Tapping into wires
 

What is good practice for tapping into wires? For example, a wire going to the engine thermistor, or a 12V wire?

In the past I have used crimp bullet connectors. Cut the wire and crimp one or more wires in with the wire of interest, then attach the bullet connector and everything is working again.

Are there any other clean ways of tapping into wires? Sometimes the loom makes it difficult and wires are pulled pretty tight.

http://tbn1.google.com/images?q=tbn:...bbig/16468.jpg

http://tbn3.google.com/images?q=tbn:...bbig/28314.jpg

Use 3m Scotchlok connectors. The don't require you to cut the line in any way, so if the wire is tout it's not a problem. Just make sure to get the right Scotchlok for the gauge of wires you are using.
Example. [PDF]

Another option is to use a tap splice, but this is much more complicated, needs to be soldered, and sealed with tape.

There is no substitute for solder and heatshrink or chem-seal.

Crimp-on taps work, but if you can't seal them, the wires will corrode. Since electricity only travels the outer-most diameter of a cross section of each thread in a wire, when corrosion occurs, the wires effectively have a reduced surface area per strand, and thus create impedance compared to the rest of the circuit.

The second-best thing to do would be to apply the crimp-tap, then be generous with some hot glue in the area, making sure to liberally seal the wire's connection and tap.

OR just get the self sealing scotchloks that are designed for unprotected environments.

PS: You can't heatshrink a tap splice, there is no way to get the heatshrink over the line wire.

I'm well aware that you can't heat shrink the wire if you don't cut it. That's why I mentioned the hot glue method to seal the wire.

I still never trust crimp taps, because they represent possible impedance in the line versus solder and heatshrink. I've done quite a few wiring modifications and even complete wiring harnesses from scratch, and I've yet to come across a loom that I couldn't tap into with solder and heatshrink.

If you're going to tap the wire and you plan on soldering it backtogether, you just cut the tapped wire at your tap point, so you can put the shrink on. You still need to seal the doubled side of the wire though, the heatshrink will not seal properly there, so you put some hot glue or something that you can shape on the wires before you put the heatshrink over that area.

when solder wicks in to a wire it creats a weak point in the wire as well, that is why they recommend not solder filling larger cables any more and instead recommend a multi sided crimp, the problem with crimp taps is the moisture that can get in there and corrode the wire, a dap of dielectric grease takes care of that, the insulation is then cut and weakend as well the way around that is to use silicone II calk, or other silicone sealent that is safe for copper, normal silicone calk has an acid base that will eat away at copper so make sure it's safe for use on electrical connections, it will keep unwanted moisure out and add some protection around your connection.

Anymore I always strip, solder, and tape it really good, or heat shrink if feasible. I used to love the splice taps but anything outside would corrode and break after 1 winter, and some, even interior ones, would work loose. The last straw was when the the signal wire to my trailer brake controller worked loose.

I prefer solder. If done properly, I believe it's less subject to failure and intermittent connection problems.

If at all possible, find some point where the wire loom/harness provides a reasonably accessible 12 in. (30 cm.) or so run. Depending on just how many wires are in the harness, open a 3-7 in. (8-18 cm.) section of the loom. Find the wire you want to tap into and remove a 1 in. (3 cm.) section of insulation (with out nicking any of the wires). In so far as possible, separate the individual strands of the exposed wire. Strip off 3/4 in. (2 cm.) from one end of the wire you're adding. Separate into individual strands. Blend the individual strands of both wires together and wrap tightly with a single strand of very thin wire (that you've previously prepared) and solder. (More or less what this guy does but don't cut the wire.)

Now we need to provide strain relief for the solder joint; repair the insulation; and repair the wire loom/harness. This will require electrical tape and a tape-wrap termination tool. As no one sells tape-wrap termination tools, you'll need to make one from a 16 in. (40 cm.) piece of 16 ga tie wire (plumbing section of Lowes, Menards, Home Depot ...). Make a 180* bend in the middle of your piece of tie wire. Starting about 4-5 in. (10-13 cm) from the 180* bend, make several spiral twists in the wire to keep the bend from spreading back open. Then bend the two loose ends of the tie wire into a tee and wrap them around any convenient piece of dowel/stick which will serve as a handle.

Illustration of a pseudo wiring harness and really crummy tape-wrap termination tool (no handle).
http://i490.photobucket.com/albums/r...ireSplice1.jpg


For illustration purposes - pseudo solder joint - A with a single wrap of electrical tape.
http://i490.photobucket.com/albums/r...ireSplice2.jpg


Wrapped down the single wire leg to wrap termination point - B and back up to the pseudo solder joint - A.
http://i490.photobucket.com/albums/r...ireSplice3.jpg


Wrapped up the dual wire leg to wrap termination point - C and made a single wrap around the termination tool.
http://i490.photobucket.com/albums/r...ireSplice4.jpg


Re-wrapped over the termination tool back to the solder joint - A.
http://i490.photobucket.com/albums/r...ireSplice5.jpg


Re-wrapped over the termination tool back up to termination point - C.
And thread loose end of tape through termination tool.
*** I flubbed this picture. You should only pull the loose tape end about 1/3 of the way through the tool (arrow), not all the way as illustrated here. ***
http://i490.photobucket.com/albums/r...ireSplice6.jpg


Detail illustrating correct threading through tool.
http://i490.photobucket.com/albums/r...ireSplice9.jpg


Use the tape-wrap termination tool to pull the loose end of the tape under the tape windings for a clean insulation repair that won't unravel as the tape and glue age.
http://i490.photobucket.com/albums/r...ireSplice8.jpg

If you have any doubts about strain relief, make an additional wrap of a longer section of the spliced wire to two or three other wires in the loom.

Finish by wrapping the exposed loom wires in similar fashion.

TestDrive, your last picture won't load, I was very confused by your method without it, but I think I get it now.

That's very clever, especially the first part about wrapping the exposed wires with a single strand of wire, all the wire splicing I've done looks like crap because it bulges out everywhere.

You should see the image now.