300SL (107) Side Light Problem

[djlew]

Problem started when I noticed the radio got servere interference when side/lights were switched on. Found offside rear side light was out, but not registering on the bulb out warning light. Changed bulbs (the torpedo shaped ones in aclamp connector) but no joy.

Have checked bulbs - OK
Checked voltage across connector with bulb out =12.4 Volts
Radio interference disappears if bulb is removed with sidelights on and returns when replaced

Any suggestion gratefully received.

 

[grober]

RF interference is normally cause by a "mini arc" i.e. tiny sparks generated by a break or poor connection. Sounds as if there's a poor earth connection on your lamp holder. Test the resistance of the bulb holder earth connection to the chassis/BATTERY earth. Make sure the bulb holder connections are scrupulously clean/free of corrosion.

 

[djlew]

Side Light update

Have fixed the problem by running a wire from the live side of the nearside rear side light to the live side of the off side side light. Side light now ok + radio interference gone.

Obliged if anyone has a better fix?

Mind you I suppose a 21 year old car is likely to have a few mysteries begin to develop.

 

[grober]

insulated break in wiring??

If the interference is gone then that points to an internal break in the wiring supplying the faulty side light causing the arc but not arcing directly to earth in this case . Only when the bulb is in place and the old circuit "on" will the arc occur when current flows and the majority of a 12V voltage drop will be across this partial break hence the light wont come on. If the arc had been due a short to earth then this should have continued if the old power wire was still "live" despite the new supply wire to the light .Since the RF is gone then the problem is solved and there is no fear of a direct short to earth. Don't know if I put that very well but glad your problem is solved.

 

[djlew]

Another Electrical

Thanks for the explanation. However, unbelievably, whilst driving to work today the dasboards lights dimmed to nothing, the radio cutout, all the other lights died and a few minutes later the engine cut out. Got the big yellow towtruck trip home where amazingly it started up, however it soon cut out again in the same way. Suspect the alternator as no load engine off was giving 11.8V and running no load just crept above 12V. Battery is high quality and only a year old. Anyone suspect what I did with the side lights might have caused the problem?

 

[grober]

UNLIKELY but this new info might point to a direct short to earth somewhere in the wiring loom which I had just ruled out before. Did you disconnect/isolate the original supply wire to the side light. I would do this if possible. time for a thorough inspection of the wiring loom. Was it raining when the fault occurred?

 

[djlew]

I did think of disconnecting the original supply and will do that tomorrow, no rain today just very cold standing at the side of a motorway at 0645hrs this morning. The fault is quite similar to when the alternator failed a few years ago. It was reconned at that time so may well be at the end of its life now. At least a non working alternator is easy to diagnose. (I hope!)

 

[television]

Sounds very much like the brush pack on the rear, and only a few ££ to change it