What is a shielded joint connector and why would it be spliced into a nonshielded ground wire?

 

IE: My front 02sensor has a ground wire coming off it it, but a shielded wire splices into it, goes through a shielded joint connector and ends up at the ECU, while the other unshielded spliced portion runs through basically the entire engine ground system.

 

I am guessing it would be shielded from radio interference, but wouldn't it recieve interference from the unshielded portion running the rest of the way through the harness?

 

Thanks.

I wonder if they're all like this, or yours was modified by someone.

my guess is that it is to protect the signal from the O2 sensor to the ECU from EMI (electro-magnetic interference). the readings can change from "crosstalk" of other wires that are in the vicinity of the cable run. more than likely it was a precautionary measure to make sure interference wasn't a problem in the future.

 

as far as the interference from the rest of the harness that shouldn't cause any problems, if you think about it the whole chassis as a ground so if the ECU ground and the shield have the same 'ground potential' then there shouldn't be any interference.

 

btw I am not an electrical engineer, just my experience of how things seem to work.

Edited July 20, 201015 yr by tractor pole
to add comment @ bottom

What is a shielded joint connector and why would it be spliced into a nonshielded ground wire?

 

IE: My front 02sensor has a ground wire coming off it it, but a shielded wire splices into it, goes through a shielded joint connector and ends up at the ECU, while the other unshielded spliced portion runs through basically the entire engine ground system.

It's a "shield joint connector", not a "shielded joint connector", and I'm not saying that just to be a stickler. Simply, it's just the place in the circuit where a wire shield connects to other shields or ground. If the connection you're describing is factory original, it's probably okay.

 

 

I am guessing it would be shielded from radio interference, but wouldn't it recieve interference from the unshielded portion running the rest of the way through the harness?

Just because the roof and siding of a house are being pelted by rain doesn't mean you'll get wet inside.

 

Kidding aside, ground wires and shields have to be run very carefully in certain sensitive circuits. They are connected in a manner that minimizes currents that could induce interference from flowing through the part of the circuit that would be detrimentally impacted. That's why sometimes an attempt to "improve" grounding can actually lead to problems such as those commonly referred to as "ground loops".

 

By the way, sometimes engineers goof, and I've seen commercial products that needed to have grounding points changed or even removed in order to obtain proper operation.

 

See http://www.metexcorp.com/emirfi_theory.htm for some info.

The signal wires for modern O2 sensors are shielded. O2 sensor output is so small (from 0.1 to 1.0V range) and changes so rapidly that any variation due to EMI or RFI can throw off the reading and cause erratic behavior or set the CEL.

Cool, thanks for all the replies. That's kind of what I was thinking. Basically, just because a wire is shielded doesn't mean the entire circut is, only that portion which would require the shielding because the wire's location gives the circut the possibility for interference.

Also usually the the shield is grounded at only one end to prevent any ground loops or stray current flow through the shield.