Altnator not chargeing bit checked ok.

The voltage regulator inside the Engine Computer gives extremely little trouble. I can help you install a 1970s regulator but first, measure the voltages on the three alternator wires. The fat output wire must have 12 volts all the time. One small wire will have 12 volts and the other small wire must have less than that but not 0 volts. Those last two voltages will only be there when the engine is running, not just with the ignition switch turned on. Holler back with those readings.

Caradiodoc

Not sure how you got higher voltage on the small field wire than what was on the output wire, but the secret is the voltage on the second small wire. It's going to be the same as the first small wire, less than that, or 0 volts. My bet is for 0 volts. That would mean the brushes are worn. If you find between around 4 - 11 volts, that circuit is working. In that case, suspect a shorted diode in the alternator.

Caradiodoc

Yup. System voltage that is too low can affect sensor readings, however, there is a set of conditions for a code to set. One of those conditions is that other certain codes can't already be in memory. As an example, you can get a high manifold vacuum reading from the MAP sensor by snapping the throttle while you're standing beside the car. The only way to get a high manifold vacuum reading for seven seconds or longer is if you're coasting with your foot off the gas. That's how the computer knows the car is moving. No pulses from the vehicle speed sensor is a legitimate condition when the car is standing still, but if the computer knows the car is moving, there had better be a signal coming from the speed sensor. If not, it will set a code. If there is already a fault code set in memory for the MAP sensor, there is no way to know what to expect from the speed sensor, so no code can be set for him.

Some codes will not set below a certain temperature, or unless the problem acts up for a required period of time. In the case of the alternator, the code will be "field circuit not switching properly".

Any code for something that could have an adverse effect on tail pipe emissions will turn on the Check Engine light. That includes charging system codes, but not because it will affect sensor readings. It's because low voltage to the injectors or ignition coil(s) could cause misfires and raw, unburned fuel in the exhaust. So the charging system won't CAUSE sensor codes to set but it could result in other codes being set.

Caradiodoc

It suggests the field circuit is ok. What I think is happening is the circuit is switching on and off about 400 times per second. The percentage of on-time varies to vary average current flow through the field circuit. That's the way vcr power supplies work. Digital voltmeters take a reading, analyze it, then display the reading while taking the next measurement. Subsequent readings might be taken while the field circuit is turned off or turned on. This all happens multiple times per second and is why the display jumps around.

What we need to do is to perform a "full-field" test to bypass the voltage regulator. That involves grounding the dark green wire between the alternator and the voltage regulator in the Engine Computer. The best way to do that, if you can get to it, is to ground that wire at the back of the alternator while the engine is running, and measure the voltage on the output wire. That dark green wire can also be grounded at the Engine Computer but I don't know how hard that will be to access the connectors so you can back-probe it. It's in pin 8. The trouble is both connectors are black, but there is no pin 8 in the wrong connector. Each one has four rows of pins with ten pins in each row. Pin 8 is in one of the outer rows, third from the end. Just be sure it's a dark green wire.

If the two wires are plugged into the back of the alternator, it will be safer to ground the dark green wire there. The wrong wire is dark green with an orange stripe. On some models those two wires go through a small black plastic block so you can't tell which wire goes to which terminal. If that's the case, I can paste a copy of a procedure that might help identify the right terminal.

When you ground that wire, watch the voltmeter or the brightness of the head lights. You'll hear the alternator strain. If the voltage doesn't go up significantly, suspect a shorted diode in the alternator.

Caradiodoc

What was the voltage on the fat output wire during the full-field test? It should have gone up to at least 16 volts.

Caradiodoc

Boy, that sure would seem like the regulator is defective. Darn the bad luck! The last thing is to find that dark green wire in the connector for the Engine Computer. I never had one apart, but I'm guessing there is a plastic cover that can be snapped off so you can back-probe the terminals while they're still connected to the computer.

Stick a stretched out paper clip in next to that wire so you can measure the voltage on it. If you find 0 volts, that wire is broken somewhere between that plug and the alternator. If there is some voltage there, you can use a jumper wire to ground that terminal and it should also cause the system to full-field. If that works, I guess I'd have to agree the regulator circuit is the problem.

If the battery was recently disconnected, that would explain the low idle speed. There's an easy fix later once the charging system is working. It just involves driving it.

Caradiodoc

That 4 volts must be coming out of the computer. It can show up from the internal circuitry. If you measure that same voltage at the alternator, the wire must be ok.

Caradiodoc

What reads 0? Where are you measuring?

Caradiodoc

Hope this diagram comes out ok. Figure 1 is what you have now. Current flows from the battery, through the automatic shutdown relay, (when the engine is running), through the rotating field winding, through the voltage regulator inside the Engine Computer, to ground and back to the battery.

In Figure 2, the external regulator takes the place of the circuit in the Engine Computer. The blue wire must be added from the green / orange feed wire to the blue wire in the regulator's connector. That is the power wire to run the regulator and it's the system voltage sensing wire. That wire exists in your current system too.

Figure 3 shows what the wire end of the 1970s regulator connector looks like. The blue wire in the middle connects to the green / orange wire. Cut the green wire from the computer's connector and connect it to the green wire in the regulator's connector. The new regulator must be bolted to the body sheet metal because the metal housing is the third connection.

Caradiodoc