Tapping into signal wires
 

What is the best (ie safest) way to tap into a signal wire? There are a few sensors that I'd like to monitor, the "signal" is usually just the voltage (eg 0V=open, 5V=closed, etc). Since taking the plug or socket apart is too hard (if at all possible), I have to somehow get into the wires. I can see three options:

1. Cut the wire, then solder it back together with a third wire for monitoring,
2. Same as above, but without cutting, only stripping some of the insulation off,
3. Insert a metal sewing needle through the insulation and solder the monitor wire to the needle.

Since I want to monitor the signal only for a short period of time, I would like to be as non-invasive as possible. Options #1 and #2 may lead to problems with corrosion in the future, so is #3 the best?

If its temporary, I'd do #3. Put some electrical tape over it after you're done just to be safe.

I'm not sure if they are available in Poland, but here the auto parts stores carry splice taps.
they clamp on to the wire and provide a 3rd port for a new wire.
when removed the insulation only has a small nick like your needle idea.

example: BWD CT617A - Terminal | O'Reilly Auto Parts

solder a lead to the pin, then stick it into the wire. Wipe a little silicone sealant over the hole(s) when you take the pin out if so inclined to restrict further oxidation.

If you can get at the ends of your wire harnesses (e.g. where they enter the ECU), you can get a narrow probe in there without piercing the insulation.

Apparently what you are seeking is a way to read the voltage of the wires by piercing but not cutting the insulation. It's called pick probing. You will need a few, very sharp dental instruments that can pierce through the insulation of the wire just enough to make contact with the wire. The handle end of the instrument will be attached by an alligator clip to the meter you are using. When you remove your pick, the pin prick in the insulation will be so slight as to need no remediation to the wire or its insulation

(I've only been using this technique for 40 years. If you find it useful a 'thank you' would be appreciated.) ;)

That's the problem - where I could, I took the plug/socket apart and added a thin wire to the pin. But in many cases the harness is waterproofed and can't be taken apart without breaking something. I've tried, but too many layers of plastic and silicone.

Dental instruments (chill runs down spine)...

Thanks to everyone:thumbup:

you could get another connector, and solder pins on the end to fit into the original connector, splicing off a wire as in #1. then when you're done, just unplug your "piggyback" setup, and plug the original connector back in. sort of a man-in-the-middle type setup, and no worries about corrosion/broken wires with method #3. just a thought.

Method #3 will neither cause corrosion, nor broken wires. But you need something a bit heftier than a pin, and a sharp, fine dental instrument is just right for the job of pick probing.

+1 for method #3, the 'pin-thru-insulation.'

...and a single wrapping of self-vulcanizing tape seals everthing up tight.

T pins or AES Wave Pin Probes
 

You mention "green fuzzies to start forming inside of the harness". If there is a harness, you can insert your probe into it without probing the insulation of the wire. But what if there is no harness near in the wire you want to probe? The pick probing technique I recommended it not at all using the fatness of a pin. The probe used is like that of a very slender needle. In fact you could probably actually use a fine needle and attach an alligator clip to it to get a reading. The "hole" made in the wire is so miniscule as not to cause a problem of corrosion or a grounding hazard.

BTW, I said "heftier" than a pin - but by that I mean stronger, not heavier or fatter.

You can use RTV to seal up any holes made in the wiring insulation. That, or F-4 tape should suffice. If possible, though, heatshrink tubing is preferred over either of these solutions.

My personal preference is to strip off the insulation from the signal wire in question, solder the other wire to it, then seal up the stripped part. Usually, I can get the splice near a wiring connector, which means I can take the wire's pin out of the connector and use a section of heatshrink.

Since the "hole" is smaller than a pin-prick it is usually unnecessary. Often the insulation itself is resilient enough to virtually close off the needle hole.

That's the situation if you want to tap into the wire. I've found that the 3-M type taps are usually OK if the wire is in the interior of the car. However if the wire is under the hood and exposed to the elements I have found the taps to be unreliable. Then doing a strip & solder as you suggested and using silicone sealant is the best way to go.

Why take chances? Besides, I had a power wire leading to the left front foglamp on my truck go corrode out and fail because of a pinprick in the insulation.

I would never use wire taps. In fact, just thinking about using wire taps gives me one of them full-body shivers...

I talked to an EE friend of mine about the taps. His opinion was that for something that is out of the weather in the interior of the car, a good-quality wire tap is a perfectly fine way of tapping into a signal wire.

I will admit that the thought of using them still doesn't make me very happy, though.

-soD

Good advice. I also heard it from an EE friend of mine, and he was correct. Anything that is in the outdoor elements of weather (read: under the hood) needs waterproofing; most interior applications don't.

I don't like crimp connectors for use in wire joining either. Between possible loosening and potential for corrosion, I avoid using them.