living on an
Join Date: Sep 2008
Location: United Kingdom
For what its worth here's my experience of this (and I am not a master technician!).
I had very similar symptoms on another car, with erratic warning lights and strange intermittent problems that got worse if the headlights were on, or if the cigar light was used or the sunroof opened/closed. Nothing made sense and all the symptoms were illogical.
However, long story short, what we eventually discovered was this. As Alfatech said, the main earth strap from the battery to the chassis was slightly corroded and not entirely bolted tightly enough to the chassis. This meant that when the car was drawing quite a bit of current due to heavier loads, like headlights, the system voltage was dropping just below the critical threshold needed for the various electronics / computers to work. From memory, the battery should be about 13.8v when charging as measured at the battery terminals, so check this first. However, due to the bad earth strap, the increased resistance under load caused by it, meant the voltage at the various computer modules had dropped to about 10.5v which is too low for them to work and they started behaving erratically, hence weird warning lights coming on and things not quite working.
If you can get the car going, measure the voltage across the battery terminals, and then compare that to the voltage from Battery +ve and elsewhere on the chassis. Then turn on a heavy load, and measure again. If you see a voltage difference of more than a half a volt or more then you probably have got problems with the earth strap or main battery terminal lugs etc.
So we cleaned up the battery terminals, cleaned up the earth strap and its chassis lug, and just to be sure, we disconnected / reconnected all the major connectors we could see.
Result = Car worked perfectly again. All errors vanished. Not further problems.
As for your comment about replacing the brake light bulb - was there any moisture in the socket and bulb assembly? Always good to clean them out too whilst you're at it.
(now donning flame suit)