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* Apply 12v to CPU EGR (Pin 4). Apply ground to CPU AFC (Pin 7). With 4 volts on “From MAF Sensor” Pin 1, you should see 4 volts on “To CPU MAF Input” Pin 2. So if I put a second, lower voltage source on Pin 1, that voltage should be also on Pin 2? This worked.
Understand for this circuit to work, it has to have power. +12 Volts has to be on pin 3, ground on pin 6 ALL THE TIME. Disconnect the 10 Ohm resistor for testing the circuit operation.
The inputs have to be "Set" as such: CPU EGR (Pin 4) has to have +12 Volts. CPU AFC (Pin 7) has to be grounded.
What ever voltage is placed on: From MAF Sensor (Pin 1), that same Voltage (or very close) will appear on: To CPU MAF Input. For testing we used 4 Volts as a reference input (on Pin 1) so you could see if each section was working by how much the Voltage decreased when activated on Pin 2. Place 4 Volts on Pin 1 for the checkout.
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* (Alt. Test, no signal gen.) With 12 volts on “From CPU EGR” Pin 4, the “TO CPU MAF” Pin 2, the voltage should be 4 volts. This did not work. I see 0v on Pin 2. I am supposed to have Pin 3 and Pin 4 at 12v, and Pin 6 grounded, with the 10 Ohm resistor out of the picture, right? But there's an incomplete sentence in the instructions: is it trying to say to also put Pin 2 at 12v?
Do the "Set" conditions above. The circuit has to have 12 Volts to work. If you had 0 Volts on Pin 2, it sounds like you didn't have the circuit powered up. If you did put +12 Volts on Pin 2 you will need a new U1, LM324. That is an output of the IC and it will burn up if power is applied to it.
With the "Set" conditions, you should see 4 volts on Pin 2. If not, something is wrong.
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* Now ground the “CPU EGR Output” Pin 4. The “To CPU MAF” Pin 2 voltage should go from 4 volts to 2.45 volts. Again, this no worky. Same reason as above, I suspect...
Probably.....
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* Place 12 volts on “From CPU EGR Output Pin 4. (Alt. test, no signal gen) With ground on “From CPU AFC output Pin 7, now place 12 volts on this pin. The voltage on “To CPU MAF Input, Pin 2, should go from 4 volts to 3 volts. This doesn't work either. I still get 0 volts on Pin 2.
Still sounds like you don't have 12 Volts to power up the circuit.
Try the above "Set" conditions and let us know.