Electrical Issue - Cranking but not starting

[schwartza3]

Hi guys,

Hope someone can point me in the right direction.
I've done a few searches but didn't find anything which seemed similar to my issue.

Issue:
My A3 (AGU) is cranking but not starting 95% of the time
When I put the key into start position before firing I've noticed the climate control in the center console doesn't light up, air bag light stays on, parking light is dim, no noise from TB or fuel pump.
Also No connection to ECU through Vag Com when this happens.

The 5% of the time when it does fire, the climate control in the center console does lights up, I'm able to connect to ECU, no fault codes.
While it's on I've checked the Measuring Blocks and all seems fine.

MAF sensor reading ok
Crank sensor is reading, I've read this can cause issues with starting but there is definitely an RPM signal
Coolant temp sensor - ok (slowly rising temp as car idles)
Battery is pretty much new (2 months), also seems like the alternator is working well - read around 14V while it's on.
Starter motor is fine, the car turns on without struggle

I'm thinking it might be one of the following
1. There might be something up with the ignition switch?
2. Power to ECU possibly?
Where is the power supply Relay for the ECU on our cars? (98 AGU 1.8t)
Some 1.8t's have a few relays at the back of the engine bay (relay J271 (100) for ECU power supply), all i have here is the fuel injection barometric pressure sensor.

If anyone has any ideas what I should check please let me know

Cheers!

 

[Stuart B]

What are the fuses and circuit like on the top of the battery?

 

[antwan64og]

My AGU does it randomly, just refuses to start. If I leave it a couple of minutes then it starts.

Also every few weeks I have the joy of it randomly cutting out while cruising on the motorway, it restarts itself within a split second and the trip counter resets.

Electrical gremlins in a 20yr old car I’m afraid.
I suspect I have a dodgy earth somewhere

 

[schwartza3]

The fuses/circuit look ok, here is a quick snap.

 

[schwartza3]

My AGU does it randomly, just refuses to start. If I leave it a couple of minutes then it starts.

Also every few weeks I have the joy of it randomly cutting out while cruising on the motorway, it restarts itself within a split second and the trip counter resets.

Electrical gremlins in a 20yr old car I’m afraid.
I suspect I have a dodgy earth somewhere

Might go all the earth points, anyone know how many there are/locations?

 

[Stuart B]

weird you have a normal 30amp in the fan fuse?

 

[schwartza3]

weird you have a normal 30amp in the fan fuse?

it's been there last 14yrs haha, no issues

I've ordered an ignition switch (the one at the end of the barrel), hoping this may be the problem.
Steering wheel has to come off to get it out.

 

[schwartza3]

Hi all,
Just to let you know what the issue was in the end in case anyone comes across a similar problem.
It was the ignition switch as I suspected. It is positioned at the end of the ignition barrel.
To get to it the steering wheel needs to come off and the plastic around the steering column need to be disassembled

Basically over time the different contact points inside the switch wear out so when you rotate the key they don't make a good connection.
This explains why my car was cranking but not starting as the coil packs wouldn't have fired, why I couldn't connect to the ECU, etc