W126 Alarm System - How To Disable?
 

I have a 1991 350SDL (original owner) my alarm system is coming on unexpectedly. How can I disable the system until I can get it fixed?

Thank you.

I know that for my 560SL, I pulled the fuse for the central locking system and that stopped the alarm. I left it out for a couple of weeks and when I put it back in, the problem was gone.

Of course, you have to unlock each door individually when you pull the fuse.

You can disconnect the two plugs into the box under the false wirewall in the passenger footwell. I think disconnecting just the one closer to the floor will do it.

The thing that usually gets flaky is the switch in the door lock mechanism.

Sixto
91 300SE
87 300SDL
83 300SD

Thanks sixto, that did it. Yes the driver door lock feels flaky.

Thank you.

Yeah what happens to me is that when I lock it from the driver's side the alarm comes on. I just unlock it from other places now such as the trunk or the passenger's side. But yeah, it's under the passenger footwell.

I still have problems. Now the car does not want to start, it is a diesel. It finally starts after I try about 10 times, it acts like the glow plugs are not working when it does start as it runs very rough and noisy until it runs some. Glow plugs and glow plug module were recently replaced and the module is putting out 12VDC to the glow plugs.

Any ideas?

Thank you.

Assuming the 603.971 glow system is similar to the 603.961 glow system:

Glow system self-diagnostics are pretty good in these cars. If the glow system isn't up to snuff the glow light will tell you by not turning on at all, staying on af the car has started or blinking.

If the glow light doesn't come on, check the fuse strip in the glow relay. Check it well because sometimes those things crack over time but still look intact.

If the glow light comes on, check the resistance of each glow plug to ground. You can do this at the glow plug relay. The connector with 6 pins has the wires to each of the glow plugs. I don't know what the acceptable range is for a 603.971 but they should be within 0.1-ohm of eachother. I mean it's unlikely that they're all bad. A bad reading could also be a bad wire. If you find one out of spec, check resistance at the + terminal of the glow plug.

It could be that carbon is fouling the tips of the glow plugs. You'll have to pull the plugs, clean the tips and use a special reamer to clean the glow plug channel in the prechambers. Ugh.

Sixto
91 300SE
87 300SDL
83 300SD

sixto, everything checks out fine with the glow plug system.

Very hard to start again this morning but once started restarts are fine. Lots of smoke coming out of exhaust when it's trying to start which indicates to me that fuel is not a problem.

I'm wondering if there is another cutoff circuit in the iginition chain somewhwere that may be acting up?

I tried replacing the glow plug module with a second new one and it behaves the same. I do notice that the new glow plug module has a soft click to it when the ignition is turned on whereas the old module had a very distinctive hard click sound?

Any ideas?

Despite what you say about the glow plugs the symptoms do sound like there's a fault here.

When the engine is cold, the fuel does not vapourise properly, so ignition is difficult. When it does fire, there will be a lot of excess fuel in the chambers, hence the smoke. Once the engine is warm, no more vapourisation problems, and it starts fine.

Have you checked for power at the glow plugs when you initiate the glow cycle?

The difference in clicks will be down to the relay.

Easy restarts doesn't absolve the glow system since you probably don't even need the glow system fora few hours after shutting down the engine.

Sixto
91 300SE
87 300SDL
83 300SD

I measure 12VDC coming off the relay to the glow plugs with the key on.

Does anyone know where the fuselink is for the glow plug system? I found one to the left of the fuse box in front of the brakes master cylinder, 30 AMP, is this it??

Thank you.

A box with a couple of fuses outside of the main fuse box is probably the AC blower fuse box.

If you remove the cap over the glow relay (behind the left headlight), you'll see a couple of plug in connectors and an inch strip of metal held by two philips head screws. The metal strip is the 80A glow plug relay fuse. At least in the 300SDL.

Sixto
91 300SE
87 300SDL
83 300SD

Can someone please tell me where the alarm for a 1985 300SD is located? Last week my car's lights all died at once, headlights, turn signals, interior, I mean EVERYTHING! When I shut the car off the headlights flashed twice and the horn blew (As if saying "See ya!). This leads me to believe the alarm is bad. I got one on Ebay (part # 126 820 08 26) and took the passenger side mat off where the seller said the alarm should be, but nothing! Where have the gods of Stuttgart hidden it?!?!

Remove the floor mat, remove the carpet. Against the firewall there should be a plastic panel held by one or two nuts. Remove the remove the panel which will expose some electrical boxes. The long narrow black box with 2 connectors each with about 12 pins and likely with Becker markings is the alarm unit. Remove one or both connectors.

Sixto
87 300D