Sharing Sensors with OEM Controllers

A. Yes, it can sometimes be done, and this is how:

• First, you must remove the 2.49k bias resistors on the MegaSquirt^® EFI Controller, R4 & R7. You do not jumper them, or replace them with other resistors, simply leave them out.
• Next, you need to determine the stock computer's bias resistor. This can be tricky because some ECUs (GM for example) have 2 bias resistors and swap them in at different temperatures to gain accuracy. Example, -40°F to +40°F and +40°F to 230°F. If your ECU does this, you can't easily hookup the MegaSquirt^® in parallel.
• If your ECU uses a single bias resistor, then you're set. Here's how to determine if you have 2 resistors and what their values are:
  First, the sensor and bias resistor act as a voltage divider. The equation is:
  V_o = V_i / (R₁ + R₂) / R₂

  Or:

  Output equals the bias voltage divided by the ratio of the sum of the combined resistances divided by the sensor resistance.

  The bias voltage is 5v for most any EFI system you'll encounter.

  So, you have 5v applied to one end of the bias resistor and the temp sensor is connected from ground to the other end of the bias resistor. The ECU reads the voltage at the junction of the bias resistor and the temp sensor to determine it's value.

  This equation can be used to find the variable resistance value in a voltage divider:

  R_o = R_b * V_ob / (V_b - V_ob)

  In english it says that the unknown resistor's value is equal to the known resistor's value multiplied by the voltage across the unknown resistor and divided by the difference of the bias voltage minus the voltage across the unknown resistor.

  You can illustrate the way to find both resistor in a diagram:

  

  If we run the equation for the above values, we get:

  R2 = 20000 * 0.19v / (5 - .19v) R2 = 20000 * 0.19v / (4.81v) R2 = 3800 / 4.81v R2 = 790ohms

  We can also solve for an uknown R₁ (bias resistor) by changing the voltages around a little:

  R1 = 790 * 4.81v / (5 - 4.81v) R1 = 790 * 4.81v / (0.19v) R1 = 3800 / 0.19v R1 = 20000

  Note, the results aren't exactly as above, they are rounded.

• So, to find out what your bias resistor value is, follow this procedure:
  Unplug the temp sensor and read its resistance, record this as R₂.
  • Plug the sensor back in and backprobe (insert paperclips or small diameter sharp probes into the silicone plug of the connector so that they are in electrical contact with the connector pins).
  • Next, turn on the ignition so that the computer is sending power to the sensor.
  • Now read the voltage across the sensor and record this as Vo.
• Now you have all the data you need to determine the bias resistor value.
  Bias Ohms = R₂ * (5 - V_o) / V_o

  So you know what the bias value is.

• To determine if your ECU switches the bias resistor during warmup, you need to perform the first test before the engine is hot, then stop the engine after it's run long enough to get to full temp, then perform the test again. If the bias resistor value you calculated is significantly different, your ECU switches them during temp sensor warmup.

  You could configure the MegaSquirt^® for the "hot" bias value and just accept what behavior the "cold" bias value will cause.

• You then need to use EasyTherm to create a new table which uses the correct bias value.

The above information is from a message (36193) posted by Perry Harrington on the Yahoo! list. If you have questions about this procedure, please contact the MegaSquirt Forums list.