I'm so stupid... Was: Head Cleaners

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Fri Jul 2 18:12:28 2004

> We had one of those haynes service manuals (it was an old car, so we'd

I have been singularly unimpressed with Haynes manuals. They seem to omit
important information, not describe certain jobs (automatic
transmissions, diffeeentials, some electrical stuff, etc are regarded as
'too difficult for the home mechanic' -- BLETCH!), and even have some
serious errors.

I try to get the offiical manufacturer's manual (I actually have quite a
shelf of them now) -- often it's expensive, but it's worth it.

The Citroen BX official manual is 3 thick binders. The Haynes manual is
one book about 1/4 the thickness of one of the binders. Hmmm. And it's
_useless_ for electical problems for one very good reason. The Citroen BX
wiring loom is in many sections, which convege on the fusebox under the
dashboard. For example, there's a ribbon cable (!) that runs down the
left side of the car. The front end plugs into the fuse box. The rear end
plugs into the rear light cluster on that side. Another ribbon cable
plugs into that light cluster, runs across the back of the car and plugs
into the RH light cluster. Other looms plug into the fuse box and go to
the column swithes, engine/front lights, etc.

What this means is that if you have an electrical problem, you get isnide
the fusebox and start probing the connections there. You can quickly
isolate the problem to a particular area and then do more specific tests.
Or at least you can if you have the pinouts of all the connectors. This
is shown in the offiical manual, it's not in the Haynes manual. Hmmm...

> bought it just in case) and after poring over the virtually indecipherable
> schematics, I traced the current flow for this lamp. (From memory:) this
> lamp was nominally connected between +ve and the output of the
> alternator. This when the alternator was charging, there was no potential
> difference, so the lamp stayed off. However, if the fuse in the +ve supply

Alternators have a 3 phase stator, the otuput of which goes to a 6-diode
bridge and then to the battery. There are another 3 diodes connected to
the non-earthed side (normally +ve) to provide an auxilliary supply for
the regulator/field winding. The warning lamp is normally connected
between the switched battery +ve line and the auxilliary line. When the
alternator is charging, the warning light has the battery voltage on both
sides and doesn't come on. When the alternator is not charging, the
'alternator' side of the lamp is effectively earthed via the regulator
and field winding. That's why worn-out brushes will stop the lamp from
coming on.

There's a reasonable component-level description of a standard alternator
and regulator in the Land Rover series III manual, BTW.

-tony
Received on Fri Jul 02 2004 - 18:12:28 BST