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I read on another forum on how to fix this with radio shack parts. Google it and it should come up. I will see if I can find the link again and post it.

 

Where do you monitor the alt output voltage?

From FSM:
With engine from idle to 2000RPM, Voltages at B should be:
13.9 to 15.1 V @25°C (77°F)
13.5 - 14.3V @115°C (239°F)
Temp is probably engine compartment temp.

If charge light comes on with Alt output within the range above, something's still wrong.
Could be the regulator starting to go bad going; It should have 12V on the yellow wire (L) IF output voltage is correct.
Could be that the connection between yellow wire, charge fuse and charged light (PLs see my sig for explanation of charging system with schematic).

Which "Power" Wire?
IG has 12V nito alt, S has 12V into alt, "B" has 12V out of alt.

 

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You need to clarify if you are measuring at battery POST itself, or at connector. This could expose poor connection due to corrosion. ALL connections should have shiny metal contacting shiny metal. Dull-gray metal does not make good electrical connection.

Your voltages seem within the good range. IF you have good connections, you would get almost same voltages on the positive lines- maybe a fraction of a volt off due to normal voltage drops across parts/wires/fuses, depending on current running through them.

ACTUAL alternator output is measured here at screw terminal on alternator. That's your "B" wire/terminal.


This is where you should have the highest voltage in the electrical system IF, and only IF, you have a properly functioning alternator/regulator & battery.

The 14.2V is normal. Most likely the same as voltage at "B" post. You need to clarify if you are measuring the 13.8V at battery POST itself, or at connector. The 0.4-volt difference sounds normal. HOWEVER, it may also mean:
1) Voltage drop across the Fusible link (6-inch long wire between bat + and fuse block), and/or
2) Voltage drop across where batt connector mates with battery post.

That's why, you need to clarify if you are measuring the 13.8V at battery POST itself, or at connector.

The cleaner and better your connections are, the lower that voltage drop will be.

When I get to my truck I'll check how much is voltage difference between "b" post and battery post.

 

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oh.. sorry, I completely forgot that this is Pre-84 forum so alt and regulator wiring are different. I hope someone with similar system could chime in.

 

if you find its not alt, reg, or batt. it could be the lamp relay under dash next to glove box, but that effects oil or brake lamp also i think. i rebuilt mine cheap and cheated toyota .