| If you aren’t getting any signal through the VR circuit at all, and you only have a multimeter and Stim (or power supply) to test the circuit with, don’t panic. This article will cover what you need to do to test out the circuit when adjusting the VR trim pots doesn’t get you anywhere.. This also applies for the MS3X cam input circuit.
First, you’ll need to power up the MegaSquirt and have a way to apply solid power and ground to pin 24. On a JimStim, you can simply run a jumper from the “24” pin to the ground or 5V pins as appropriate. Here’s a diagram of the V3.0 main board VR circuit.
The circuit from VRIN to pin 2 of U7 is essentially a filter. It tracks the input voltage to the circuit, but absorbs noise. In steady state testing, it will produce a few changes to the signal, but what you see on the input (VRIN) will be very similar to what you see on the output (Pin 2 of U7). So, to test this one, apply ground and 5V power to the input (pin 24 of the DB37 connector, VRIN for a V3.0, or pin 1 of JP1 for a V3.57), and check the voltage at pin 2 of U7. You should see about 5V at this pin with 5V applied to the input, within about 0.5 volts. When you ground the input, pin 2 of the U7 should have from zero to 0.7 volts. If the voltage is significantly higher, Q22 is the most likely part to fail. The U7 chip compares the voltage at pin 2 to the voltage at pin 3. Adjust R56 to get between 1 and 2 volts at this pin. The output (pin 1) of U7 is low when you have more volts on pin 3 than pin 2, and high when you have more volts on pin 2 than pin 3. Once you have adjusted R56, apply ground to the input. You should see between 4.5 and 5 volts at pin 2 of U7, and between 3.5 and 5 volts at pin 1 of U7. The MC34072 should be closer to 5 volts; older kits that used the LM2904 may be closer to 3.5. Next, apply 5 volts the input pin. This should give you from 0 to 1 volts at pin 1 of the U7. If you see the correct input voltages on pins 2 and 3 of U7, but the output voltage does not change as it should, you need to replace U7. If you’re testing the MS3X, the schematic is similar, but the pins are different. The test points are as follows:
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