Author Topic: 8V with interesting fault, diag discussion  (Read 2154 times)

Offline jonno74

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8V with interesting fault, diag discussion
« on: 06 September 2011, 18:45 »
Hi, I have a 1995 GTI 8v, with an interesting fault....
The OBD is throwing up a very occasional immobiliser fault whilst driving, which cuts the engine. I have initially traced it to either the ECU, or the immobiliser unit. All wiring and relay operation is fine - however, as an experiment I have noticed that when the engine is running and I disconnect the immobiliser unit (either power supply plug or key/coil sensor plug),there is no affect, the engine keeps running - only when you turn off and try to re-start will it recognise there is a problem; this leads me to think the immobiliser is doing its job, sending the correct signal to the ECU on start up, and for some reason the ECU is developing a fault which it resets/interprets as an immobiliser problem, and then forces shut-down.
Does any one have experience of the protocol between the two units i.e. the type of signal/timing that the ECU is looking for from the immobiliser unit? Any findings from a similar investigation would be appreciated,
Alternatively, has anyone got a cheap siemens ECU, or one I can borrow to confirm or eliminate the ECU? I am located in Portsmouth area 

I'm reluctant to just swap out the ECU/immobiliser/coil/chip/key for the sake of it without understanding the cause of the problem!
Otherwise it could be a cheap project for someone, would prefer the space for one with more valves and less doors!

Thanks

Offline gasketface

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Re: 8V with interesting fault, diag discussion
« Reply #1 on: 06 September 2011, 19:14 »
Relay number 30 is on it's way out  :laugh:


Offline jonno74

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Re: 8V with interesting fault, diag discussion
« Reply #3 on: 06 September 2011, 19:28 »
thanks for the info! much appreciated ... so in your case, the ECU relay (30) was causing the system to report an immobiliser fault on the OBD scan?

Offline gasketface

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Re: 8V with interesting fault, diag discussion
« Reply #4 on: 06 September 2011, 20:20 »
Yes it was,guy i bought car off threw money at mechanics who tried everything but car got steadily worse up to a point where it would just fire and die.

If you look at the last link in the 3 i gave you can see what i mean.

It could obvioulsly be other things but the symptoms you give sound the same as mine.It's your cheapest option,new relay was under £17 from VW dealer.If you are close to a breakers,even cheaper !.

Offline jonno74

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Re: 8V with interesting fault, diag discussion
« Reply #5 on: 06 September 2011, 20:48 »
ok thanks, yes I've read your thread, great info, i'll give it a try! I did try bypassing the main switch contacts of the relay, with no change to the fault, but without a detailed schematic i don't know the interaction of the other pins (86,86a,85). 

Ordered new relay from euro car parts , £11 free delivery, will let you know how it goes! 

Offline jonno74

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Re: 8V with interesting fault, diag discussion
« Reply #6 on: 09 September 2011, 18:23 »
 Its not the relay! pinned it down to the ECU, it appears to have an internal fault, which with a bit of encouragement can be induced at will (i.e a big thump)! .

Just need to find one before Cadwell Park on Tuesday!! 

has anyone got a 037 906 025 H ECU available in the Hampshire area?

cheers,

Jon

Offline gasketface

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Re: 8V with interesting fault, diag discussion
« Reply #7 on: 09 September 2011, 18:31 »
You will need matching key as well with the chip in it.

Offline jonno74

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Re: 8V with interesting fault, diag discussion
« Reply #8 on: 09 September 2011, 18:55 »
hi, I read that the the immobiliser box is coded to the key, not the ecu?