[Mgs] Soliciting advice on an electrical problem

[Barney Gaylord]

The only things needing to be grounded on the 
dash panel are the map lamp and the turn signal 
indicator lamp. The only switch to be grounded is 
the wiper switch which has a black harness wire 
for grounding.  Each instrument should also have 
a black wire for grounding (mostly for the 
illumination lamps).  The only electrical 
instrument is teh fuel gauge with power in, 
signal wire to tank, and grounding through the mounting stud with a black wire.

The dash panel does need to grounded somehow to 
make the two noted lamps work.  That ground might 
be through the dash mounting screws and brackets, 
or it might be through the instrument mounting 
brackets in turn to the black wires.  If those 
two lamps do not light up, you may need to remove 
a bit of paint somewhere.  If anything else on 
the dash doesn't work (like the heater blower or 
wipers for instance), it's not a paint problem.


At 06:21 PM 7/25/2018 -0700, Max Heim via Mgs wrote:
>I had time to do a few checks with a test lamp.
>
>With key on I confirmed I have power to the 
>white leads at the fuse box. I removed the radio 
>blanking plate and was able to test some 
>switches. I have power to the back of the heater 
>fan switch, for instance, but it doesn’t come on.
>
>Are these switches grounded through the dash 
>frame? If they are, I think I know the problem — 
>the powder coat is acting as insulation. TThe whole thing is probably isolated.
>
>I was wearing good clothes so didn’t check the fuel pump
>--
>Max Heim
>'66 MGB


>>On Jul 24, 2018, at 12:26 AM, PaulHunt73 
>><<mailto:paulhunt73 at virginmedia.com>paulhunt73 at virginmedia.com> wrote:
>>
>>Ohmmeter is no good, it needs to be voltage 
>>when the things are turned on. If you have a 
>>bad connection then by its very nature it will 
>>be variable, and what might look good enough 
>>with the microscopic current passed by an 
>>ohmmeter can suddenly get much bigger when you 
>>pass real-world current through 
>>it.  Paradoxically the reverse can also be 
>>true, when testing a switch that hasn't been 
>>used for a long time for example, it can go 
>>high-resistance over time, lead you to think it 
>>is faulty, but will work normally when full 
>>voltage is applied which can burn through oxidisation.
>>
>>Points oxidising is a case in point, although 
>>in that case they can get beyond being 
>>'self-cleaning' when voltage is applied, and 
>>need attention. I've had a new SU pump and in 
>>the box was a slip of paper saying to clean the 
>>contacts before returning it as faulty.  I've 
>>got them going again simply by bridging the 
>>points with the tip of a screwdriver, which 
>>energises the solenoid and opens the points, 
>>then when they are released they wipe together 
>>which starts the cleaning process.  A couple of 
>>seconds gently prodding with a screwdriver may be all that is needed.
>>
>>You also need to check the earth to the pump is 
>>good, by looking for 12v on the pump body when 
>>the ignition is on.  If you see any voltage 
>>there - when you have 12v on the spade 
>>connection - then the earth is bad, it should be zero.
>>
>>As long as the two ignition whites at the tach 
>>are connected together the engine should 
>>run.  These should have male and female bullet 
>>connections on later versions (a continuous 
>>loop of white wound round an external pick-up 
>>on earlier), the 12v fed to the tach should be 
>>a spade and is not needed for the engine to run.
>>
>>PaulH.


>>----- Original Message -----
>>>Thanks, that was the direction I started 
>>>looking at. I checked continuity using an 
>>>ohmmeter between the 4-way and the fuse box, 
>>>and between it and the ignition switch (pin #2 IIRC).
>>>
>>>I also found 12V at the fuel pump feed. So 
>>>that is a little baffling. Fuel pump is from 
>>>car A and was formerly working. Maybe the 
>>>points stuck in the 2 years it has been out of service.
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