[TR] Soldered Wire Connections in Cars

[Dave1massey at cs.com]

In a message dated 7/11/2008 1:05:47 PM Central Daylight Time, 
Brian.L.Jones at gsk.com writes: 
> >  My soldering is very bad plus I've been told that wiring in  cars ought 
> not be
> >  soldered.  Soldering does not allow minute vibrations of the connection 
> thus
> >  yielding broken wires. 
> 
> Is this urban myth, or is there truth to it?
> 
Good question.  As I like to say, you can prove anything you want with 
anecdotal evidence.  I suppose that some poorly executed wiring with soldered 
connections have failed.  But then I am sure that some poorly executed non-soldered 
connections have failed as well.  

The two potential failure modes with soldered connections that come to mind 
are:
1) residual flux causing corrosion, and 
2) mechanical stress transfer to the end of the solder wicking.
The first on is the result of using acid flux and not washing the residue 
away.  (This has been touched on before) Rinsing can be problematic as it can 
wick up the wires underneath the insulation where rinsing it out is not very 
easy.  Always us rosin or "No Clean" flux which leaves a benign residue.

The second is the result of the mechanical flexing of the harness 
transmitting all the forces to the point where the solder, that has wicked into the 
strands, ends.  This can cause stress hardening of the wires at that point and 
fatigue will cause the wires to break.  The preventative cure for this is to 
secure the harness at a point close to the connection that will absorb the 
mechanical forces and keep them from stressing the soldered part of the wire.

On a TR3, especially the early ones, there are a lot of connections that use 
screws to clamp the wires to the terminal.  In these types of connections a 
tinned, stranded wire will act as a solid wire except that the lead/tin alloy is 
less resilient than is copper.  That means that as the connection heats up 
and cools down the compression forces will vary.  If the force heats up enough 
the lead/tin will flow (since it is malleable) and will permanently change 
dimension.  When it cools back down the connection forces are much reduced.  
Copper will spring back to a greater degree than will lead/tin and will retain 
connection pressure much better.  This is the aluminum house wiring situation from 
the 60's all over again.  One could (or should) retorque these connections on 
an annual basis to stave off problems here.

Good, reliable wiring doesn't just happen, it is the result of attention to 
detail.  Just like everything else.

Dave