Please note this is not my write up! I have shared it from the T4 Forum.
Part of the key ignition switch goes to relay 109 “engine control module power supply relay”, 109 “coil” side goes to ground Via the ECU. 109 “switch” side goes ECU fuse and then into the ECU. SO if 109 has no earth it can’t “switch on” power to the ECU so it’s not doing it’s “thing” when engine is cranked over = no start. TDI None Start.
A multi meter and spare wire/cable will be very handy!
This is what I did ON A T4 65KW 2.4 TDI AJT engine.
Check fuse’s inside the van, below right of the steering wheel and fuses inside the engine bay normally in front of battery. (In a black cover/box) if they all ok.
Find relay 109 (2nd row up 3rd one in from left) remove it and test Voltage at slot 4, slots should be numbered. (Terminal 30 slots into 4) It should read 12V. If not it’s power supply to the relay. (can’t help you on this one!)
If 12V there, take the removed relay 109, there should be a little circuit diagram on the side, get a small length of wire and join it to terminal 30 then terminal 87, so you got a small loop, basically you bypass the switch in the relay. Put the relay back in the fuse block/box.
Put in key and see if the glow plug light comes on and try starting.
IF it works it be relay 109 OR relay (fuse) box OR wiring.
If not it’s fuse box/ wiring to the ECU fuse. (Goto REF A for further T/S)
Simple test remove wire loop from relay, connect one end of wire to terminal 85 of 109 relay. the other end of wire put to ground (so long bit of wire be good) you should hear a click, turn key and see if glow plug light is on and start van. It still no go, looks like the relay is U/S (you could take cover off relay and hook it up to 12V to double check).
If the van started, it’s the earth side of the relay = fuse box/wires
Quick check the wires/plug on the back of the fuse box. You looking for plug G1 it’s white (middle one of 3 white plugs, bottom row) THIS MIGHT NOT BE YOUR YEAR/MODEL but give you a idea. It could be a loose wire, look for a black and white wire.
If looks all ok, remove the white plug, best to unhook the side clips on the fuse box and let it hang, there is a little lever on the right side in-between the two row of plugs slide/pull it to the side. (Release’s the lock on the plugs)
VW TDI None Start.
Now we check the continuity of the circuit inside the fuse box. Remove relay 109, put one wire of the multi-meter in relay hole #5 (relay pin85) to pin 7 in terminal G7. (Make sure you on right pin)
If all ok it’s not fuse box. TDI None Start.
Next step is to check the continuity of the wire (black & white) going from Plug G1, pin 7 to pin18 T121 which is the ECU. Bigger of the two connectors into the ECU (in front of Battery, low down)
I got access to back of the plug to see the wires and found which pin the black and white wire goes into. So if there is continuity looking like you ECU and you need to get a 2nd opinion!
REF A, continuity test, relay hole #8 (relay pin87) to pin 10 in terminal G7, to check fuse box. If ok check wire from plug G1 pin 10 to ECU T121 pin 1 and 2 (via ECU fuse) Black and Yellow.
If the Black and White wire failed the test, it is the wire, it is a long wire loam that comes in the engine bay drivers side then over the front/top of engine etc to the ECU. Just below the brake master cylinder is a common place for a break. Remove wires from the cable wrap/protector and fine the black and white wire and trace/insp it.