[Mgs] Soliciting advice on an electrical problem

Max Heim mvheim at sonic.net
Wed Jul 25 19:21:23 MDT 2018


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. The 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 <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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