[T3] ECU's was Two Tests

bobsnotch at aol.com bobsnotch at aol.com
Sat Sep 21 09:57:32 PDT 2019

Ok, just checking. I was also going from memory and we know how bad of an 
idea that can be at times. 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.
Glad to know Dan and Neena got their issues worked out on their car. I even 
sent them an ECU and an A or B MPS unit, since I had no use for it.

-----Original Message-----
From: Jim Adney <jadney at vwtype3.org>
To: type3 <type3 at vwtype3.org>
Sent: Fri, Sep 20, 2019 4:38 pm
Subject: Re: [T3] ECU's was Two Tests

On 20 Sep 2019 at 17:02, bobsnotch at aol.com wrote:

> I always thought you kept telling us that IF the FI worked, it's not an ECU 
> problem. However, we've learned over the years, the the #19 ground circuit 
> can die in them. So is this another possible place that they can go kaput?

I was able to repair one ECU where one lead of the transistor that ran #19 
and rusted away. That came from Alabama, and it looked like high humidity, 
and a poor choice of transistor spacer got to it.

What I've always tried to say was that the ECUs have been extremely 
reliable. OTOH, I've probably been too forceful on this at times. I have 
personally never encountered a bad one on a car I was working on, but I 
have several which are bad, that came from other people. The problem with 
those is that I don't have any way to know if they died on their own or died 
due to some sort of accidental abuse by the owner or a previous mechanic, 
or even if they were actually bad at all before someone tore into them.

I'm sure there are a number of ways one could kill an ECU, but that's 
different from one dying on its own.

I always keep in mind Daniel's experience: He accidentally miswired his FI 
trigger point connector, and then started noticing that his engine wasn't 
running right. He got a number of loaner B brains, some of which worked 
and some didn't (with his car) Even though he'd been assured that they were 
all good. He and Jessican traveled across the US and back in that car, 
finally stopping here in Wisconsin, so we put this system on my Bosch 

When we got to the point of testing the trigger points, I was turning the key 
while Daniel watched the tester. There was a long pause from the rear of the 
car, but finally Daniel said, "They both move, but they move differently." So 
we switched places.

I don't recall what the meter showed, but it was something I'd never seen 
before and it wasn't right. I think the next thing I did was get a piece of wire 
to take the place of the trigger points, and that didn't test right either. It was 
then that we pulled back the rubber boot and looked at the numbers on the 
wires and compared them with the wiring diagram. We realized that Daniel 
had mixed up wire 21 and 12, which is really easy to do when the numbers 
are upside down.  

After correcting the wiring, the tester registered right, the engine ran better,

AND: Now ALL the B brains Daniel had borrowed worked perfectly!

I don't know why some worked before and some didn't. Perhaps a revised 
trigger circuit, but most likely just slightly different tolerances.

So, the moral to the story is: "Even brains which seem bad, may be perfectly 

And that's why I remain reluctant to blame the ECUs. The same goes for the 
pressure sensors. People tend to jump on both of them when they get 
confused, just because they're a black box that they don't understand.

The problems are almost always elsewhere. Not always, but almost always.

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

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