all the lights, wipers, etc work fine.
 

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Discussion Starter · #8 · (Edited)
Don, after much research. I keep getting one common thing, the "L" connection must hook through a light or a 50 ohm resister. I connected mine to the " alt. excitor" wire on the aftermarket wiring harness. Have no idea where this wire is connected in the harness, but possibly just a 12v source, but definitely not through a light. I am going to the shop to check the voltage on that wire right now.
I added the wiring through the gen light, but may be to late, as it still hums.
So, I may have had the alt. connected without a 50 ohm resistor or light bulb on the "L" post.
From what I've read, if you hit the "L" post with more than 3 v (which I'm assuming must be what the resistance source brings it down to) you could instantly fry the alt.
So, I may very well have fried it???
Any thoughts?

Just checked and the "alt excitor wire is a direct 12v source, so I definitely put 12 v to the "L" post.
I also read you can use a 12v source on the "F" terminal so long as you aren't using the "L" terminal?
 

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Discussion Starter · #9 ·
Bought a replacement from a guy on epay that had it left from a crate engine he bought. This time I installed it with an inline 50 ohm resistor. Everything working great now.
This one not only knew the words, it's singing a tune...
 
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Just info to consider.

As most know, I don't like the CS series alternators, would rather have a SI series. That said, I was surprised the firs time I changed the alternator on my 1986 GMC Safari van. It has a 4.3 V6, front and rear A/C, and factory 5 speed trans.

The CS series alternators I have replaced on it have all been factory single wired, from the inside. They have NO wires, other than the large gauge positive BATT wire, going to it.

The van has a volt meter, and no charge light.

Just as more info, I have been though a lot of SI series alternators, I use them a lot on just about everything, and through a few CS series alts. The CS has an internal fan, and isn't easy to get set up correctly as it goes back together. It is super easy to get that internal fan in the wrong place, or, bent, and hitting other parts inside. And, if it is an early CS, it might not have had a later, "improved" rear end frame case.

Hope this helps.
 
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