MoTeC Global Forum

I have a M130 in my car where the knock inputs currently are connected to two temp sensors and used as temperature measurements.
As I would like to install knock sensors on my engine and connect them to the appropriate inputs on the M130, I'll have to move the temp sensors. All my AI inputs on the M130 are already used, so I've aquired an E888 expansion unit and plan to move the two temp sensors there.
The two temp sensors are currently connected to sensor 0V-A on the M130.

I am questioning if it is advisable/possible to connect the sensor 0V-A on the M130 to the sensor 0V on the E888 to have a common sensor 0V.
I would like to do it this way if possible to avoid having to split up the existing wiring harness to add an aditional wire for the sensor 0V from the E888.

Hi,

I would highly recommend against doing this. Each device should supply the 5V+ and 0V to the sensors that are connected to it, and this should not be shared across multiple devices as you can have a situation where one device will effect the other if a fault occurs in the device or sensor.

Hi Dean,

thanks for your reply.
I understand that from a reliability and integrity point of view this is not an advicable solution.

But if the consideration is limited to the electrical design of the two units, could the connection of the the sensor 0V's cause the unit(s) to fail?

While on the subject of multiple sensor supplies, the M130 have an A and B supply (0 and 5V). What is the reasoning for this? Are the A and B supplies independent of each other, so a fault on one group will not affect the other group?
Does the same rule apply here that the sensor 0V A should not be connected to 0V B?

Hi,

In normal operating conditions, having both units sharing a common 0V will not cause a failure, it does leave open the possibility of both units being damaged if a failure does occur that induces a higher voltage on the 0V inputs that triggers the reverse polarity protections. Wiring them together also means that a sensor failure has the potential to affect the operation of multiple devices.

The M1 ECU's have either two or three separate 5V+ and 0V supplies, an ideal wiring solution is to have sensor connected to the same 5V+ and 0V supplies (note that the AT's are internally connected to the 5V+ supplied through 1K ohm pullup resistors) so that if a sensor fails, the M1 is able to try and maintain the operation of the other sensors on the same supply by compensating their values to accommodate the change in the ratio-metric values.