[Supras] ECU year interchangability

[berniek at technicaldevelop.com]

Hi list.

Anyone on the list into electronics, who has gotten inside turbo engine 
ECUs?  I opened up a couple of spares to try to determine 
interchangeability factors.  Certainly there must be small software 
differences regarding fuel and ignition mapping, but the hardware 
appears to be slightly different between '89 and 90-up years.

I have both an '89  and a 91 5 speed ECU as spares.  The main and 
processor boards are the same, but board stuffing of two diodes and one 
100 ohm resistor differ between the two.  Specifics are as follows:

1:  100 ohm resistor inputting "ECT" (from ECT computer) is not stuffed 
in the '89, so no signal from the transmission ECU is used in the engine 
ECU.  It is stuffed in the '91.  Seems strange that the resistor would 
be stuffed in any event since the ECU is intended for a 5 speed transmission

2:  A 1N4001-like diode is stuffed in the '89 with the cathode connected 
to the "LP" pin, which is connected to the headlight relay.  It is not 
stuffed in the '91.  Presumably it is to increase idle speed slightly 
when the headlights are on, but the '91 omits it. 

3.  The same type of diode is used in the '89 for "DFG", a connection to 
the defogger relay.  It is not stuffed in the '91.  Reason for inclusion 
in the '89 would again appear to increase idle speed when the defogger 
is used.

The '91 I bought last month is not running at the moment, but I did 
start up the '90 with a BHG for a few minutes.  Sure enough, the idle 
speed drops when the lights or particularly the defogger is turned on.  
Has anyone got any ideas about why Toyota deleted those components after 
'89?  Alternator part numbers from the fiche are all the same, all 80A 
except for a 70A version used in some N/A cars.  Any comments about ECU 
interchangeability in general would be appreciated as well.

Thanks.

    BernieK