As I said, I am no expert on TD brake lights. But if the switch controls
the 12v supply to the lights then what I said is correct. The only time in
a properly working system that you would see 12v on one side of the switch
and 0v on the other, with the switch open, dropping to zero volts both sides
with it closed, is if the switch controls the ground feed to the lights and
not the 12v feed.
If the switch were not passing enough current due to internal resistance
then you would see a proportion of the 12v across the switch terminals *when
it was closed* and the remainder across the lights, which may cause them to
glow dimly. But once the switch resistance gets high enough, which will
only be a few tens of ohms, you will see virtually all of the voltage
dropped across the switch and none across the lights, i.e. similar to when
the switch is open.
A relay is beneficial in some circumstances, like when one has to purchase
the crappy hydraulic switches that seem to be available for the MGB these
days, but if the relay were connected to the wiring at the switch in the
original posters case it would do no good, as his problem lies closer to the
ignition switch like at the fusebox or connectors in between. It also
requires a diode to be connected across the relay winding terminals,
opposing normal current flow, as the large inductive spike that is generated
as the relay is denergised is also enough to knacker the switch.
PaulH.
----- Original Message -----
From: "CHRIS KOTTING" <ckotting@wideopenwest.com>
To: <MGs@autox.team.net>
Sent: Monday, August 09, 2004 6:14 PM
Subject: Re: TD Brake pressure switch gone bad??
> If the switch is still in circuit on both sides (i.e. both the from-the-
> ignition-switch and the to-the-lights wires are connected) then the
voltage
> dropping to 0 when the switch is actuated may be perfectly OK. IF the
wiring
> to the ligths is intact and the bulbs are in place, then when the switch
is
> actuated the voltage potential between either terminal of the switch and
> ground will drop to near zero (possibly too small to measure on a 12 volt
> voltmeter.) However, voltage isn't the entire issue.
>
> What I suspect is that the switch is closing the contacts, but isn't
passing
> enough current (amperage) to light the lights. Solution 1 is to replace
the
> switch, and do it again when that switch degrades to the same point.
>
> Solution 2 is non-original but remarkably effective. Put a relay in
circuit
> so that the brake light switch only has to pass the few milliamps required
to
> fire the relay, and the amps required to light the lights pass through the
> relay contacts.
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