I have to wonder how true this is. I have used early ECUs in some later
models without a problem. Since my spare ECUs that are wired for the
Digitool are both early types, I use them when troubleshooting. I may
have only used them in up to 89s though.

The 2 added wires appear to me to be simply for the ECU Malfunction
light that VW briefly added to some 88/89 models. 90/91 did away with
the light but the wires are still in the harness.

97.123  shows ECU pin 20 wired to dash and connected to an
ECS-Malfunction light and switch (88/89)

97.205  shows ECU pin 20 still wired up to dash area but not connected
to anything anymore (90+)

97.105 shows ECU pin 20 not used  (87) , same for both 86 wiring versions

Pin 23 is also used in (88/89) and (90+). It connects a secondary power
source to the ECU when the key is on even if the ECU power relay is not
functioning. I presume this is to allow for the ECU to detect missing
main power and allow error codes to still be displayed, such as a code
for "bad ECU relay power". A change was made to the relay and it's
wiring to accomplish this.

As far as I know, no error codes were actually implemented in our
Vanagons. OBD was coming and VW seemed to be getting ready for it's
implementation.

Mark

 Eric Zeno wrote:

> YES.      There are several differences.  IF you look at the wiring,
> alone
> you will see that there is , IIRC,  2 added wires  to the ECU.   An older
> sat, 1986 ECU will not run a  1990  model.
>                                  Eric