82 jota wiring fault

[merlin]

when I use the indicators on there own they work ok ,but when indicating and braking all the rear lamps flash ,my thoughts are a bad earth spent the day checking as much as I could cant find a fault ,i disconnected various things . any ideas as what to look for or try
thanks
dave

 

[Rob]

So what did you check?  You're right in thinking poor earthing - so you need to check for earth continuity from each affected lamp holder.  Also check the rear stop/brake lamp wiring is connected the right way round

 

[merlin]

I checked the rear light and indicator earths all ok , someone has fitted an aftermarket brake switch tried reversing wires ,checked all earths I could find ,and changed some household connectors used by a PO , found the ign switch plug had welded itself together ,new switch on way ,also found the 9 pin multiplug from the left switch was not in good condition new plug ordered , I disconnected and tested things as I went round tried everything I can think of
dave

 

[Rob]

Ah, from that list the problem could lie pretty much anywhere!  However, the root cause is a poor earth connection in the vicinity of the affected lights.  Instead of flowing straight to earth from the lamp filament (are you absolutely sure the bulb holders/lamp connections are corrosion free, as well as having a very low resistance path to earth?), the electricity finds another way back .... through another lamp filament to its earth.  Typically the side of the metal cap on the bulb doesn't make good contact with the metal strip that provides the earth contact/continuity...or the soldered/crimped connection looks OK but isn't.  If you're handy with a multimeter you can use that to pinpoint the point of high resistance ... or find volts to battery earth where there shouldn't be any on the earthy side of the lighting circuit.