A few electrical questions

[soutie]

On my 1989 rebuild I have a few questions from a non sparky like me, like should the white ignition switch positive wire read negative with my Sealey continuity tester when the ignition is off? When the ignition is on it reads positive.

Plus,  I have an exhaust gas temperature gauge which positive wire, when not connected to the ignition feed wire gives a negative reading when I test it with my continuity tester. When connected up it gives a tempreture reading. Does this look normal?

[Eightpot]

The ignition switch gets its power from the brown permanent live and distributes it to the white wire and others when you turn the key. 

An EGT sender is a resistor which is permanently connected to earth (your manifold). The level of resistance changes with heat and the guage translates that into a temperature reading (it's basically a volt meter).

[Snagger]

White wires should only be powered with the ignition on, with earlier vehicles and possibly later vehicles needing the key to be at the II position.  The green wires should be live with the key at I or II, but all should be dead with the key at O.

It sounds like you may have the wiring connected to the wrong terminals on the EGT gauge - you should not be getting negative deflections.  With only one wire attached, it shouldn’t be deflecting at all unless it is earthing through the casing, which would suggest an internal fault.

[soutie]

Thanks for the replies. I am using the continuity test as per the photo. The gauge has a plastic casing and is digital. I have checked and I had wired in the positive feed and negative correctly. The wiring for the probe has a red wire with a blue terminal and a yellow wire with a red terminal. I have assumed the red wire is the positive. But to make sure I also wired them in the other way round. Both ways round the gauge works. I then rewired it with the probe red wire the positive plug. Still not sure which way round they go.

I have check the instructions and it does not mention the wire colour and only had a positive or negative on a black and white diagram.

[Escape]

Depending on where you're measuring, it is very possible that you are measuring continuity to negative through some other component. With all the different circuits in parallel, you always need to make sure you isolate what you are trying to measure. To test the ignition switch for example, disconnect all but the brown feed wire from it to see what gets power in which position.

[FridgeFreezer]

Jeebus I'd splash £9.99 on an actual multimeter and use that, at least you know what you're measuring then. I'd guess the Sealey tester is just an LED in a box, I really hope you didn't pay the £47.99 RRP that sealey are quoting on their website as that's shocking - total value of that thing is about 47p on a good day. If the sealey thing is not super well made it could well show "negative" voltage the moment there's a tiny amount of volt drop in a wire that takes it below the ground reference connection - and as Escape says that really depends where you're measuring and what else is "downstream" of it.

EGT probe may well be is a type-Kthermocouple that generates its own voltage (but no current) across the probe wires, so you may see (very small) voltage on the probe wires all the time. It's written on the back of the gauge

[soutie]

FridgeFreezer these are the two multi meters that I have. How do I use them to test continuity and what is positive and negative?

[FridgeFreezer]

 

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