(note: this info is all on one page on purpose)

The following is an outline of the entire engine side wiring harness for an 88 325is (Motronic 1.3 - Ecu #173) and its interface with a 1979 Euro 323i (other E21's are probably similar)

E21 323i Engine Plug

Engine plug:
E21 Plug #	E21 Wire color	Motronic 1.3 Wire color	Purpose
1	Brown/Green	Brown/Green	Oil pressure switch to instrument cluster
3	Green/Black	Green/Violet & Black/Green	Power to Fuel pump from Fuel pump relay and starter
5	Blue	Blue	Alternator Charge (D+) to instrument cluster
6	Green/Violet	Green	Coil 15+ / O2 Sensor Heater Relay
8	Green	Green	ignition switch to Motronic #27
10	Black	Yellow/Black	ignition switch (start) to starter
13	Brown/White	Brown/Violet	from injector rail (temp sensor) to instrument cluster
15	Black	Black	from Motronic #6 to tachometer

Motronic ECU Pinout:

Pin #	Color	Purpose/location	Elsewhere?	details
1	Black	Ignition Coil (timing -)	Yes	Lead to diagnostic connector
2	Brown	Ground	Yes	Lead to diagnostic connector
3	Brown/Green	Fuel Pump Relay (85)	No
4	White/Yellow	Idle Control Valve #3	No
5	Brown	Purge Canister (trigger -)	No
6	Black	Tachometer (engine speed out)	Yes
7	Grey/Yellow	Air Mass Sensor #2	No
8	Black	Cylinder ID sensor (spark #6 lead)	No
10	Brown	Oxygen Sensor (ground)	No
12	Grey/White	Air Mass Sensor #3	No
13	White/Yellow	Diagnostic - RxD (Motronic receive data line)	No	Lead to diagnostic connector
14	Brown	Ground	Yes	Lead to diagnostic connector
15	Grey	Check Engine Light (- triggered)	No
16	Brown/White	Injectors 1, 3, 5 (- triggered)	No	+ lead is red/white
17	Brown/Yellow	Injectors 2, 4, 6 (- triggered)	No	+ lead is red/white
18	Red	Power (direct battery lead +)	Yes
19	Brown/Orange	Ground	Yes
22	White/Green	Idle Control Valve #1	No
23	Brown/Green	Oxygen Sensor Relay (85)	No
24	Brown	Ground	Yes	Lead to diagnostic connector
26	Grey/Blue	Air Mass Sensor #4	No
27	Green	Ignition Switch Lead (run position)	Yes
28	Black	Oxygen Sensor (+ read voltage)	No
29	Black/White	Vehicle Speed input	Yes	from Cluster Speed Output
31	Yellow	Cylinder ID sensor (spark #6 lead)	No
32	White/Black	Present Fuel Rate output (to instrument cluster)	Yes	Lead to diagnostic connector
36	Brown	Main Relay (85)	No
37	Red/Blue	Main Relay (87 #1)	Yes	Same as red/white
39	Green/Blue	Programming Voltage	No	Lead to diagnostic connector
40	Black/Grey	A/C Compressor ON to Motronic	No	A/C system
41	Violet/Grey	A/C System ON to Motronic	No	A/C system
42	Red/Blue	Park/Neutral switch to Motronic	No	Auto Transmission only
44	Grey/Violet	Air Mass Sensor #1	No
45	Brown/Red	Coolant Temp sensor (not cluster)	No
47	Black	Pulse (engine speed)/Crank Ref Sensor (+)	No
48	Yellow	Pulse (engine speed)/Crank Ref Sensor (-)	No
52	Brown/Blue	Throttle Position Sensor #1	No
53	Brown/Black	Throttle Position Sensor #3	No
55	White/Violet	Diagnostic - TxD (Motronic transmit data line)	No	Lead to diagnostic connector

Connector mappings:

Plug Pin	Color	Purpose	Motronic 1.3 Pin
Injector rail connector:
1	Brown/Red	Motronic Coolant Temp Sensor	45
2	Brown/Orange	Motronic Coolant Temp Ground	N/A
3	Brown	Cluster Temp Sensor Ground	N/A
4	Brown/Violet	Cluster Temp Sensor Sender	N/A
5	Red/White	Power to Injectors from Main Relay	N/A
6	Brown/White	Trigger, Injectors 1,3,5 (-)	16
7	Brown/Yellow	Trigger, Injectors 2,4,6 (-)	17
Diagnostic Connector:
14	+ Red	Battery + (unfused)	18
15	+ White/Yellow	Diagnostic - RxD (Motronic receive data line)	13
7	White/Green	? Not connected	N/A
6	White/Black	Present Fuel Rate	32
16	Green/White	power to O2 heater relay	N/A
12	Blue	Alternator charge (D+)	N/A
1	Black	Coil Timing (-)	1
11	Black/Yellow	Starter Solenoid	N/A
18	- Green/Blue	Programming Voltage	39
20	- White/Violet	Diagnostic - TxD (Motronic transfer data line)	55
19	- Brown	Ground	N/A / 2, 14, 24, 19
Oxygen sensor plug:
1	Brown	Oxygen Sensor ground lead	10
2	Black	Oxygen Sensor volt lead	28
3	Brown	Ground	N/A / 2, 14, 24, 19
4	Green/Blue	Sensor Heater power (from O2 relay)	N/A
Ignition coil:
1 -	Black	Timing (-)	1
15 +	Green	Power (+)	N/A
Air Flow Meter:
1	Grey/Violet	Air Inlet Temp	44
2	Grey/Yellow	Air Inlet Input	7
3	Grey/White	Air Inlet Degree	12
4	Grey/Blue	Air Inlet Closed	26
Throttle position switch:
1	Brown/Blue	Closed/Idle Switch	52
2	Brown/Orange	Ground	N/A
3	Brown/Black	Wide Open Switch	53
Idle control valve:
1	White/Green	Idle Control Voltage (-)	22
2	Red/White	Power from main relay	N/A
3	White/Yellow	Idle Control Voltage (-)	4
Cylinder Id Sensor:
1	Black	Pulse Input	8
2	Yellow	Pulse Input	31
3	Clear	Shield/Ground	N/A
Crank Pulse sensor:
1	Black	Engine Speed (+)	47
2	Yellow	Engine Speed (-)	48
3	Clear	Shield/Ground	N/A
Oil Level sender:
1 (blue/white)	Blue	Dynamic Oil level (closed when oil low)	N/A
2 (brown)	Brown/Orange	Ground	N/A
3 (blue/violet)	Green/Yellow	Static Oil level (closed when oil full)	N/A

Engine Relays:

Relay diagrams:
Relay Connector	Color	Purpose
Main Relay:
30 (input)	Red	from Battery + (unfused)
85 (- trigger)	Brown	from Motronic #36
86 (+ trigger)	Red	from Battery + (unfused)
87 (output, switched)	Red/White	to injectors, purge solnd, fuel pump relay, idle valve
87 (output, switched)	Red/Blue	to Motronic #37
Fuel Pump Relay:
30	Red	from Battery (unfused)
85	Brown/Green	from Motronic #3
86	Red/White	from Main Relay
87	Green/Violet	to fuel pumps
Oxygen Sensor Relay:
30	Green/White	from ignition switch
85	Brown/Green	from Motronic #23
86	Green/White	from ignition switch
87	Green/Blue	to O2 sensor heater