My 1989 SL300 Auto is undriveable. The old style cobra radio remote key does not unlock the doors (green button), nor does it operate the immobiliser. Nor does the yellow button operate the panic alarm. The red LED on the dashboard is permanently off. When I unlock the car door with the mechanical key, the intruder alarm does not operate. When I insert the mechanical key into the ignition and turn it on, there is nothing. My whole car is dead. Under the bonnet/hood, I have turned the key on the siren to the off position, but still no joy (this would only de-activate the siren, i believe).

Is it possible that either a) the battery back-up to the siren/immobiliser is dead? or b) both radio keys are faulty (both have new batteries)? I don't really want to spend £500 on a new alarm system.

SL300 1989 Automatic, 140,000 miles (and running like a dream), Old style cobra alarm (Cat 1) 4-S-5320-STC: link to the black with green and yellow button radio key style: http://www.drivenincar.co.uk/shop/products.php?g1=ca379f

Not sure about the alarm at all. Is the car battery possibly dead? You're saying the engine won't even crank, correct? Is anything electrical working?
Gilly

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A bit of history of when and how this first happened might help, it may just be that the main fuse for the alarm unit has blown.
Do you know where the alarm control unit is fitted ,if it's a Cobra it's not an original fitment and could be anywhere, they are fitted in a location where tools are required to gain access for security reasons.
You are right about the key in the siren ,this only de-activates the siren.
Really your best bet is to get a car alarm fitter to have a look as they know the usual locations on all cars, or your local car thief .
Geo.

Seems to me that if the whole car is dead, that it might make sense to focus on the whole car, not isolate the immobilizer system. If the dome lights for example do not work, maybe, just maybe the battery is dead? If the dome lights do come on, but nothing happens when turning the key, these early SL's have that "cassette" style ignition switch which fails so much, so maybe the ignition switch is totally gone. Usually you have some contacts which will operate if the switch fails, but maybe the contact is bad that brings power into the switch, resulting in a total failure.
Gilly

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When the battery in our 1998 E320 failed recently the dome light came on, but the starter motor wouldn't even click. A new battery solved the problem ... after 4 years +/- they're usually shot.

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Fred Hoelzle

Thank you for the replies.

History: I have been having trouble activating and de-activating the cobra alarm/immobiliser system for the past 2 months. I thought it was faulty radio keys, so i bought a new key battery for both of them. Still problems and I just guessed it may be a faulty switch/contact within the radio keys (they seem old). Reason: the green button (immobiliser/central locking) only worked 50% of the time, but the yellow panic button seemed to work 100% of the time. So, I go to Holland, Italy and then France with work for 2 weeks, leaving my car parked outside at home in London. When i return, I try to gain access to the car, but no joy. Now, the yellow panic button doesn't work. When I open the door with the manual key, the alarm should trigger (motion sensors). I believe my alarm system has it's own battery back-up: perhaps this has failed? The way my immobiliser works, is that when it is immobilised, nothing should come on when i turn the key, no dome lights, nothing. However, what is also odd, is that the red LED on the dashboard is off (when the car is immobilised, the red LED should be lit on). So, I'm guessing it is the alarm system battery that may be dead.

Sorry for the long post! I'll call a local alarm fitter and get them to figure it out for me, though any other helpful suggestions would be greatly appreciated.

Thanks,

Phil

Like was first suggested, perhaps you should pull the battery and take it somewhere and have it tested?

OR, hook up a charger and see if your lil red light comes back on?

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

To clarify, I think what Terry and myself are suggesting is to test the CAR battery, not the immobilizer battery, or can you simply try jump-starting the car?
Gilly

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It was, indeed, the car battery that was dead.

I got a jump and went for one final drive with the soft top down before putting the hard top on for winter.

Thanks for all the replies and help!

Phil