Hi Gang,

Once again I am at my wits end here, I need to start with a little background. First I want to make sure its understood this is a 4 door model and the front and rear dome lights work both together and independently according to the demand. When you unlock the car from the remote all the lights come on, when the care is running and you open one of the front doors only the front dome light should come on and when you open one of the rear doors only the rear dome should come on. Also when you turn off the car and remove the key both dome lights should come on.

Ok here we go, a few months ago I started noticing when I'd unlock my car via the remote and then get in and close my driver side door all the courtesy lights would fade out EXCEPT the rear dome light! Sometimes during the day even after driving for various amounts of time I'd happen to look in my rear view mirror and I'd notice the DAMNED LIGHT was on! Interestingly I could do one of 2 things to get the light to go out, I could either reopen my drivers side (front) door and shut it again, or I could hit the overhead rear dome light control button located by the sunroof switch. THIS IS CRAZY!!!

Now another tie in to this, A month ago I moved from beautiful sunny warm Florida to insanely cold Minneapolis. Well one night shortly after the move I happened to awaken to the lovely sound of my car alarm blaring. I ran to the window and I see the car is just fine except for one thing, guess what.. the damned rear dome light was on!!!! I'd let the alarm reset the light would go off and I'd think things were gonna be ok, couple hours or so later it would happen again! After a couple of nites of embarrassment I go out in -20 cold and start fidgeting with the door jam switches thinking ok dammit this has got to be the culprit. Well I went to both rear door switches and pushed them in as far as they would go and the light stayed on. I EVEN REMOVED BOTH SWITCHES and the light stayed on!!! After pulling the last hair off my head I said the hell with this and I removed even the front door switches and finally the light went off.

Well, that seemed to take care of the alarm issue, and I decided fine I will just drive around with no f$%#!^&g door jam switches in my car! Problem with that is you leave yourself vulnerable to break in's and also you can end up locking your keys in the car "due to the auto re-lock feature. SO.. I decided to experiment, I replaced the front drivers door jam switch, ONLY. I left the other three unplugged. Things seemed to be ok for a few days and then guess what... last nite I am driving along and I look in my rear view mirror and the f%^&(#g rear lite is on again! I hit the overhead button and it shut off.

I need help guys, this is insane, is there some damned control module that I can unplug or replace that controls the dome lights? And if so why is only the rear light malfunctioning? CLEARLY it has nothing to do with the door jam switches. My fear is there is a short somewhere that is making things unstable and of course that will never be tracked down "unless" there is indeed a module of some sort and the short is IN the module itself.

At this point all I can say is HELLLLLLLLLLLLLLLLLLLP!!!![:@]

 

Ouch. I had a 1990 300E. The alarm would go off for no apparent reason from time to time when the driver's door was opened with the key. Two mechanics couldn't duplicate the problem and I sold the car (for other reasons) shortly thereafter. I feel your pain.

I agree with the short. Try taking out the driver's switch but put in the rear switches and the passenger switch.... see what happens in a week or so. After that, you may have narrowed it down to the driver's switch. It's at least worth a shot. Sorry about the alarm in the middle of the night... that sucks. Good luck.

 

Hi and Welcome.

How nice to have a comprehensive problem description. Very often you get posts with a simple.... Rear light won't go off sometimes!

I think you have actually diagnosed the problem correctly. Its the PSE module and the atmospheric conditions are important.

Your car uses digital circuitry to provide the functionality that exists for the interior lighting system. Simply put there is a brain that deciphers what condition the car is in and then controls the lights (independently) in accordance with those conditions against a set of rules.

The interior lighting system is also linked with the security system since several of the inputs (i.e. are doors open / shut) are common.

Now the door switches COULD be the problem but you seem to have eliminated this by ensuring that they are either fully pressed in or even disconnected.
Unfortunately disconnection of input sensors may be picked up by the module as an open circuit and sometimes shuts the whole system down or reports a fault.

Now the module, known as the PSE, Pneumatic Security something or other contains not only electronics, but a pump and relays.
These units are a known source of trouble either due to an electronics, pumping or electrical reason.
The most common is electronics due to moisture ingress into the circuitry. Thius is most prevalent on the SLK but also occurs at a lower incidence on other models.

If you PM me with your email I will send you some pics of the sort of issue that you are dealing with.

Bottom line is that you likely need a new PSE.
Easy DIY job BUT the short COULD be elsewhere on the digital highway (CAN) and not the PSE

I can only be about 70% confident in the cause of this problem.

Incidentally I'm not sure where your PSE is located on your model other than it will be behind the trunk lining or under the rear seat.
Lots of yellow pneumatic lines go to this "black box"

 

Greetings,

I had a similar situation. The rear dome light would not turn off on clsing the doors. Figured it was probabaly the door switch but it wasn't. Proceeded to check the connectors at the PSE and in the process when I removed the PSE I found the PSE plastic casing was wet and further the lower half of the sponge casing (box) that the PSE sits in was seriously damp, however the floor area was dry. I figure the sponge was soaking up the water. My indy thought it prudent (though, more expensive) to install a new PSE. Everything works fine...for now!!!

In the process of opening the PSE I found the following:
(1) The unit had been opened before (telltale screwdriver marks on the plastic housing and some non OEM soldering on the circuit board).
(2) The whole thing had been wrapped in a plastic bag (definately not OEM!!!)

This leads me to believe that water had found its way into the area and the previous owner/mechanic had repaired the PSE and then wrapped the thing in plastic to protect it from water damage in future. Over time the plastic tore and water got in again. Unfortunately, the source of water into the area was never plugeed, else I would not be posting!

So this is where you'll could help me out...Where do you'll think the leak could be from? There are no telltale water marks any where in the area.

Thanks in advance.