Strange Talbot electrical gremlin

[wuvvum]

OK, I know I'm shit at wiring, but this has really got me baffled. 

 

The screenwash on the Solara stopped working last week.  As this is (AFAIK) the only thing standing between it and a shiny new MoT, one of the jobs for this weekend was to fix it.

 

I checked the wiring with a multimeter and it showed 12.5v when the switch was pulled, so I diagnosed a seized pump.  I popped to the local motor factor's and bought a universal pump, but that's when it started to get odd.

 

I plugged the new pump in and pulled the switch - nothing.  I pulled the wires off and checked them again with the multimeter - still 12.5v.  As soon as I plug them onto the pump and pull the switch, the voltage drops to about 3v.  I tried this several times, with the same result every time.

 

What's going on?  It's definitely not a duff pump as I tried it on the Volvo and it works fine.

[cobblers]

There's high resistance somewhere.

 

Test with your leads on a known good earth (engine block or something) and the positive terminal with the motor plugged in and stalk pulled - if the voltage is still 3ish then the problem is on the positive side (Switch/wiring etc). If the voltage is now 12.odd then the problem is with the earth.

 

Then it's just a case of working your way along the respective problem wire until you find the issue.

[KruJoe]

It's a funny one.

Are you 100% sure the switch is ok? I know it works when no real current is drawn through, how about a headlight bulb or summert?

[tooSavvy]

... you need a 'Bleach Bottle' M8.... 

 

tooSavvy

[Joey spud]

It sounds like you have got a bad case of VD, aka voltage drop.

 

This was the first thing that was drummed into me at my new boy training many years back.

Google 'voltage drop testing' it has saved me countless hours of misery at the roadside over the years.