Megasquirt Support Forum (MSEXTRA)

Working on an MS3 setup. Already have some RCA cables running the length of the car and I’d like to run a couple 5v pressure sensors in the trunk area off them. The wire gauge on these RCA connectors is TINY. I’d guess 22-24g? Is there any real amperage requirement on a pressure sensor? I was going to run the 5v supply and sensor input off the 2 wires in each RCA cable and ground in the rear.

I'd be worried about ground offset problems unless you ran the ground to the ECU.

22 to 24 gauge wires have very little mechanical strength; they're easily broken if you accidentally pull on them. But the current is in the milliamp range, so you're not going to burn the wires out.

They are very similar to this. 2 tiny wires per cable and encased in a thick rubber. I don’t think the wire breaking would be an issue unless it was at the terminal. I have 4 tiny wires I could work with. I had planned on combining the 2 wires per L/R RCA cable for a little more strength for the 5v and signal. I guess I could dedicate one wire to a shared ground up to the ECU if that would work best?




Thanks!

Very odd RCA cable.It's normally a co-ax cable, designed for unbalanced audio, etc.

DaveEFI wrote:Very odd RCA cable.It's normally a co-ax cable, designed for unbalanced audio, etc.

I believe they are just to carry the sub signal from the head unit to an Amplifier. Then you use the monster wire from the amp to the speakers.

That non-coax config isn't odd in the USA. The RCA/phono connector has been used in countless different ways since the '40s; with simple parallel wires, twisted-pair, and coax. There really is no standard for an RCA "cable", only the connectors.

PS: in this application I would be worried about the lack of shielding.

You’d think an audio cable would be sensitive that sort of thing. But I suppose if it’s a bad idea in general I can just zip tie some real wires to the RCA cable pull it through the chassis under the carpet.

Thanks for the opinions. The easiest route never seems to be the best!

It depends what you mean by "audio cable". In the detection or pre-amp areas the signals can be fairly low/high-impedance and sensitive to EMI; but the output to speakers is often very low-impedance and quite immune to EMI.

Twisted pair cables are better at noise rejection (noise cancelling to be technical) than co-ax based cables. I've installed car audio since the early '90's and the best cables I've used have been twisted pair, sometimes I've made them myself with some 18AWG trigger wire. Usually unshielded twisted pair works better for noise rejection than shielded, in my experience.

That being said, I'm not sure I'd use RCAs for the sensor connections, not because I think there'd be any issue with the signal, though as suggested the 5V reference and ground connections need to be made to the ECU, but simply because i'd prefer to just run new dedicated wires for the sensors.

Twisted pair should work fine as far as EMI pickup is concerned, IF you use each pair for the signal and ground.

You MAY have acceptable results with using one pair for signal from one sensor, and +5v for both, and the other pair for signal from the other sensor and ground for both.
Use the sensor ground from the ECU in either case.

David

I gotta ask... we keep talking about a 2-wire RCA connector/cable, but a "5V sensor" will require three wires. How is the other wire going to be handled? Yes, twisted-trio wire can be used, but sure won't work nicely with the RCA... Besides, the OP's whole point in using the RCA is because there are already existing RCA cables, which I assume are (existing) 2-wire.

Oops: I just re-read the first post, the OP was going to ground the sensors "remote", in the trunk (to the chassis?). Yeah, that is asking for trouble.

billr wrote:I gotta ask... we keep talking about a 2-wire RCA connector/cable, but a "5V sensor" will require three wires. How is the other wire going to be handled? Yes, twisted-trio wire can be used, but sure won't work nicely with the RCA... Besides, the OP's whole point in using the RCA is because there are already existing RCA cables, which I assume are (existing) 2-wire.

Oops: I just re-read the first post, the OP was going to ground the sensors "remote", in the trunk (to the chassis?). Yeah, that is asking for trouble.

The 5v and sensor ground can be shared between more than one sensor. That's exactly how most factory harnesses are wired.

For twisted-pair to be effective the signal and ground for each signal have to be in the same "pair", and the ground for each signal should terminate at the MS. Yes, the 5V can be shared, but not the grounds if you want the best EMI resistance. It all depends how good you want to avoid any possible EMI problems. Like the OEMs, I make compromises in my wiring, but am aware of what I am doing and accept the results.