[T3] ECU question .

[Jim Adney]

On 8 Jul 2016 at 14:58, William J wrote:

> this other ECU does have the EGR thermo switch on it and the bracket . 
> Wouldn't this make it a 73 or 72 late ECU?

Yes, it could be either. Some '72s came with EGR; I used to have one. 
Those were all the X prefix engines.   

> I don't understand why late units can replace early but not the other
> way around. 

There's a certain amount of speculation in this, but here's my 
thinking:

If late ones were not a replacement for early ones, they would have 
given them a different part suffix. So the new E parts must be 
suitable replacements for all cars that came with E parts. So late 
can replace early.

That's the easy part.

There were 3 changes going from '72 to '73: distributor, pressure 
sensor, and brain. VW clearly viewed these as a new "set" of parts 
that would work well together and meet the '73 EPA emissions 
standards. If the earlier E part would work just as well in the later 
cars, there would have been no reason to make any changes in it, but 
we know that it was changed, since it got a different Bosch part #.

To me, this means that VW felt that the early parts were not suitable 
for use in the later cars. BUT, what we don't know was how many of 
the early parts were still in VW dealership and distributor parts 
inventory when the design changed. If VW recalled all those early 
parts and replaced them with later parts, that makes their intensions 
clear. But if they let the old parts mingle with the new ones it 
means that they didn't think it was important enough to make a 
difference.  

Note that it's also possible that the later parts were introduced 
into the VW parts distribution system sometime within the '72 model 
year, so that all the early inventory could be gone by the time the 
'73 cars came out. This would have been perfectly legal and would 
have assured that dealer repaired '73s and '72s would continue to 
meet their EPA requirements. If VW was smart, that's what they would 
have done, because they clearly knew what the '73s were going to come 
with long before those cars hit the showroom.

It's may also be important to note that in the Bosch parts 
literature, where the VW part numbers were not used, they continued 
to call out different Bosch parts for '72 and '73 at least for the 
next 20 years, which is as late as my Bosch catalogs go.

Finally, the Bosch test booklet, which has the instructions for 
testing the FI using the Bosch D-jet tester, has listings for 
different brains what can be substituted for each other. This lists 
the B as a suitable replacement for the A and the D as a suitable 
replacement for the C (if the temp switch is replaced), but there are 
no substitutions listed for either E brain.

In general, I come down on the side of prefering to see factory 
compatable parts used together. Certainly there are other 
combinations that you can get away with, but there's no way to know 
what the consequences might be.

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