Re: dead radiator fan in Coupe Quattro

[steveb@newkla.kla.com (Steven Buchholz)]

> <<<<<SNIP>>>>>
> mount bolts and move the motor/fan about in the shround while gingerly working 
> around the ABS lines.  Finally got it out.  Found that the ground (brown) lead 
> on the three-lead connector had shorted! 
> 
> Needless to say the connector was junk.  After cleaning the slightly browned 
> male connectors on the fan, tested it with an in-line fuse on the battery.  It 
> worked!  Cut back the harness about 4 inches, butt-spliced and shrink sleeved in 
> 3 -8 inch lengths of 12 gauge stranded wire.  Replaced the connector with three 
> female spade connectors, ensured the connection with solder, and shrink-sleeved 
> the connectors to the end (Insurance against future shorting) Used the old 
> connector rubber boot to recover the new/old connectors.  Put the car back 
> together.  Got the car "blessed" by my mechanic the next day.
> 
> Question: Has anyone ever had a GROUND wire short out???? I didn't think this 
> was possible.  The fan didn't work because the melted connector created an open, 
> and therefore incomplete electrical path.
> 
I find the word *short* to be one of the most commonly misapplied terms when talking 
about electricals.  From your description it sounds like you actually had a high 
resistance connection that caused the failure.  High-wattage lamps, radiator cooling 
fans and fuel pumps are a few of the most common offenders here because they have high 
current demands.  It turns out that any time there is a voltage drop across a connec-
tion there is a certain amount of power dissipated there as heat.  Let's say that your 
fan was drawing 15 amps of current (I don't know if this is even close to being correct).  If there was a small amount of contact resistance, let's say 0.05 ohms, then the amount 
of power dissipated would be 11.25 watts (and the voltage drop would be 0.75 volts).  
All of that power would go into heating up the connector, but 11 watts probably would-
n't do much more that warm up the connection a little bit.  Over time the contacts 
could get oxidized and cause the resistance of the connection to increase, causing the 
amount of power dissipated to increase, along with the temperature of the connector.  
As the connector gets hotter it oxidizes even more and you end up with a positive feed-
back situation.  Eventually the connection gets hot enough to melt the plastic of the 
connector.  It seems that in many cases the melted plastic works its way into the conn-
ector and actually causes the circuit to open, meaning that your fan or headlamp or fuel 
pump will no longer operate.  This type of failure can happen in the ground lead just 
as easily as the supply lead.  

You did the right thing ... replace the connector and clean up the contact to minimize 
the contact resistance.  

The only caveat here is that the failure may be a sign that your fan may be going bad.  
I had a similar problem with the fuel pump on my '78 Audi Fox.  After I repaired the 
connector it wasn't too much longer before the pump failed.  It was contributing to the 
contact failure becasue it was drawing over 10 amps instead of the nominal 8 amps.  I'd 
recommend that you measure the fan current and compare it to the nominal value to see 
if you have the same thing happening to you.

My last issue is my personal opinion regarding the soldering of lugs.  If at all poss-
ible it is best to simply crimp the lugs ... soldering the wire makes it stiffer, more 
like a solid than a stranded wire.  And never *NEVER* tin the wire end with solder and 
then crimp the lug on the end.  If the wire that you are attaching to a lug is solid or 
has been tinned it is best to solder the lug to the end of the wire than to simply 
crimp it.  

Hope this helps!

Steve Buchholz
s_buchho@kla.com