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Sun Jul 27, 2014 11:16 am

This seems like the dumbest question but I have fried 2 LED test lights and after nearly an hour of searching can't find a definitive answer, so I'm querying the hive mind:

I am test running my engine on the pan and want to wire in a light for oil pressure. It was mentioned by Dale in one of his awesome test wiring posts but it doesn't have the full info. Obviously the ground will be supplied through the engine case, but where would I connect the positive to? I was thinking that the 15 terminal on the coil would be the obvious choice, but two ruined lights later, I'm not so sure.

Any help is appreciated.

Thanks!

Sun Jul 27, 2014 1:27 pm

Since you want to run an idiot light, the power must pass through that bulb, where it is situate. You need to run power from key on power at the dash, through the idiot light, to the rear sensor, and the grounded through the sensor.

Sun Jul 27, 2014 2:07 pm

Ok, would there be another way I could rig up real quick? I currently have no dash or wiring in place as I am testing the engine on a rolling chassis. I only have the rudimentary wiring necessary to turn over the starter at this point.

Sun Jul 27, 2014 2:15 pm

Hook the positive (clip) to the positive on your coil or MAIN battery starter wire.
Test probe to the oil switch.
Or vica-versa
Test lights are dumb.. they just need a power and a ground... from the same power and ground source...
In other words...
Just be sure the power source you are using is also the same source for the ground and those have to be shared with your engine and chassis..
.

Sun Jul 27, 2014 2:34 pm

LEDs usually burn out if they are hooked up backwards.

Sun Jul 27, 2014 2:42 pm

KTPhil wrote: LEDs usually burn out if they are hooked up backwards.

Good point... .. I hate them (LED test lights)

Use the vice-versa method I mentioned above.....
Clip (ground) to oil pressure tab and probe (power) to coil power..

.

Sun Jul 27, 2014 2:45 pm

LEDs require a resistor to limit current. What sort of led are you using?

Sun Jul 27, 2014 7:35 pm

Thanks for the info all!

I was just using a cheapo LED indicator light from an auto parts store. I will rig something up with an incandescent bulb. Just wanted to make sure my fundamental understanding of DC electricity wasn't at hazard here!