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