Installation & Wiring Questions

[Fred]

Have a read of this thread first, it may answer some of your questions: viewtopic.php?f=54&t=1349

Ask whatever you want here. If you think your problem deserves its own thread, that's fine too! :-)

Fred.

[oden]

Where are people getting their shielded cables from?

What temp should under bonnet cables be rated for?

Looking for shielded multicore cables (from element 14) and only PVF2 (Polyvinyl Fluoride) goes above 80 degrees.
PVF2 are rated to 150C, but they can only be purchased by the reel and cost in the thousands.

Digikey site allows you to filter based on temp range and there seem to be more options but again, purchasing by the roll.

[Fred]

My engine bay has some silicone mic cable in it, and it's still doing well after 4+ years and a few burnouts :-)

It'll depend on which part of the engine bay.

I know some people buy rolls from china. If you're on a budget you can get auto grade coax from oem looms on various parts like knock sensors.

You can also just not use it and upgrade later if required.

Try audio places for pro grade mic cable if you want 2 + shield.

Fred.

[Preston]

Just a note: The wire rating refers more to the temp inside the wire, caused by the current flowing through that wire. The outside jacket maybe able to withstand higher ambient temperatures. Factors that need to be taken into consideration are cooling/heat dissipation.

There are many ways of shielding the wire against ambient heats, which sounds more like your problem, rather than the temperature rating of the wire. Example of shielding below.

http://www.heatshieldproducts.com/category/automotive/5

On a side note there are usually safety factors built in, technically speaking if it says 150deg max, that means it should work all day every day at that temp and be ok (we know that from experience that this is not always the case, product quality being the biggest factor causing failure) Also should be capable of temperature spikes beyond the rated temp for short periods of time with NO deterioration, again based upon the heat generated via current in the conductor and its ability to dissipate heat.

Preston.

[oden]

Cool, Mic cable it is then. Thanks Preston, good info.


In the case of IAT sensor (as an example) there are 2 wires, one being ground. Does this get connected to the shield?

Read somewhere else in this forum that only one end of the shield should be connected to ground, and the other should be left unconnected.

[Fred]

You're definitely best to use 2 + shield cable and only connect the shield at ONE end :-)

Don't look at the other pin as a "ground" look at it as a floating pin, which it is. The fact that it is probably close to ground at the OTHER end of the wire doesn't matter. Ground the shield at the EMS and connect the two inner cores to the sensor. Keep the shield as close to the sensor as you can without risking it touching/shorting, and use some shrink sleeve to keep it safe and reliable :-)

BTW, cheap mic cable is just rubber/pvc and doesn't handle soldering well let alone engines.

A good test is to get a 6 inch sample of each that you're considering and tin the end, excessively, ie, get it really hot with the soldering iron. A shit wire will quickly shrink away and melt as the heat rises. A good wire might discolour a bit in the end, but wont quickly shrink away.

Good silicone stuff doesn't do that, it's nice. Mine was made in japan and sold cheap as end of line stuff from dick smith years ago. They couldn't sell it retail because it cost too much, but upon clearing time it became a bargain :-)

Fred.

[jharvey]

The silicone coatings are good. Largely because they have good UV and long term out gassing characteristics. So I would recommend them if you can get them. The others like PVF2 will out gas the plastic over time, often becoming brittle after a couple years, cracking, ect. They are often designed for conduit installations. However the silicone based coatings do not have those issues. Even better you can boast with your friends that you sprung for silicone implants in your project

About coax, for most automotive signals is not a good choice. Largely because of impedance mismatching. Knock signals are about the only potential exception I can think of. However, most knock sensors are above 1kOhm impedance, so you really need to use some really special coax for that to work the way it should. I would typically suggest most knock sensors for most applications, you use shielded twisted pair.

About pushing your limits in temperature, don't do it. Go larger diameter wire when you can. The limits are fire prevention minimums. So a wire rated for 80C ambient will fail under full electrical load. However, it will still fail at 100C with no load. The failures don't happen over night, it's more of a longevity issue. I would recommend getting a higher temperature rated wire.

A potential 3 wire silicone jacket wire could be Newark's 74M8464

[Preston]

About pushing your limits in temperature, don't do it. Go larger diameter wire when you can.

Agreed where the temperature source is the current in the wire.

The limits are fire prevention minimums.

Absolute rubbish. The temperature ratings ‘aka’ limits as stated in this case are put in place to maintain insulation characteristics for the wire, it has nothing to do with fire! a common misconception.

re So a wire rated for 80C ambient will fail under full electrical load. However, it will still fail at 100C with no load.

Again not true.

The failures don't happen over night, it's more of a longevity issue.

Agreed, hence the ability to go beyond ratings for x period of time while maintaining the wires electrical insulation properties. ie temp spikes XX% above wire temperature ratings is possible without damage.

I would recommend getting a higher temperature rated wire.

I think the point of my post has been missed.

• If the heat source is internal, then yes you need to look at a different wire whether it have a larger conductor, lower resistance per meter or higher temperature rating.
• If the heat source is external ie Engine bay temps. Then there are other ways of dealing with the problem, an expensive high grade, high temperature wire is not necessary. A low cost wire with temperature sheathing would be adequate.

So as Fred has stated that cheap mic cable works well electrically, around high externatal heat sources shield the wire and it will perform just as good and as long as any expensive cable with high temp ratting.

Preston.

[Fred]

Furthermore...

About coax, for most automotive signals is not a good choice. Largely because of impedance mismatching. Knock signals are about the only potential exception I can think of. However, most knock sensors are above 1kOhm impedance, so you really need to use some really special coax for that to work the way it should. I would typically suggest most knock sensors for most applications, you use shielded twisted pair.

How many cars have you wired exactly? :-) Also, what do you and don't you know about coax and the impedance of it?

Coax impedance is at RF frequencies!!! Knock sensors, TPS, thermistors are at audio and sub-audio frequencies and present a VERY low impedance at those levels down to the resistance when hit with DC which is effectively what the signals we're talking about are. IE, the above is false. As is much of the stuff you wrote in OT's thread.

Sorry dude, but I can't accept you giving people wrong advice, even if you mean well!

Fred.

[oden]

Thanks guys, will try and find some silicone mic cable