At 04:19 PM 8/8/04 -0400, Roger Wilt wrote:
>I tried short circuiting the switch and still got nothing on the brake
>lights.
Brobably not a switch problem then.
>I was under the impression if the switch was bad the brakes would light up
>. (yes- the ignition was on)
Nope. Switch is normally open circuit and switches to closed with
pressure. Common failure mode is for internal leakage past the diaphram to
allow fluid behind the diaphram, which equalizes pressure on both sides and
prevents switching motion of the diaphram (remaining open circuit with
pressure). I have never heard of one doing a closed circuit failure, or an
internal short to ground.
>I determined the wire that went up to the engine area from the switch,
>went to the turn signal box
That means your TD has the same brake and turn signal circuitry as the MGA
1500. For color coded wiring diagram see here:
http://mgaguru.com/mgtech/electric/circ_f2.htm
>(turn signals are fine).
That's interresting. This means most of the brake light wiring is proven to
be okay, as the turn signals use the same rear bulbs as the brake lights.
>My confusion is that when the brake is depressed, the Ohm reading between
>the 2 contact points of the brake switch, drops to 0 - Signifying it is
>working correctly.???
Yes working, unless it is also doing a short to ground on the switch
housing at the same time.
>The volt meter however shows + voltage in the hot wire contact (other
>tester tip is grounded to the frame) before the brake is depressed,
Perfect.
>but drops to 0 after the brakes are depressed. HUH???
Look for a short to ground in the green/purple wire between the switch and
the relay box terminal 5. Such short would most likely be at one of the
wire end terminals.
>I've checked/tested the ground in the driver's rear frame and it seems to
>be ok.
>
>Curious - are the running lights and the turn signals grounded separately
>from the brakes??
Nope. The turn signal and brake lamp are the same filament in the same bulb.
>Just a bad switch?? ....
Probably not. If switch is open circuit at rest and closed circuit under
pressure, the only possible switch failure would be an internal short to
the housing when it switches (and I've never heard of that failure mode).
For additional diagnostic, disconnect the wire from terminal 5 of the relay
box, apply 12 volts to terminal 5 with a jumper wire, and the brake lights
should light up (since you said the turn signals work).
If they don't light up, touch the hot jumper to terminals 3 and 7, either
of which should make the brakes light up. With the relay in rest
contition, terminals 5, 3 and 7 should all be connected together via
internal relay contacts. If these three terminals are not connected, you
have a relay contact problem.
Assume the relay is okay and the brake lights work with power at the relay
terminal 5. Reconnect the wire to terminal 5, disconnect the same
green/purple wire from the brake switch, apply power to the wire at that
end, and see if the brake lights work. If not, the wire is broken or shorted.
If that works, then you may have a bad (dirty or corroded) high resistance
connection in the green wire feeding power from fuse terminal A4 to the
brake switch. A high resistance connection can show full voltage with no
load and zero volts with a load applied (that makes the load look like a
short to ground).
If you get this far you should already have the problem solved. Otherwise,
give another hoot.
Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com
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