T18 is the connector close to the aux-12v post for emergency starting. It connects several functions to the wire loom, and sometimes gets brittle or connections get oxidized.

Here's an overview of the different layouts that were used over several years..

 

1979-1984 (?)
=========
1 solid blue. goes to the Alternators B+ terminal
2 blue/blk. Oil pressure sender unit
3 blue/yellow. coolant temp sender
4 blue white. oil pressure sender unit
5 blue/green. coolant temp sender
6 blue/brn. engine oil switch
7 2 red/whites'. WUR and Sup air valve
8 unused
9 black. AC and AC pressure switch
10 blk/yellow dot. Starter to resistor
11 unused
12 unused
13 unused
14 2 yellows. Large yellow-starter to fuse block; small yellow- to cold start valve
 


1987/1988
========
T18 belegung: Ich geh mal davon aus das dein Auto ein 1987/1988 ist. (1989 ist ein wenig anders belegt). Der T18 hat Verbindung für Ansaugbrücke Sensor (10,13), Klima Kupplung (9), Starter (14), Öl Lvl Sensor (6), Kühlwasser Sensor (5,3), Öldruck Sensor (4,2), Generator Feld-Strom (1),

1 = blue (blau) Alternators B+ terminal (Starter Feld-strom)
2 = blue/black (blau/schwarz) Oil pressure sender unit (Öldruck Sensor)
3 = blue/yellow (blau/gelb) coolant temp sender (Kühlwasser Sensor)
4 = blue/white (blau.weiss) oil pressure sender unit (Öldruck Sensor)
5 - blue/green (blau/grün) coolant temp sender (Kühlwasser Sensor)
6 = green/blue (grün/blau) engine oil level switch (Öl Lvl Sensor)
7 = open
8 = open
9 = black (schwarz) AC compressur clutch (Klima Kupplung)

10 = green (grün) Intake temperature switch (Ausaugbrücke Temp schalter)
11 = open
12 = open
13 = brown (braun) Intake temperature switch (Ausaugbrücke Temp schalter)
14 = yellow (gelb) Starter (Anlasser / Starter)
 

 

1990 model:

1= blue. E21 to Alternator Terminal B+
2= blue/black. Oil pressure sensor Terminal WK to E12
3= blue/yellow. coolant temp sender to E14
4= blue/white. oil pressure sensor Terminal G to E22
5= blue/green to coolant temp sender, brown/green to E15
6= green/blue. engine oil level switch to E13
7= brown/green. oil pressure sensor Terminal M to E15
8= unused
9= black. AC pressure switch to compressor clutch
10= green to intake temp sensor, brown/blue to fan control unit
11= unused
12= unused
13= brown. intake temp sensor to ground @ MP II
14= yellow. C12 to starter solenoid



1991
=====
T18 - 14 pins:
#, Wire color, wire size (notes)

1 - Blue 0.5
2 - Blue/Black 0.5
3 - Blue/Yellow 0.5
4 - Blue/White 0.5
5 - Brown/Green 0.5
6 - Green/Blue 0.5
7 - Brown/Green 0.5 (Brown/Red on other side)
8 - NC
9 - Black 1.0
10 - Green 1.5
11 - NC
12 - NC
13 - Brown 1.5
14 - Yellow 4.0