1971 alternated brown wire only has 4.4 volts

[Big 71 Jimmy]

New to the forum 1971 Gmc Jimmy running 350 with external regulator, battery not charging checked alternater not charging jumped F and 3 on external regulater, then alternator charged at 16.5 volts. Changed ext regulater. No help. Found brown wire coming from switch supposed to be at battery volts (12 volts ) and it is at the key switch but when the wire gets to the regulator I only have 4.4 volts not sure where I am loosing voltage at. Also found my circuit board on back of gauges going to my ammeter was fried, so I have ordered one of these. Anybody have any idea what I should do next?

[VetteVet]

The brown wire you are referring to shares the ignition switch terminal with the 12 gauge accessory power brown wire that runs to the fuse panel.
It is a 24 gauge 10 ohm resistance wire at the key switch and is brown and white or brown and yellow. It runs to the firewall block under the fuse panel and connects with the 16 gauge brown wire that goes to F on the regulator.

If you are getting 12 volts at the key switch accessory terminal, then you may not be making a good connection in the firewall block, with the brown wire in the engine bay side of the firewall block.

You could run a jumper from the battery positive post to the brown wire on the F terminal and see if you get charging voltage at the alternator or the battery. You can't leave it though because you need the 10 ohm resistance to excite alternator fields when it first starts charging. Also you need it to prevent the engine from running after you shut the key off because the alternator will feed back enough voltage to keep the ignition system firing.

It will tell you that your voltage drop is in the firewall connection block. If it charges at 14 .5 volts.

[Big 71 Jimmy]

Vettevette
Thanks a ton for your help, I just got back to the garage to attemp to fix this again and have eliminated a lot including the firewall connectors. So something that is not making sense if I disconnect the ground wire from the regulator then my volts of my brown wire go from 4.4 back up the 12 volts and if you start the vehicle the alternater starts charging Like it should be. Any other thoughts? thanks again for your help!
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[VetteVet]

Your check on the brown wire should be with the regulator unplugged and you should get 12 volts on it and with the key on and zero with the key off. You should also get 12 volts on 3 (red wire) key on or off.

If you are not grounding the alternator to the engine block and one of the diodes has shorted, it might be grounding through the regulator ground. The regulator might not work as it should and the battery could overcharge.

I think I would take the alternator and have it checked at Autozone or wherever you do business for car parts. Make sure they put a load on it, I think there machine does that automatically when it checks output.
If it turns out the alternator is bad and you replace it, get a 12 SI internally regulated alternator for a mid 80s camaro and we'll convert you. That will solve a lot of problems.
If it is charging correctly now you could watch it and make sure the battery wasn't overcharging.

[Big 71 Jimmy]

Thanks vettevet
So when I had this problem the first thing I did was replace the alternater. So I am more than happy to get it tested/ replaced but it is a new alternater.
If I scrap this one and go to the an internally regulated alternater would you go with a 12si or a cs130? Or something else?
It is just a 350 soon to be a 383 and aftermarket AC but we have aftermarket stereo with speakers so if we replace / upgrade the alternator just want to upgrade to the correct one
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[VetteVet]

Well I hate to see you not use the alternator you bought, but in all honesty, you are going to need the internal regulated one for the stuff you want to run. The 12 SI is 94 amps and will probably work, but if it were me I would get the CS and go with the 144. It seems to be more reliable. The conversion is very simple, just follow my diagram below.

The CS needs a bigger resistor in the brown wire from the key switch which you show 4.4 volts on. You will have to resolve that to 12 volts no matter what alternator you use.
Then you run the brown wire to the L pin on the CS alternator with the resistor soldered inline. You can find the CS alternator plug off a junker,1990 or newer, or they sell the adapter online for about $25. Maybe O'riellys sells one.

Then connect the red wire from the old regulator to the S pin.

Then just run the output wire from the back to your main junction where the other red wire goes and you're done. You will want to upgrade the output wire to an 8 gauge, to handle the increased amps the CS puts out.

The CS 130 is plentiful but the CS 144 has a better reliability reputation. Check online for one.

If you have any questions just ask.



If you want to keep using the battery gauge in the dash cluster, you will have to wire the two gray (black) wires like the diagram shows. One wires to the battery positive, and the other wires with the alternator output wire at the main junction block.

[Big 71 Jimmy]

VetteVet
You are a wealth of knowledge! Thanks for the replies and your ideas. I will convert it in the next few days. And I'll let you know how that goes. Thanks again
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