Extending engine bay harness - solder seal or heat shrink butt connectors?

 The one thing I really enjoy about having male to female spade connectors is for diagnostics.  You can very easily cut the heat shrink and pull the spades apart to test for voltage or continuity, I am a service technician by trade, therefore I always build my vehicles with diagnostics in mind.

Soldering or connectors works great. However, solder will be a little better with vibration, so if you have a dedicated track car or an off-road vehicle that vibrates like a sex toy, you might want to use the solder. However if you have a vehicle that uses a lot of wires and is prone to needing service, I would use male female spade’s with the heat shrink. 

Heat shrink over the spades means it won’t come apart, until you want it to. 

Solder joints are very brittle, and crimped butt connectors are not.  My Miata is basically held together with the Amazon heat-shrink crimp connectors in the OP (and lots and lots of zip ties and safety wire), I haven't had one fail yet.  It's worth emphasizing though- pay the money for a good quality ratcheting crimper, worth every penny.

I did some Audi factory training a couple of years ago that suggested that crimps were preferable over solder because they didn't create as much localized stress in the wire that would eventually lead to breakage. 

I honestly hardly ever solder anything anymore. I use those heat shrink butt connectors for everything. For automotive work there's almost nothing that's not acceptable to repair this way, with the rare exceptions being where you're going to disturb shielding etc, not really having to do with actual resistance in the connection. 

rslifkin said:

A good crimp is better than a soldered joint in terms of vibration resistance and such.  However, I've never had a properly done solder joint fail in a car.  And it's easier to get a good solder joint than a good crimp, especially if you're working on wiring that's still installed on the car.  

If you're really paranoid, crimp it (with an all metal crimp connector), flow solder into the joint to fill any gaps left by an imperfect crimp and then heat shrink it.  

I've been doing just this the past couple of weeks as I prepare a car to return to the living that's been asleep for 30 years. I've had to relocate  the alt. and re-wire the ignition system with an electronic system in place of the "condenser & points" system the car came with.

Vigo said:

I honestly hardly ever solder anything anymore. I use those heat shrink butt connectors for everything. For automotive work there's almost nothing that's not acceptable to repair this way, with the rare exceptions being where you're going to disturb shielding etc, not really having to do with actual resistance in the connection. 

Solder is for copper pipes and PCBs, not for wiring harnesses. :)

When I made my harness for my Alfa Romeo spider conversion, I soldered all the wires take the harness.but this is how I did it, stripped 8 mm of wire, solder the wires, bent each side into a J shape. Hooked them togeather,a little crimp.then soldered those two I sections to each other, then heat shrink cover. All the wires were the same length as I built my harness ( got 4 sections about 35 inches long from doner cars)  this way I can cut to length when I install the harness. Bulit it over long .

Oh, yes, I used the oem connections.to make the harness, looks really nice. Photos later.

Keith Tanner said:

I like a good solid mechanical crimp so that's what my cars have and what I buy for my own cars. Use a ratcheting crimper and you'll never have a failure. As noted, they are easier to install because you can crimp one end and then the other.

I"m with Keith on this one. Good ratcheting crimper and heat-shrink connectors with the "glue" inside and you really shouldn't ever have a failure.