Urgent Help Required - Cannot disarm Alarm system on W129

[IBN]

Hi Guys

I have a really annoying problem with 300SL (W129) .... I cannot fully turn off the alarm system and I urgently need help /advice.

The car is a 1990 (US Spec) with what I think is an infra red (as opposed to radio) plip .... it has sensors by each of the doors (and by the boot lock) which register green when you are unlocking the car and red when you are locking it.

A couple of weeks ago, I decided to take the "old girl" for a spin, so I removed the covers and disconnected the CTEK (which I leave permanently connected). I got into the car and started her (she fired first time) and selected reverse gear to back her out of my garage .... As soon as I selected reverse the alarm went off and headlights started flashing etc. I thought this was strange because the doors weren't locked and therefore the alarm was not enabled. So I stopped the engine, got out of the car and disabled the alarm using the plip (pointing it at driver door sensor) This disabled it until I got back into the car, started it and selected reverse gear when the same thing happened.

I also seem to having problems with arming/disarming the alarm using the boot and passenger door sensors. If I lock and unlock the car using the boot sensor, the alarm is triggered immediately I open the passenger door, if I then disable the alarm using the passenger door sensor, I can open /close the door without triggering the alarm but cannot open the boot without doing so.

My problem is I cannot now drive the car because of this issue and I seem to be going round in circles trying to diagnose the problem. I live in Northern France and my nearest dealer is nearly 50 miles away so if possible I like to sort it out in situ rather than pay for the vehicle to be transported to the dealer.

I have tried disconnecting / reconnecting the battery (per something I read on a US website) but this doesn't solve the problem.

Your advice / suggestions would be much appreciated!

Thanks
​

 

[A-AvantGarde]

Have you got the key for the siren? You may not even know what it was for but you may find you have a tiny key (looks like a key for a filing cabinet etc). I believe the early alarms can be disabled by turning the key switch on the siren located in the engine bay.

 

[rees_A]

Check in the glove box, some alarms can be disabled via a button in there(I think its a car wash mode or something)

 

[IBN]

Thanks for responding Guys

Unfortunately, I don't have a siren key and and passenger airbag is where glove box was.

 

[grober]

There was an issue concerning the connections to the central locking pump being mixed up meaning the central locking is effectively disconnected from alarm system and this turns off alarm system arming/disarming from the boot lock but I don't think this is your problem.
In terms of your alarm going off when you select reverse most alarms work by sensing changes in voltage. What is happening when you select reverse is voltage is dropping somewhere. Likely candidate is the brake light switch or reversing light switch or some sort of short in the reversing light circuit.

 

[IBN]

Hi
Updated Information. Today, I started the car with the CTEK still attached and I guess charging to the battery and I had no problems with the alarm even when selecting reverse. Does this confirm Grober's suggestions regarding voltage dropping?
If it does, how do I go about diagnosing where the problem exists?

 

[Dec]

Is it the case that you have an ongoing issue with the battery going flat?

Dec

 

[IBN]

Hi Dec

Not that I'm aware of. I bought a new battery about a year ago ..... replaced an ancient one that was worn out. And because the car is for summer use only, I always attach a CTEK battery charger / conditioner to keep the battery up to scratch.

Normally, before starting car I disconnect CTEK but yesterday I didn't.

 

[Spinal]

Sounds like you have a voltage drop; which given that it's related to you selecting reverse, would suggest the reversing bulbs....

Leave the CTEK connected, put the car in reverse, and have someone look at the reverse lights. Are they both working? If not, may I suggest you trace the wire - my guess is either you have a frayed wire (short circuit) or your battery is on its way out

 

[Dec]

When you unlock the car, you also disarm the car alarm so the use of any consumer shouldn’t rearm the alarm and so it shouldn’t be going off.

With some alarms, if you disconnect the car battery and then reconnect it, the alarm will rearm itself.

What might be happening is that the battery is not in good condition despite the charging, there is enough power in the battery to start the car, after starting the battery is depleted, the alarm sees this as a disconnection, re-arms itself and is triggered by the irregular voltage as the car is running.

Check that the alarm is actually being disarmed when you unlock the car, try locking yourself in the car, wait 60 seconds for the alarm to set itself up then unlock the car in the usual way, then open all the doors to verify that the alarm has indeed been disarmed when you unlock the car.

Dec

 

[IBN]

Thanks Dec - Sorry for delay in responding, I've been away from home (and computer)
until today.

 

[grober]

When you unlock the car, you also disarm the car alarm so the use of any consumer shouldn’t rearm the alarm and so it shouldn’t be going off.

With some alarms, if you disconnect the car battery and then reconnect it, the alarm will rearm itself.

What might be happening is that the battery is not in good condition despite the charging, there is enough power in the battery to start the car, after starting the battery is depleted, the alarm sees this as a disconnection, re-arms itself and is triggered by the irregular voltage as the car is running.

Check that the alarm is actually being disarmed when you unlock the car, try locking yourself in the car, wait 60 seconds for the alarm to set itself up then unlock the car in the usual way, then open all the doors to verify that the alarm has indeed been disarmed when you unlock the car.

Dec

I think Dec and I are saying the same thing- voltage supply drop triggering the alarm. I am saying due to a wiring fault-Dec is saying due to a battery on its way out. IIRC the reversing light switch is incorporated into the electrical unit associated with the gear selector lever.

 

[Dec]

Could also be back up batteries…if you have them… inside the siren or water damage or other damage inside the siren PCB board.

Not sure about this but I think if there is more than 2 wires going into the siren then there will be back up batteries inside it.

If the alternator wasn’t charging the battery, could that do it as well?

Dec

 

[m7m]

How about removing the reverse light bulbs? This will stop electric current flowing in that circuit. Hence, will help you narrow down the cause of the problem.

 

[IBN]

UPDATE

I've an update on this post. A couple of weeks ago, someone suggested that I should
clean the fuses (in both boxes) and this may sort out my problem. When I started this, I discovered that all the fuses were oxidised / corroded, so rather than muck around, I decided to replace all the fuses which required a trip to the local motor factors.

I cleaned all the contacts and replaced all the fuses - unfortunately, I still had the problem and the alarm still went off whenever I got into / started the car ... So back to the drawing board!

BUT 3 days later, when I again tried to start the car, to my amazement the alarm was behaving itself and has been acting properly ever since.

Can anyone throw any light on why the alarm is now working correctly?

Thanks

 

[Dec]

Perhaps water/damp somewhere that has now dried out with the hot weather.

Dec

 

[IBN]

Thanks Dec - never thought of that, you could well be right.

Ian