As I'm driving at times multiple items lose electrical power at once. For example, the radio turns off, windows & sunroof stop working, wipers if on stop in place and the heated seats turn off and power seats do not work all at one time. Now when I notice the SRS light coming on for a few seconds, then everything works fine but soon afterward all items no longer work again.

There is no consistent pattern except that everything works for a short duration after the "SRS" light momentarily comes on while driving and only movement and not time changes the situation. It is quite a nuisance when you park and can no longer raise up the windows or while driving the radio shuts off. The car is a 1995 E420 with 58,000 miles. I have changed the combination relay as I had erratic wiper movement which was resolved but now the wipers again have no power except for a minutes after the SRS light comes on. Could the convenience relay be the culprit for some of the issues? Interestingly I took the convenience relay out to note any changes and the power seats still worked with it out.

Secondary problem is the low beams permanently do not go on, but high beams and fog lamps work fine. I checked voltage at the fuse for low beams and it read none, but high beams had normal. If with a screwdriver I touch between low and the high beam fuses, low beams will light. Could it be the current is not getting past the combination switch on the low beam circuit...thus faulty switch?

Thanks ahead for any advice.

 

I would suspect your ignition switch might be faulty. It's the only thing that I can think of that is related to all of those systems. The SRS light coming on is a clue, whenever you turn the key to position 1 the SRS system is powered, it is the only warning lamp in the cluster that comes on with the key in the accessory position. The other warning lamps and various powertrain computers only receive power in position two and three. That would explain why the engine isn't shutting off from the faulty switch, only the accessory position is bad. I suppose it could be a short in the cable going from the ignition switch to the fuse box, but I think that is unlikely. Do you have a lot of keys or key chains on your key ring? These ignition switches are known to fail when there is excessive weight on the key ring. I'd also risk a bet that if you were to get into an accident while all of these systems are inoperable the airbags would not deploy. I would get this taken care of quickly.

On your E420 the convenience relay has nothing to do with the seats, it only controls the windows and the closing and opening of the windows\sunroof with the key in the door lock.

As far as the low beams, I'd have to look at the wiring diagram.

 

I've attached a pdf of the headlight circuit, the low beam circuit is outlined in yellow. If your getting no voltage at the fuse it looks like it is the combination switch. If the headlight switch was faulty you wouldn't get high beams either.

 

[QUOTE=midnightlsc;3233827The car is a 1995 E420 with 58,000 miles. [/QUOTE]

Bad wiring harness?

 

I doubt it would be the notorious engine wiring harness, as all of the affected systems he mentioned are on the interior wiring harnesses. It wouldn't hurt to check that harness though.

 

A weak battery can also have unusual effects on MB electronic systems.

 

I definitely agree with you on that.

 

I would do the standard changing all the fuses from the aluminum to the copper first.

The aluminum ones can look good but really be eaten away by the copper they reside in.

Bruce

 

I would check your grounds for starters. Then it might be the base module. They are touchy.

 

Thanks for your sound input and especially for the helpful headlight wiring diagram. The engine wiring harness has already been replaced. Good analysis in finding something (ignition switch) that is tied to all these accessories. I will give attention to the ignition switch but never have much weight from keys hanging on it. Interestingly inserting key in the door lock works to lock/unlock but not to open/close windows/sunroof as once did. Battery voltage is 12.2 volts at the battery with engine off—I assume that is okay. I've checked various ground connections around the car and even inside the console—everything looks solid. Thanks.

 

When you say it might be the "base module", do you mean the "convenience module" located under the rear seat cushion on the driver's side?

 

I hope my info leads you in the right direction. Keep us posted, it would be nice to know the solution.

As for you windows\sunroof working from the key, have you tried to control them from the trunk and passenger locks. If only the drivers lock has lost the ability to control the windows it's probably the micro switches in the lock cylinder. If that's the case your alarm may not be arming when you lock the car as I believe the convince relay and the alarm module use the same micro switches. If the windows can't be controlled from any of the locks it might be a fault of the convince relay. Just food for thought.

 

I think whipplem104 was referring to the base module located in the module box on the right side of the engine compartment. It serves as a power supply and surge protector for the LH control module and injection valves on the V8's. It may also feed other things related to the engine.

 

EVERYTHING WORKS PERFECT!! Yes it was the ignition switch at fault. I replaced the electrical portion of the switch and also the combination switch for the low beams to work again. After much observation of the SRS light pattern of coming on by itself during driving and thus the car's accessories resume functioning indicated as you said led to the ignition switch accessory position being faulty as the culprit. Thanks so much for all your help.

 

I glad I was able to help.