[T3] ECU's was Two Tests

Jim Adney jadney at vwtype3.org
Sat Sep 21 10:19:19 PDT 2019

On 21 Sep 2019 at 16:57, bobsnotch at aol.com wrote:

> I've had a couple go bad thru the #19 wire though. Just didn't know if
> the IAD sensor could be a problem in these. I know the "C" MPS units
> have an issue with the diaphragm plate cracking, as I've had a few of
> those over the years. 

Yes, we get quite a few complaints about wire 19 and the fuel pump relay, 
but most of those end up being wiring problems. A few come down to that 
transistor in the brain that controls wire 19, but I just looked at the schematic 
from Rennlist last night, and I see that Bosch did a really clever thing with 
that circuit, which should keep that transistor from blowing even if it is fed 
from a direct 12 V source. I don't know if the early years got that clever 

It's also interesting that those transistor leads are plated iron, so they can 
rust away. [Not Bosch's fault, that's pretty standard practice.] That was the 
problem on the one I fixed. Bosch used a sponge pad under that transistor 
for some period. That sponge tended to absorbe moisture and promote 
corrosion. Outside that time, they used a clear plastic 3-legged standoff, 
which caused no problems. That was a much better solution, but at some 
point they added a heat sink to the top of that transistor and they must have 
thought that they needed something thinner underneath, to keep the heat 
sink from sticking up too far and shorting out to the enclosure.

The cracking diaphram in the '70-1 pressure sensors is a known longevity 
problem. That's Bosch's fault, but we need to keep in mind that they still 
lasted over 40 years, which is much longer than anyone expected those cars 
to live.

It's worth noting that the aneroid, the expanding bellows that all the pressure 
sensors have, sees the same kinds of stresses, but they almost never fail. 
I've never seen a bad one. I'm guessing that this was something that Bosch 
purchased from a maker that had been doing these for decades, and 
understood the problem in depth. The diaphrams and the aneroids are 
slightly different colors, so they must be different copper alloys. The material  
that the aneroid is made from is clearly the better choice. I'm guessing 
Beryllium Copper.

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

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