There are a number of analog inputs available on the 1G and 2G ECU. Many are used for critical engine operation, but some are available for datalogging aftermarket sensors. The problem is that some of these inputs are incompatible with some aftermarket sensors. In particular, the 1G EGR temperature input and the 2G baro input have pull up/pull down resistor configurations that some sensors are not capable of driving properly.
The solution to this is to “buffer” the input so that the sensor is not directly driving the ECU's input. The circuit and buffer board described below can be used for this purpose. The picture below illustrates the buffer board available for purchase from ECMTuning.
01/01/2013:Due to low demand, we've decided to discontinue this product. Many common sensors now include this type of functionality internally, negating the need for this board. The Omni 4-bar sensor that we sell on our website, for example, has this functionality built in. Most, if not all, wideband kits these days do too.
Connect “+5V from ECU” to the ECU's +5V sensor power, pin 23 (Green/Red) on a 1G ECU or pin 81 (Green/Yellow) on a 2G ECU. This is also present under the hood at the MAF, TPS and MDP sensors.
Connect “GND to ECU” to the ECU's sensor ground, pin 24 (Green/Black) on a 1G ECU or pin 92 (Black) on a 2G ECU. This is also present under the hood at the MAF, TPS, MDP and coolant temp sensors.
If the sensor is a 5V sensor that draws little current, like a MAP sensor, connect the sensor's power, signal and ground connections to the input side of the buffer circuit assembly. If the sensing device, such as a wideband, has its own power and ground, connect only the sensing device's signal wire to the input side of the buffer. In either case, you still must provide +5V and GND to the buffer circuit and you must then insulate any unused wires.
The following table lists a set of ECU inputs and their internal pull up/pull down resistor configuration.
|Baro||float/100k down?||3.3k down|
|IAT||2.2k up / 39k down||2.2k up / 39k down|
|Coolant||2.2k up / 39k down||2.2k up / 39k down|
|Rear O2||-||220k down|
Again, the EGRTemp on a 1G and the Baro input on a 2G are problematic for some aftermarket sensors. When feeding certain wideband kits into the EGRTemp input on a 1G, for example, you may see normal A/F ratio readings above 11.5:1 but then the readings flatline there and won't show any value below that.
For 2Gs, feeding an aftermarket pressure sensor into the Baro input can produce a cap on the upper value read by the sensor. A 3.3-bar sensor, for example, might flatline at 2.5-bar. For the 2G Baro issue, there's a fairly simple solution if you're handy with a soldering iron (or two). The 2G baro input workaround page describes this solution.
In either case (1G or 2G) and for just about any input, you can also consider using a unity buffer to drive the ECU. Such a device presents a low-load input for connecting the external sensor and then produces a higher drive output for connecting to the ECU's input. The following schematic can be used to build this device.
Note that this is not necessarily an easy thing to do. The op-amp part referenced in that diagram is extremely small and difficult to work with if you're not familiar with SMT techniques or have access to appropriate equipment.
If you can't wait and working with a SOT23-5 package is just too small for you, then consider this circuit/chip instead. It's a bit easier to work with.