Help someone!, Mk1 P reg has failed MOT as the back lights dim slightly every time the indicators flash. Bad earth i am told, so i have cleaned the connectors to the lights, cleaned the bulb holders, cleaned the earth contacts in the rear boot, cleaned the earth contact under the battery and cleaned the battery earth. Still no better. Taken all the indicator bulbs out to see if any corrosion under them but no. Anyone out there with experience of this sort of problem? otherwise its going to be an expensive trip to the auto electricians..... Only thing not pulled out yet is the indicator relay.

All lights are checked individually during an mot so why are your lights on when hes checking your indicators. Whatever the case, im sure thats an advisory not a fail.

Thanks Matt68, apparently not, there is a specific failure code they tell me, its written on the failure form. I did argue the point cost they dont dim a lot but they maintain that is now part of the test. Yeah all lights work fine indivdually but with the rear lights on and indicator going they do go dimmer reckon its less than 1-3v drop. Tried switching on some high drain stuff, heated r/w, blower fan etc without the indicators on, there is no appreciable drop in rear light levels, suppose this means an indicator circuit fault but Im b if i can diagnose where!

do other combinations of lights dim? for example you headlights go dimmer when fog lights on? possibly not getting enough power from the alternator?

checked all the earths in the engine bay? earth on the alternator?

you have already checked what it usally is, when its a bad earth if you indicate then put brake lights on all the rear lights flash like a disco, does this happen?

Is that all its failed on? cus id be major pissed if i had to pay for a re-test like u do now with the new changes, if its just that.

have you tried making a new earth connection from the bodywork directly to the earth plate inside the cluster?maybe something like internal corrosion of the earth wire in the loom?? causing the poor earth.

dave said:

you have already checked what it usally is, when its a bad earth if you indicate then put brake lights on all the rear lights flash like a disco, does this happen?

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That kept happening to mine but garage sorted it apparently the electrics were a miss. Cost next to nothing but I was having other work done at same time. Has done it since but very very rarely I just take everything out put it back together and that solves the problem for another 6 months.

Thanks folks will try that new earth, Dave i was hoping you would come up with a wonder cure! No don't get any alien like light show, in fact when brake lights are on there is no appreciable difference in the dimmin. Yes am majorly pissed off cos its not as if the lights dim so you cant see them, even in daylight! Think I'll try the flasher unit next thats all i can think of as good earths on cluster, headlights don't show any dimming whatever is put on (unless i start the car!). Will let the forum know but if anyone has ideas I'll try em rather than pay an autoleccy ££££!

have you checked battery and alternator are chucking out the right power? no one fitted to high a watt head light bulbs?

Thanks Dave, Yes i did fit some high output lights from Halfords, (standards are soo dim!) good call. Thought i'd check it out this morning but flat battery, was good 3 days ago but car not used daily now as no MOT. Will check the battery situation, ie good or not (assume its pretty much ok as no apprciable difference on headlights before when revving engine with all electrics on) , then the lights, then see if theres a low voltage drain to earth (which I assume would cause same effect as poor earth and cause battery drain). if not then its off to autolec. Will post up the result for others to benefit!!

This sort of fault was normally cured by fitting new light boards as they were known for causing the 'pulsing' effect on the rear lights and failing the MOT.
I would seriously consider changing them.