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Wiring Harness Splicing

I am fully aware that the large majority of people recommend soldering a splice connection & I agree that is likely the best method . That being said , can anyone definitely say , that a quality , properly installed heat shrink butt connector is more than likely to be problematic at some point & time ? Nobody needs to tell me that since I know soldering is better , why don’t I solder the connections . I just want to know if the before mentioned butt splice method is known to be a problem . If you say it’s a problem I’d like to know if you did it yourself & know it was done properly & still turned out to be a problem . Any yahoo can butt splice two wires together but that doesn’t mean the proper butt splice was used , the proper amount of wire was stripped , a proper crimp with a proper crimping tool was made & the proper amount of heat was applied to heat shrink the splice .

Butt splices are fine when, like you just mentioned, all the parameters are met.
Especially when soldering is not an option, like in tight spaces.
Matching sizes of wire to splice and all that you already said.
AFTER crimping the splice. . .give it a tug.

the splices I absolutely HATE are the ‘‘scotch lock’’ style that you squeese down over an unstripped wire, often used to tap a line. Even if you have a hot circuit today, give it a year and you’ll go nuts trying to find the break in you circuits.
Someone put a stero in my 79 that way and I had to re-do every single scotch lok splice !

I think the biggest problem with butt splices is making a proper crimp on the wire. Using a well made crimp tool and the proper size splice should pretty much eliminate that problem. If the connection is exposed to the outside then sealing the connection (using heat shrink or tape) is important also. When those things are done right, you won’t have any issues.

The only guarantee is if you use a hermetically sealed butt splice. Otherwise the wire is exposed and will oxidize over time. No, heat shrink alone is not hermetic but they do make heat shrink and butt splices that do result in a gas tight seal after heating to melt the sealer. That being said, many people get by with simple butt splices every day and they can last the life of the vehicle. I’ve had some underhood splices fail after many years but my environment has a lot of road salt and long winters…

Actually a properly done splice is better than a soldered splice, even if it is not hermetically sealed.

The butt connectors I’m considering using are themselves heat shrink , didn’t know they were made until recently . I’ve been familiar with the heat shrink tubes for years . I will also give the splices a good electrical tape wrap if I go this route .

Actually a properly done splice is better than a soldered splice, even if it is not hermetically sealed.

Mechanically yes. However, exposed tin-lead is less susceptible to oxidation than pure copper…the solder will wick up under the insulation. A butt splice always leaves some exposed bare wire…

A proper butt splice with stranded wire is superior both electrically and mechanically and more corrosion resistant.

Years ago an independent distributor of shop supplies offered a great deal of high quality merchandise. He had a line of heat shrink crimp connectors that had a tab of solder trapped in the metal tube that would melt at the temperature that shrank the plastic shell. A mini blow torch device with a shielded burner tip made it possible to get in relatively tight spaces. But absolutely, a proper crimping was necessary to make a good connection. Details such as the length of the bare wire and the position of the crimping tool on the connector were critical to ensure a reliable repair. And a proper crimp connection without solder can be reliable.

There’s also the POSI-LOCK connectors.


All the terminals on every car are crimped, not soldered so a proper, sealed crimp connector should be as durable. The 2 key words are "properly and sealed. That can be tough to do. Under-car connections, I want soldered with heat shrink to seal it. Inside the car, crimps are OK. My preferences.

Oh the lowly butt splice . . how valuable.
I just repaired a harmonica neck bracket with an 8ga butt splice.
You know , that harmonica holding device that Bob Dylan wears around his neck so he can play the guitar and harmonica together. This one broke at the bend just below the spring.
Took out the metal sleeve from the splice and , BINGO, a useable harmonica brace.

I’ve had problems on auto wires carrying high current (10 amps peak or more) using crimp connectors and twist connectors (the kind used to wire houses). When I used those as a temporary fix or experiment, I had to go back and solder all those again b/c they eventually failed, the resistance in the connection was too high.

Edit: I’ve had problems with the removable spade connections oxidizing and failing too for higher current circuits. If available, I’d prefer gold plated version of spade connectors, but I’ve never seen them. Anybody know if such a product is made?

Standard crimp connectors really aren’t rated to carry high current but they do have other higher quality crimp connections that can handle way more than 10 amps of current and of course they cost more. The car audio supply folks do make some gold plated power connections. You might try checking Parts Express to see what they have. I’m not sure if they have spade type but maybe they do.

Depends on the harness. In our shop right now there are two vehicle with heavy damage to the main body harness. Replace is the correct action.

Again, it depends on the severity and which harness.


You mentioned problems with twist connectors and automotive use

Those are NOT meant for automotive use, anyways

I’ll take solder and shrink wrap any day of the week.

Unless it’s the case of a biodegradeable European wire harness for which there is no tolerable repair short of harness replacement… :frowning:


I’m finding that many non-european wiring harnesses are also prone to getting brittle and breaking down, once they’ve got a few years on them

Perhaps it has less to do with the name of the car, and more to do with the age of the wiring itself, and perhaps the supplier of the wiring harness :fearful:

That said, the european wiring harnesses from the 1980s and 1990s seem to have degraded faster, and more severely

But nobody is immune in the long run, either

I’m also a little conflicted

I also worked on plenty of Benzes from the 1980s and 1990s that never had a wiring harness problem. It’s kind of like a tsb. While a potential problem exists, not every car will actually experience that problem.

When you want trouble free connections…of course you solder… That being said I have probably used up half the worlds supply of Butt connectors in my day. Done correctly and using the right tools…they seem just fine. Ive got systems installed with butt connectors still functioning today…15yrs later.

The majority of my butt connecting…lol… is usually inside the vehicle however. When exposed to the elements that butt connector will attract all the worst so… If its sheltered and done correctly using proper tools…you should be fine. If these are important connections…you already know the best method…and if you use butt connectors…it might be nice to leave yourself a little note on which wires were played with for future diagnostics to help yourself remember what you fixed…in case it acts up.


The only time I solder is when I have to splice two different size wires together. Butt splices don’t accommodate wires more than one gage different, i.e. 12 and 14 ok but 14 and 18, no. When I installed the backup camera in my car, I had to splice a 24 ga. wire into the 16 ga. harness.

I was an electronic technician in the Navy working on aircraft weapons and radar systems. When I first joined in 69, soldering was the rule of the day. Then as research was done on why wires tended to break at the connections, it was discovered that crimp splices were much more reliable than soldered connections.

Early on, the butt splice crimped with a “Champ” tool was how it was done. The Champ tool is the typical cheap yellow handled crimp/strip/cut tool you find for a couple of bucks in any hardware or auto parts store. The jets operated in a salt water spray environment under high loads. Eventually more research was done and the modern precision crimp tools and environmentally sealed splices came about. These have found their way into most hardware stores today.

At least on the car or truck, you can use any heat gun to shrink the splice sleeve, on aircraft, you have to drag out the big cumbersome HT900 heat gun.