1990 300SL - door locks and alarm system
 

Need help - the remote stopped working on all the door locks this week. (Batteries are replaced in the remote.) Now I have to use the key to get in the car in the last couple of days now. I unlock the door, start the car, and when I put it in reverse or drive the car alarm goes off. The only way I've found to shut off the alarm is to shut the car down, climb out and put the key back in the door lock and turn it. Can anyone help with fixing this problem. I also came out the other morning and found my emergency flashers going off "all by themselves" of course this created a dead battery, which jumped no problem. What followed is the door lock problem but I'm not sure those are connected.

Thanks for your help

You need to resyncronize the locking system after replacing the battery in the remote. Page 118 of the owners manual.

Thanks - I'll try that and let you know if it works

Door locks and alarm
 

Well that hasn't worked - we are thinking its two separate problems now. The alarm system still goes off and we know the remote works (there's a light) but the resynch didn't work....nothing happens at all.

The door locks - well when you use the key in any lock it doesn't unlock or lock all the other doors. Just the one the keys in and no green light.

Any ideas? We are stumped at the moment.

Are you saying that the red and green lights in the doors flash in response to the remote, but the door locks / trunk lock don't respond? The doors / trunk open with a physical key, but only the lock into which the key is used? The whole lock system runs on vacuum supplied by a small vacuum pump in the trunk above the battery. Does the pump run? If not, check the fuse. ( It may look OK, so replace with a spare. ) You can usually hear it run when needed with the trunk open. Can you lock everything by using the lock level inside on the driver's door? That would eliminate the vacuum pump as a problem. On the pump is a vacuum manifold / distributor. Are all the lines connected? A vacuum leak in the system will cause problems. The vacuum locks and unlocks the doors, the two rear storage compartments, the center console, the trunk, and in some models the hood. If any of the actuators are leaking, binding or damaged, you can have problems in the whole system. Each actuator can be tested with a small hand grip type vacuum pump attached at the lines on the manifold.

Actually there are no red or green lights in the door - it's when we tested the remote. They only doors/trunk open only in the lock in which is used-correct. We'll check the pump. No I have to lock each door individually using the lock level inside the driver's door.

We will check everything you indicated and let you know what we find out. Thanks so much for your advice.

My car is a 1991 300SL and I have the red and green lights in the door handles and near the trunk lock. I thought all SL's 1990 thru 1994 had this style locking system. Anyone else with a 1990 SL have any knowledge on this? The lights flash Red when you lock the car with the remote or key and Green when you unlock it with the remote or key. The alarm system is activated by locking any door or the trunk with either the key or the remote. It is turned off by unlocking any door or trunk via any key or the remote. There is no light to indicate the alarm is armed. The alarm will sound if the radio is removed while the sytem is armed. If someone has replaced the original radio with an aftermarket unit and not jumped the alarm circuit, it can cause the alarm system to malfunction. There are also switches to set the alarm off in the glove box and eye glass box. Lot's of places to look for trouble. BTW. The alarm flashes the exterior lights, so it's possible that's what caused your battery to run down.

Yes we have the red and green lights in the doors and trunk but they are not working. It gives no indication that you have pushed the button on the alarm key fob (we get a red light on the end of the fob which indicates the batteries are good in the fob). The red and green lights on the car and alarm worked perfectly normal until we changed batteries in the fob a week ago. Now the alarm system has a mind of its own - it may go off when you put the car in reverse or drive, in which case you get out and put the key in the drivers door lock and it shuts off. Then you begin driving again and the alarm may go off again while you're driving down the road. In which you case, you get back out and put the key into the lock and it shuts off again.

Yes it has an aftermarket radio however that was done 3 years ago and it hasn't been a problem.

We appreciate your input and we'll keep on trying to fix this thing.

Well, this is truly weird, and I've not much else to offer. The alarm system is totally confused. I've changed the batteries in my FOB's without incidence. Fuses 5 and 6 in the trunk fuse block are for the alarm system. Fuse 6 handles the infrared locking receiver, trunk lamp,vacuum pump and power antenna. Fuse 5 is for the headlamp low beams, parking lamps and anti theft alarm. I've had aluminum fuses on my car go bad. Not blown, but just open from corrosion. I've changed them out for the newer brass fuses. As the alarm circuit is always "hot" even with the key off, I'd try pulling fuse 6 for a few seconds and see it the alarm receiver resets. You may have to go through a resync after that. What I find so strange is that it worked fine until the FOB was "disturbed". That's the clue and that's where the answer is.....

Well we have very good news - we changed the fuse as you described and it worked. We've owned this car since January and are loving it (both of us have owned Mercedes previous but this is our first SL model) and did not know there was a fuse box in the trunk. Definitely still learning this car. BTW-do you know where we could purchase a service manual for this?

Thank you so much for sharing your knowledge with us. We appreciate your willingness to help and if there's ever anything just drop us a note. It's still amazing how this stumped my husband - he's a very skilled mechanic and retired owner of a auto repair/body shop that specialized in rotary engines and more. Mr. Fix-it for sureAnyways, the two Mercedes specialists we spoke with here locally couldn't even tell us. So kudos to you. :D Have a blessed week. - Kenny & Kathy

1990 300SL

That is good news....I'm glad I was able to help. I've owned my 91 SL for 6 years and it is my first Mercedes. This car is unlike any other vehicle I've ever owned in so many ways. This forum and the collective knowledge of it's members have helped me fix many problems unique to this vehicle, particularly with the electronic components and the soft top system. I didn't know there was a fuse block in the trunk either until one of my windows stoped working. I was ready to take the door apart until someone on this forum suggested I check the fuses "in the trunk". I never would have thought to look there for fuses. It's a wonderful car; just a little quirky at times. Tod