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S40 T5 won't start - Ignition switch?

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Old Aug 23rd, 2018, 19:04   #11
FunkyMelon
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The SCU is just a middle man receiver that passes the code along to the next module for starting as far as I'm aware.
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Old Aug 24th, 2018, 10:23   #12
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For the benefit of future readers (and me) what's an SCU ?
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Old Aug 24th, 2018, 10:29   #13
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Originally Posted by Welton View Post
For the benefit of future readers (and me) what's an SCU ?
the ignition switch ..
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Old Aug 24th, 2018, 10:35   #14
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Quote:
Originally Posted by FunkyMelon View Post
The SCU is just a middle man receiver that passes the code along to the next module for starting as far as I'm aware.
Yep, you are right, I got confused with SCL I think (that module for locking the steering wheel). SCU just powers up RFID chip in key, reads it and sends "OK to fire it up" to CEM.

Oddly though, I would have thought you wont be able to start the car if SCU is dead (its basically like trying to start it without the key, no?), but you managed to. Maybe it means that SCU is somehow working (like detecting the key, disabling immobiliser) but just doesnt send up POS3 starter command ?
dunno, just thinking out loud...
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Old Aug 24th, 2018, 10:43   #15
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The ignition switch on mine sometimes doesn't 'engage' and I'll get a warning message about the key and 'try again' or something.

The dirty fix for me has always been a confident 'tap' on the back of the key!
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Old Aug 24th, 2018, 10:43   #16
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Originally Posted by LizardOfBodom View Post
Yep, you are right, I got confused with SCL I think (that module for locking the steering wheel). SCU just powers up RFID chip in key, reads it and sends "OK to fire it up" to CEM.

Oddly though, I would have thought you wont be able to start the car if SCU is dead (its basically like trying to start it without the key, no?), but you managed to. Maybe it means that SCU is somehow working (like detecting the key, disabling immobiliser) but just doesnt send up POS3 starter command ?
dunno, just thinking out loud...
the scu ( ignition switch) has sets of contacts just like an old fashioned one , but they don't carry current just send signals to the CEM , The scu also has the antenna ring for the immobiliser chip in the key .
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Old Aug 24th, 2018, 10:48   #17
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The ignition switch on mine sometimes doesn't 'engage' and I'll get a warning message about the key and 'try again' or something.

The dirty fix for me has always been a confident 'tap' on the back of the key!
The car is trying to tell you something , if you don't get it fixed you are going to be marooned in the most isolated place .. guaranteed !
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Old Aug 24th, 2018, 11:05   #18
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Quote:
Originally Posted by LizardOfBodom View Post
Yep, you are right, I got confused with SCL I think (that module for locking the steering wheel). SCU just powers up RFID chip in key, reads it and sends "OK to fire it up" to CEM.

Oddly though, I would have thought you wont be able to start the car if SCU is dead (its basically like trying to start it without the key, no?), but you managed to. Maybe it means that SCU is somehow working (like detecting the key, disabling immobiliser) but just doesnt send up POS3 starter command ?
dunno, just thinking out loud...
I've not had a look yet but my assumption is each of the ignition positions sends a different signal to the CEM to work from. My theory is it sends 4 different signals...

1st - Key is inserted, fob/car codes checked amongst all modules, CEM talks to SCL and unlocks the wheel
Ignition Pos I - Basic power to car, radio etc
Ignition Pos II - Power to rest of car, lights, rest of console/modules etc
Ignition Pos III - Sends signal to CEM to activate switched power line to relay/starter circuitry to fire engine.

I kinda confirmed this in VIDA before dropping it off, VIDA shows you ignition positions, I and II were recognised, I got nothing on III.....

I've asked Volvo to put my old SCU in the boot so I'll strip it apart when I get the car back and find out how it actually works.

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Originally Posted by Welton View Post
The ignition switch on mine sometimes doesn't 'engage' and I'll get a warning message about the key and 'try again' or something.

The dirty fix for me has always been a confident 'tap' on the back of the key!
That's definitely a sign of yours going. Get it replaced ASAP as you'll end up like me one day where it just won't start. I was lucky I was parked up at work, not somewhere far away and unknown.

You could do as I did above to get you going and short those pins out but be very confident you know what your shorting as you could potentially take out your CEM if you get it wrong.
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Last edited by FunkyMelon; Aug 24th, 2018 at 11:16. Reason: Typo.
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Old Aug 24th, 2018, 11:11   #19
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The key thing has only happened maybe 2 or 3 times in the last 9 years, I guess if it does it again that's when i'll be motivated to replace.

This is a good source of info in any case.
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Old Aug 24th, 2018, 11:29   #20
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Quote:
Originally Posted by Clan View Post
the scu ( ignition switch) has sets of contacts just like an old fashioned one , but they don't carry current just send signals to the CEM , The scu also has the antenna ring for the immobiliser chip in the key .
Ok, so to summarise it up , let me know if thats correct:
SCU can go bad both ways:
- mechanically, where for example contats degrade and no longer recognise key position
- electronically, where components goes haywire and for example antennae no longer sends/receive signal or such signal is no longer passed from SCU to CEM.

In the 1st scenario, like FunkyMelon, you can hardwire the car as everything works OK bar the actual "START" position
In the 2nd scenario, you would not be able to even disengage SCL nor start the car as CEM would still be in "immobilised" mode.

Dose that seem right?
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