alternator and voltmeter question (1 Viewer)

nat

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Sep 20, 2005
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Los Gatos, California
 
 
 
1972 FJ55
SBC 350
3 wire GM type alternator with only a wire from the Batt post to the + post on the battery

The other day after work, my rig was dead, hardly any juice in the battery. I thought it was strange, so I had some buddies push start me and I was off. After driving for an hour, I stopped, and when I tried to start her up, it was dead again. I pushed started it and home I went. On the way, my lights dimmed, turn signals and brake lights stopped working.

I pulled the alternator and it bench tested ok at 2 different places. I put in the battery from my FJ40 and tried it out. After driving for a while, my lights dimmed, signals stopped working, wipers slow.

I figured maybe some weird thing is up with the alternator so I bought a new one. Now when I run it, my voltmeter (aftermarket gauges) shows just under 13V and if I run the headlights and turn signals, the volts drop to under 12V. Is this Normal? Any ideas on what is up? Thanks for any help. :cheers:
 

Coolerman

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3 wire alternator as in it has an external voltage regulator? Check the regulator. Sounds like you are not charging. You should have 12.8 - 14.3 volts when charging.
 

nat

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It is supposed to be an internal regulator. Everything has been running fine since the V8 swap 2 months ago. I just bought the alternator this morning. I have a nice big gauge wire from the alternator to the battery. Should I hook up the other two leads? How would they get wired?
 

Coolerman

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If your alternator has the following terminals here is what you need to do.
This is for a CS series alternator.

S - connect this right to the "+" on the Alt. with a ring terminal
F - Not needed
L - Connect to 12V source with a 3-4 Watt bulb in the circuit
P - Not used

The bulb provides the excitation current needed to jump start the alternator. Some alternators are self exciting. :)

If you have an SI model alternator then you need to hook it up as follows:
Three wire SI: One BAT, one field and one sensing. The field wire goes to one side of the "idiot light" and then from the other side of the idiot light to the 'RUN' position of the ignition switch. The sensing wire either goes to the 'BAT' side of the ignition switch or the 'RUN' side depending on the application. When the ignition is in the run position the sensing terminal is energized with battery voltage and the field terminal is energized with a current limited voltage through the lamp. When the alternator starts charging, the field voltage raises and the voltage differential across the lamp reduces and the lamp goes out. The charging voltage is 'sensed' through the sensing wire and the internal voltage regulator regulates the voltage to about 14 volts.

What is the model # of your alternator? (I should have asked that first.)
 
Last edited:

nat

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Sep 20, 2005
Messages
1,999
Location
Los Gatos, California
 
 
 
Duralast Gold
DLG7127M

It has 3 leads:

Bat
R
F

A guy at a shop said to hook the R to a keyed ignition source (he put a diode in the line)

The F to the + battery

and the Bat to the + battery
 
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No. Ogden, Utah
 
 
 
 
Dude, I'm havin the same struggle over here. Good reading HERE and HERE but you'll have to dig around a bit on those sites for different wiring methods. Helpful to determine what type you have. The connector that Coolerman was referring to is for a CS130.
 

nat

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I forgot to mention that as my lights and such were dimming, my voltmeter would stop reading anything. Don't know if that helps.
 

Coolerman

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Got to go get a Christmas tree with the wife. :) I'll research this a bit as I'm going to be upgrading my stock alternator soon. I'll post my findings later if someone doesn't get you an answer soon.
 

MrMoMo

That's not rust, it's Canadian patina...
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~Kingston, ON, pero soñando de Panamá
 
If you only have one wire going from the alternator to the battery, you must be grounding through the block, and frame... I would check that all those connections are good. When you have a bad connection, you get higher resistance and that would cause your alternator to effectively be putting out full power, but the power is not neccisarily getting to the battery. After a while of running your battery charge level goes down enough to make your lights dim. Check your grounds as it sounds like you are not getting a complete (good) circuit Use your multimeter set to OHMs, measure from the batt negative post to the frame, and from batt neg to the block. Then batt. neg to alternator case. All of these readings should be very close to zero, not to be confused with "OL" Perhaps the ground from your block to the frame is going?

HTH

Bruce
 

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