Black99rt wrote:While we are on this topic, what is the best TPS wiring scheme? Should I twist all three wires together? Shielded three conductor?
3 factors make a signal source prone to noise contamination:
- small voltages
- high source impedance
- signal has wide bandwidth ... this means you can't lopass filter the signal aggresively.
All these are relative, of course. But of all the sensor signals, I'd say the knock sensor (mic type as opposed to piezo tuned type) far and away wins all 3 factors, followed by the TPS and MAP.
CLT and IAT are very slow signals so they can be lopassed very aggressively to get rid of any noise.
A shielded wire for the knock sensor is de rigueur. Factory wiring uses shielded wire.
For the TPS and MAP a twisted-wire pair would be good. In my own miata I use the factory wiring for the TPS, and I used shielded wire for the MAP sensor.
Shielded wire for the TPS may be needed if the wiring goes past some extra noisy crap like the ignition coils.
The proper way to use shielded wire is to have 2 wires inside a shield. The - wire goes only from the sensor - to the ECU signal gnd, and isn't connected anywhere else. The shield is grounded to the head or chassis at one end only. The other end of the shield is left floating.
You can use a single shielded cable to carry multiple signals. Say your MAP sensor and TPS are mounted close to each other. Use a shielded cable with 3 wires inside. One is connected to the MAP and TPS - side, and one wire for each for their + side. At the ECU the - wire goes to sensor ground. Again the shield is grounded to chassis or head, at one end. The other end of the shield is left floating.
All this will be defeated if there is *shared impedance noise coupling* (aka "ground loop" - a name I hate) inside the ECU grounding as I alluded to earlier.
Again the current paths of the signal ground return and any other high current ground returns (e.g. MOSFET and IGBTs), *must not share any length of conductor*. The 2 should be connected together at only a single point - in most cases, inside the A/D IC or uC's itself. Many A/D IC's have a signal gnd pin and a digital ground pin, internally connected with bond wires. All circuitry associated with the analog stuff is returned to the signal gnd pin and all digital stuff is returned to the digital ground pin. On the PCB the 2 grounds should not be connected to each other.
In an automotive ECU, all sensor grounds should be connected to the signal ground. (With the possible exception of hall sensor outputs). For protection the signal gnd input at the ECU connector should have a low-impedance, low-current PTC (auto resettable fuse) in series with the connection to the A/D analog ground. If a user connects the sensor ground to a chassis ground, a high current will be driven through the said A/D internal ground blowing it. The PTC will protect it.