[T3] 73 type 3 FI ignition condenser ?

[Jim Adney]

On 19 Mar 2019 at 18:15, William Jahn wrote:

> I do have an issue with an erratic idle 

You've mentioned this before, and I've struggled with trying to understand 
what you were talking about, until I realized that my '73 had some sort of 
problem at idle when cold. It's not really a consistent miss, but, instead, it 
seems to sort of stumble on idle when cold. As you have mentioned, it drives 
just fine, it's just the cold idle that's annoying.

I've developed a theory about this. It started when I noticed a note on one of 
the schematic drawings for one of the later 914 d-jet brains. To follow this, 
go here:  

https://www.sw-em.com/bosch_d-jetronic_injection.htm

Scroll down to the 3rd schematic and look at the note near the lower left 
hand corner (index M,11.) This refers to shorting out R471 except at idle. 
You'll find R471 near the lower left hand corner (index G,28) of the 2nd 
schematic on that web page. R471 is a 300 Ohm resistor (in this 914 version 
of the brain) in series with the cyl head temp sensor, thus it adds 300 Ohms 
to the cyl head temp sensor resistance when the engine is idling. This 
richens the idle mixture.

My theory is that this is TOO rich when the engine is cold, which leads to 
poor idling when cold, like what you've described and I've also experienced. 
However, it remains to be seen whether our '73 d-jet brains have the same 
circuitry.

This is all consistent with my observations that the stumbling idle goes away 
as soon as the throttle is opened a tiny bit, and that the engine runs much 
better, and runs much longer with the pump shut off, with the throttle slightly 
open.

Fortunately, there's a simple test for this. Open the engine cover so you're 
all ready to go, make sure your IAD temp sensor is plugged in, then start the 
engine and note the poor idle. Then go back to the engine and unplug the 
throttle valve switch. Does the idle clear up? Does the idle go bad again 
when you plug the TVS connector back on?

If so, I think we've found the problem.

There are several possible solutions.

1) You could leave the TVS disconnected. That would cure this problem, but 
also disable the FI shutoff during overrun, and also disable the "throttle 
pump" and full throttle functions of the TVS.

2) You could disconnect the idle connection wire from the connector. That 
would fix the problem and also disable the FI shutoff during overrun but 
leave the "throttle pump" and full throttle functions working. (Late FI Type 3s 
were delivered in some markets in this condition. The wire was there, but 
just folded back under the boot in the wiring harness.)

3) You could open the brain and reduce the resistance of R471 to the point 
where it no longer caused a problem. That's the approach I would take, but it 
would take some experimentation to determine a good resistance value.

It's still too cold here for me to want to do much outside testing, but if you 
can do this test and let me know the result, that would be a big help. I could 
go ahead and open up a late 311 E brain and try to find that resistor. Once I 
find that resistor I can check to see if it still has its correct resistance or has 
it drifted up (unlikely.) Then I can get ready to try different resistors in place 
of that one.

It will be interesting to find out if this "feature" is part of our d-jet systems and 
if it is even in brains earlier than the '73, late 311 E brain.

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