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Re: [Mgs] MGA 1600 rear light intermittent short

To: "Robert J. Guinness" <guinness@stclegal.com>, MG List <mgs@autox.team.net>
Subject: Re: [Mgs] MGA 1600 rear light intermittent short
From: Barney Gaylord <barneymg@mgaguru.com>
Date: Sun, 11 Apr 2010 06:06:22 -0500
This sounds really wierd, but I'll take a shot at it.  First, if 
something lights up it is not a short to ground, but some errant 
connection (or short) to power on the input side of the lamp.

On the 1600 the turn signals are independent from the brake 
lights.  Parking lamps are connected in parallell, and ditto for 
brake lamps, so if one lights up the other should light up along with 
it.  Having only one light up rather implies that it is a turn signal 
circuit.  So my first guess is that you have something miswired so a 
turn signal wire is connected to the bulb for the parking lamp and 
brake light.  But that still does not explain power for light when 
everything should be shut off and dead.

When ignition is off, battery power should be connected only to the 
horn and to the lighting switch, no power to the turn signal 
circuit.  The idea that the light comes on later, some significant 
amount of time after switch off, is even more confusing.  The only 
relay contact in the stock 1600 electrical system would be in the 
control box (for generator), and that's even more wierd.

There must be come wiring error.  You could grab the schematics and 
check every wiring connection in the whole car.

A more expedient approach would be to follow the faulty circuit to 
find the source of power.  When the lamp is erroneously illuminated 
remove the lense and use a test light to determine which wire (by 
color code) is supplying the power.  Then follow that wire back to 
the preceeding connector in the harness, test again to be sure you 
have the right wire with power, and check colors of other wire or 
wires in that connector.  For any connector in the entire car there 
should be only one color of wire.  Never connect two different 
colored wires together.  Continue following wire or wires with power 
in this manner working your way back toward the power source 
(battery).  Somewhere along the line you should find a mismatched 
wire connection.

1600 turn signal and brake lights: 
http://mgaguru.com/mgtech/electric/circ_f3.htm
Lighting circuit: http://mgaguru.com/mgtech/electric/circ_d.htm
Regulator circuit: http://mgaguru.com/mgtech/electric/circ_c.htm
Master electrical diagram: http://mgaguru.com/mgtech/electric/circ_a.htm

Happy hunting, and let us kow what you find.

Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com


At 01:52 PM 4/10/2010 -0500, Robert J. Guinness wrote:
>The passenger rear running light intermittently goes on by itself on 
>my MGA 1600.  This is the lower bulb that is also used as a brake 
>light.   It happens most often after I have turned my back on the 
>car and closed the garage door, only to come back later to see it 
>on.  Any advice on what and where the likely culprits are, and how 
>to diagnosis this short.  ....
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