Stumped on Electrical Issue

[Bob60]

Any electrical engineers out there. . .?

I'm generally pretty handy with electrical repairs - you pretty much have to be with several British cars but this one has me stumped.


Item: Spring loaded rocker switch that will make a circuit only when pressed then snap back to off - used on a Jensen Interceptor to activate a solenoid which opens the fuel filler flap. It had begin to only work occasionally.


Diagnosis: Removed switch, hooked up 12v power supply and tested - sometimes switched full 12v, sometimes only switched something like .8v. Figured it was grungy inside so I carefully disassembled switch and cleaned all the contacts with metal cleaner - See photo 1 - and reassembled.


Problem: Now, 3.1 volts bleeds through the switch even when in the "off" position. Disassembled again and took the guts out to make sure I hadn't put it back together incorrectly. The two spade terminals are not connected in any way, yet 12v supplied to one terminal will feed 3.1v to the other terminal. See photo 2.


I've done all my own Lucas electrical work for 40 years and I do not understand what is happening here?


Any opinions?


Photo 1


Photo 2 - how can this be when these are not connected in any way?



Thanks,
Bob
'73 Jensen Interceptor
various others

 

[Roger]

Well, of course they are connected, although that which connects them is supposed to be an insulator. I imagine it no longer insulates fully - maybe something on the surface you could clean off, or just maybe the insulation properties of the switch material, bakelite or whatever, are breaking down.
I had a rotor arm do that to me once - start leaking to earth through the distributor.

 

[Bayless]

Did you add a little grease inside? If so maybe you used the wrong stuff, conducting vs insulating.

 

[Bob60]

Good thought! I'll clean it thoroughly again and retest.

 

[David_Doan]

your dvm has very high input impedance - meaning it can measure voltage across a very high resistance. the cleaner you used is probably slightly conductive but nothing worry about on a 12v system. To verify, disconnect the 12v source and just measure the resistance across the switch, it will probably be greater than 1M ohms.

 

[TR3driver]

your dvm has very high input impedance - meaning it can measure voltage across a very high resistance. the cleaner you used is probably slightly conductive but nothing worry about on a 12v system. To verify, disconnect the 12v source and just measure the resistance across the switch, it will probably be greater than 1M ohms.

I agree entirely. Assuming your DVMs has a 10 Megohm input resistance, we can even compute what the switch resistance at approx 30 Megohms. That means the leakage current through the switch will be way down there, less than a microamp. Nothing to worry about.

To verify this, hook up something as a load, perhaps a small test light.

(PS, I'm not exactly a EE, but close.)

 

[Bob60]

Update -
I hooked up a small 12v test light and, as you predicted it would not illuminate. But the DVM still read 3.1v. I put a little carb cleaner on a q-tip and cleaned the contacts, dried with compressed air, and cleaned it again with denatured alcohol and again dried with air. Hooked up DVM and .01v bleed!
Amazing how a miniscule amount of cleaner residue can conduct current.
Thanks for the suggestions. Now I can get my fuel filler flap working again.

Bob