Subject: [Mgs] wiring gremlin in 70 MGB

From: "davewillner" <[email-masked]>
Date: Wed, 30 Jul 2014 19:16:23 -0400

Ok, so last fall my turn signal lever on my 70 MGB melted down, the end of the
lever started to melt. I did a quick replacement with a used one I found at a
show, it was for a later year (71+) as it didn't have the horn button. I'm not
sure if this is all coincidental or not, all my fuses are ok and in tack. The
battery has been going dead, the ignition key buzzer goes off when the
driver's door is opened even without the key left in. And the tach has stopped
working awhile ago, I think around last fall when I had the initial issue.
Today the signals just stopped working also. I just ordered the correct signal
lever with the horn button, but curious as to what to take a look at first?
Appreciate any insight and help, thanks

Dave
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From: "PaulHunt73" <[email-masked]>
Date: Thu, 31 Jul 2014 15:12:18 +0100

Jeez, where to start!  Any or all these symptoms could be connected or 
unconnected.

A melted switch implies a serious short and that needs to be investigated 
first.  It could have damaged any number of wires and circuits, so that's 
what you need to be looking for first of all, and fixing any damage found.

The tach needs two inputs to register - a 12v supply and earth on the one 
hand, and the ignition signal on the other.  Either could be missing, and 
indeed the tach itself could be faulty.  But if the engine runs, and you 
have the original points ignition wiring, it's safe to assume that the 
signal part is OK, so check for 12v and earth between the white wire that 
goes to the spade on the back of the tach, and the tach body.  Also give the 
glass of the tach a sharp rap with a knuckle with the engine running and see 
if it starts registering.

With the ignition on and the turn switch moved up or down do the dash 
tell-tales light?  If not there is no power reaching the turn signal switch. 
This comes from the green fused circuit, through the hazard switch in the 
off position, and through the turn flasher, to the turn switch.  Check the 
green wires on the hazard switch and turn flasher for 12v with the ignition 
on, and the light-green/brown wire on the turn flasher for 12v.  This last 
wire goes to the turn switch, so check both sides of the multi-way plug on 
that wire, as well as at the switch itself.  Somewhere along that run you 
will probably find 12v no longer appears.  If there is no 12v on either 
green at the hazard switch then there is a break back towards the fusebox. 
12v on one green and not the other means the hazard switch is faulty.  12v 
on the green but not the light-green/brown at the turn flasher means the 
turn flasher is faulty, and so on.

If the dash tell-tales light you have power through the turn switch, so 
check the corners to see if one or other isn't lighting.  If both are 
lighting it could be the turn flasher has failed, or it could be bad 
connections anywhere in the circuit reducing the current.  With each corner 
lit measure the voltage on the red wire of each light unit with respect to 
earth.  If much less than 12v that shows bad connections somewhere.  Then 
measure the voltage between the body of the light unit and a good earth 
elsewhere, anything above 0v shows a bad earth.  If one or both corners 
aren't lighting there is a break in the circuit from the switch out to the 
corners, or the bulbs are faulty.  Check the green/red (left) and 
green/white (right) circuits for 12v with the switch moved to that side at 
the switch, multi-way connector, four-way bullets by the fusebox where the 
main harness joins the rear harness, and at the corners of the car.

The buzzer sounding with the doors open but the keys out implies an earth on 
the purple/pink between the buzzer and the ignition switch.  If removing 
that wire from the switch itself stops the buzzer, then the ignition switch 
is faulty.  If not then that wire is shorted to earth somewhere between the 
ignition switch and the buzzer.

PaulH.

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