S500 W140 immobiliser issue

teddykan

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Hi
I'm new to this forum and wish to seek help with my S500 W140.
Its registered Aug 1993 and according to previous owner she served in the UK Qatar embassy. She's been sat outside unused for about 10yr. It was a dud fuel pump. Don't understand the logic of having 2 pumps in series. The dead one prevented the still working one from getting fuel out to the engine. I replumbed to the working pump and got her running. The ride is sublime and she ran ok. Then I played with the infrared key whilst sat inside and pressed the central locking button on the console. The immobiliser suddenly kicked in and shes been immobilised. It's been immobolised for 3weeks now. I've had her battery disconnected for days, ran a lightbulb across her battery leads without battery for days (to try drain computer internal charge), tried the resync procedure as per owner handbook, but she won't crank over. The blinkers flash and the horn blares. Each time upon reconnecting battery and entering the car, either via IR door entry or ignition on, the immobiliser kicks in. No crank.
Ive tried looking for a K40 relay in engine bay but no luck. Of those relays I had a oom all the soldered joints look as good as new. The realys also look spanking and no sign of corrosion that could make them seize. Looked behind passenger footwell steel plate and no luck, although I've spotted a red/yellow wire I can't find a black/purple one, nor a small box these wires go in and out of.
I so wish to defeat this immobiliser as it's such a pain. Any ideas please?
 
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teddykan

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Here are some of my investigations looking for suspect relays. One relay appeared to have been broken into (the casing was cracked and then taped up). All the solder joints look fine to me, with no hint of dryness or overheat burn marks.
Ideas folks?
 

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LostKiwi

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I'm sure you'll understand but given youre asking how to defeat a security device and are a new member we don't know don't be too surprised if you don't get a lot of assistance.
You probably need to get the car to a good Indy/autoelectrian or to MB.
 
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alexanderfoti

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Do you have two diffferent remote fobs or just the one IR? MB's of that era were often fitted with an alarm/immobiliser at the dealer, and will be hidden from view.
 
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teddykan

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Just one key, a remote fob with one flick out keyblade and one button that works the IR send function.
 
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teddykan

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I made enquiries of auto electricians in Glos. Two of them, upon thinking that the immoboliser may be factory-fitted, declined to help (as that's beyond their capability). One electrician ventured out to have a look. He ran a full diagnostic and found the following:
1. a few fault codes all of which were cleared one by one
2. there was only 6 or 7 volts to the engine ECU, when it should've been at least 12V
3. the alarm hazard lights and horn eventually switch off, but will reactivate as soon as car start is attempted
4. one of the boxes (either Remote Central Locking) or alarm box in the rear driver side wing (above battery), when left dangling from the harness, seemed to make alarm circuit work - no hazard lights upon attempt to start car
5. car won't start as, whilst all lights are lit on dash, starter doesn't crankt
6. some electrical contacts suffer corrosion. There may well be some poor contact somewhere in the loom

He concluded that my best bet is to check out the starter.

I wasn't overly happy with the diagnostic, especially item 4, which suggests a broken or intermittent contact wire within a wire loom that looks perfect (also it's in boot where it's benign environment). This is a short section of loom which I may probe into if I can repeat what happened during his battery of tests.

On point 5 I had thought he could then intercept the starter relay and check that - but he didn't.

Guidance and ideas welcome!
 

LostKiwi

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Just noticed the age of your vehicle. In 1992 Mercedes changed their wiring insulation to a new biodegradable plastic which unfortunately biodegrades very quickly when subjected to heat. This usually affects the injector loom and throttle body loom (both sit above the engine) but may affect other looms as well.
Have a look at the wiring where it leaves the connectors for cam timing and temperature sensors. If you can see exposed wires the loom is shot and needs replacing or repairing (Sileck in Kent).
 
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teddykan

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Yes thank you for the warning about the loom. I had caught it on Youtube where a Merc expert explained that the bio-degradable wiring proved to be an Archilles Heel.
However upon a cursory glance at the wiring in the engine bay the loom seemed ok. I'll have another closer look at what you mentioned too.
Regards
 
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teddykan

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Incidentally, a dodgy loom doesn't seem to explain why and how the car ran OK for about 30min before the immoboliser got activated.
 
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image.jpeg I'm having same trouble with my 1996 s320 w140.....immobiliser stuck on...and it's getting on my TITS as no one seems to no the answer or it's a secret ....my car only has 75,000 on it and the mot is due in a month..l don't want to scrap it
 

LostKiwi

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View attachment 49546 I'm having same trouble with my 1996 s320 w140.....immobiliser stuck on...and it's getting on my TITS as no one seems to no the answer or it's a secret ....my car only has 75,000 on it and the mot is due in a month..l don't want to scrap it
The problem at this age is there were many variations of immobiliser. It's nearly impossible to diagnose without hands and eyes on.
 
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Who can sort this..because mercedes ain't interested"being the best car ever built" it's a joke...maybe Mercedes owners should file a law suit!!!l have always been a Mercedes owner and don't wish to change brand and go and by a BMW lol
 

McDonald

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Who can sort this..because mercedes ain't interested"being the best car ever built" it's a joke...maybe Mercedes owners should file a law suit!!!l have always been a Mercedes owner and don't wish to change brand and go and by a BMW lol

Take it to MBS in Southampton. They're active on this forum. They have been very good with my car and offer real value.
 

LostKiwi

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Any decent car security firm or auto electrician should be able to sort it.
 
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teddykan

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I read somewhere on a forum that the W140 key transponder comms with the security device on the car (induction coil around ignition barrel) in not reading the base code, but also shares a rolling code programmed into their memories. Will a mobile auto locksmith be able to read that stuff?

I suspect that when I first had the car running the fob battery was dead hence it didn't send any IR signal to the car to lock or unlock. Once I put new fob batteries in I suspect the commsgot out of sync. The owner's manual explains a resync procedure which I think I'll repeat this weekend.
 
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teddykan

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I asked my friendly garage who is d to be AA patrol and he iad they aren't that well equipped to defeat immobilisers by the roadside. They'll more like recover you toneed a specialist who could.
 

Craiglxviii

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RMK in Birmingham are the people. Replacementmercedeskeys.com
 

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