[T3] Clock Question-Internal coil

Jim Adney jadney at vwtype3.org
Sun Jun 5 15:52:07 PDT 2022

On 5 Jun 2022 at 16:23, oktype3tim at cox.net wrote:

> Interesting,  I found another 6V clock.  It winds, but doesn't run. (Running
> is a whole different problem, not pertinent to the question at hand.) The
> winder in this 6V clock does not have a resistor at all. 
> Thus, does it make sense that in the conversion from 6V to 12V they kept the
> same coil, but knocked down the voltage by adding a 100 ohm resistor across
> the coil?

I don't think so. The resistor is there to absorb the inductive kick that comes 
when the points open. It's the same kind of kick that makes spark plugs fire. 
In this case, the resistor is there to absorb that surge and minimize the 
sparking/wear in the contacts.

I use a diode, instead of a resistor, to absorb that inductive kick. In this 
application, this is called a "freewheeling" diode. This limits the voltage 
across the contacts to about 0.7 V, which is much better than what a 100 
Ohm resistor will do. That diode costs less than 10 cents, but that's still 
probably about 10x what the resistor cost VDO. You have to install the diode 
in the direction that it WON'T carry current when the points are closed. It 
only carries the reverse current after the points open.

I've never had a 6 V clock to work on, so I don't know what differences there 
might be. I'd like to see several to get a better idea of what VDO changed. 
Some changes are likely to have come from experience: what works, what 

> Another interesting fact; 12V clock covers have a sticker that says use
> fusing solder for the thermo fuse that has a fusing point of 120 Deg C.  The
> 6V clock cover says fusing point of 90 deg C.

That may be one of those experience changes, or the fact that low melting 
point solders often contain Indium, which is expensive, so economics may 
have had a role. I resolder them using the existing solder and a fairly "dry" 
iron, to avoid changing the melting point much.  

> With regard to my particular problem.  At some point in history someone
> possibly put a winding coil from a 12V clock into my 6V clock hoping to make
> it work; which as previously reported---it didn't.
> What do you think Jim?

Can you tell if the wires in the 2 solenoids look to be the same diameter? I'd 
expect the 6 V wire to be larger. It's also possible that someone removed the 
resistor in the hope that it would increase the current thru the coil. (That 
would have negligible effect.)

You could probably wind a new coil with larger wire. A 50% increase in 
diameter would probably be enough. Is there room?

With cars this old, it's always important to consider what previous repairs 
might have been done. Every Type 3 I've ever bought, after the first one 
which was bought new, needed a lot of going thru to undo previous poorly 
done repairs or upgrades. I've found very few upgrades, other than the ones 
VW did in the course of production, to actually be improvements. We have to 
be very careful about this.

Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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