mk 4 golf no crank.

[nevo]

Having a spot of bother with the wifes mk4 tdi agr,battery has been on the edge for some time starts but a bit sluggish,anyway changed over to another one,connects up,come to start,nothing,some head scratching decided to put the other one back in again nothing.Put a jumper to the solenoid,starter turns ok,plugged in vcds and I'm getting no communication,my question is, is there a fuse or ing relay in the starting system or ecu relay,and nope I haven't connected up the wrong way.

[nevo]

Just an update if I run a jumper to the starter solenoid it will start and run so I'm wondering if the ignition switch is burn out,there's no voltage at the connector when turning the key to start.

[nevo]

Just an update,got the ignition switch out jumped the 50b ,it cranked so opened up the switch itself looked all ok nice bright contacts put it back together tried it again and hey presto it cranked on the key but no start,so scanned it and 1 fault for the 109 relay took that out opened it up all ok,back together plugged in,cleared the fault,car starts ok,all this just changing the battery,vw electrical gremlins,so my question is does the 109 relay power anything else besides the pump solenoid.just another aside the no communication with vcds was the usb connection on my laptop,another thing that got me head scratching.

[turboextreme]

Relay 109 supplies power to
- Glow plug relay
- Fuse 32
ECM
Fuel quantity adjuster
- Fuse 34
Cold start injector
Waste gate bypass regulator valve (N75)
Change over valve (N239)
EGR vacuum regulator valve (N18)
Mass air flow sensor (G70)
- Fuse 43
Clutch vacuum vent valve (if equipped)
Brake vacuum vent valve
Relays for preheating coolant (if equipped)
Crankcase ventilation heating element

The ECM supplies power (and sometime ground as well) for:
Fuel cutoff valve
Cruise control switch
Manifold (intake) air pressure/Intake air temperature sensor
Needle lift sensor
Engine speed sensor
Engine coolant sensor
Closed throttle position switch (part of "go" pedal)
Kickdown switch (part of "go" pedal)
Throttle position switch (part of "go" pedal)
Fuel temperature sensor
Modulating piston displacement sensor

The ECM also communicates to the instrument cluster electronics and transmission control module (on automatics).

As a result of this, you can see why if the relay starts to cause power to flicker, or voltage drops, for all of these components, the results can be "unpredicatable" and none of them desireable.

[nevo]

Thanks for that turboextreme,as I stated above I traced it to the ignition switch giving no power to the solenoid,while I was in there I pulled the 109 out and back in and I think putting it back in it wasn't making a good contact,which threw me for a bit.