[T3] : Re: 73 Type 3 Ignition Switch

Jim Adney jadney at vwtype3.org
Thu Oct 22 09:07:56 PDT 2020

On 22 Oct 2020 at 1:01, William Jahn wrote:

> The primer switch runs the pump key off and so gets all the air or most
> of it out of the fuel ring , meaning from both the pressure side and
> return. 

Depending on which primer switch you have, and how it is wired, you may be 
able to prime the fuel system with the key either ON or OFF. I recommend 
ON. We don't really care about the return side, because that no longer 
affects the engine.   

> The 72 and 73 have a TVS or TPS that with the key on you hear 20 clicks
> which I thought basically overrides the dist position. Once running the
> dist will fire two pairs at a time. I thought the basic enrichment when
> you accelerate was done via the switch as well as decell when the
> switch cuts off the fuel via the two position micro switch which
> determines the circuit board zipper and also brings in the WOT at about
> 3/4 travel.

The "zipper" (nice term!) enrichment only enriches the 2 injectors that are 
enabled by the position of the trigger points in the distributor. The brain 
doesn't care whether the engine is running or not. You can tell this on '70-3 
cars by moving the throttle with the ignition ON while feeling the injectors 
with your other hand. You will find that only 2 & 3, or 1 & 4, are firing when 
you do this.

> The 72 and 73 have a 5 pin switch and the Pressure sensor with one
> adjustment  has no WOT enrichment like the earlier 71 and 70 have and
> even the 68 & 69 which has the vacuum controlled switch.  It seems if
> you are at a steady speed the TPS is in a position for the zipper
> contacts are engaged by the micro switch  and the dist then fires two
> pairs  until you let off the throttle then you get no fuel from the
> injectors. 

At steady speed the zipper contacts aren't moving, so you only get the 
normal injector firings, without enrichment.  

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

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