I have thought of this route BUT this problem ONLY happens after the
engine has warmed up. This leads me towards an electrical problem and
your thoughts about a poor earth will be checked. If a poor earth
should this not have shown up when the battery and alternator were
checked?
The unit that controls electrics has just been replaced with no change
to the problem

"Stewart DIBBS" wrote
> "yourpete" <none@000.com> wrote in message
> news:951271_8f4af7ca4a55d1930a8829b577d7fdf6@autob oardz.com...
> > During night driving I have noticed that the head lights
> vary in
> > brightness. They brighten up at about 2500 to 3000 revs and
> dim when
> > the revs drop.
>
> OK, lets look at what causes a visible variation: change in
> voltage. Since
> you say the alternator and battery have tested OK, then its
> likely a
> mechanical rather than an electrical problem.
>
> What causes an alternator to vary voltage? A slipping belt is
> the usual
> reason. A bad ground is another.
>
> > But when in say 4th or 5th gear at about 1500
> > revs(Diesel model) and I lift off the accelerator the lights
> > brighten up...
>
> Have a good look at all the alternator mounting brackets. Is
> anything loose?
> eg the main pivot bolt on to the block. Check all the engine
> block to
> chassis ground links.
>
> Is the crank pulley loose or is a two part balancer type?
> Could be the
> pulley is separating and moving the belt around causing
> slipage or lateral
> movement in the alternator.
>
> SD

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