If someone who understands automotive electrical systems can clear up a mystery for me, I would appreciate it. This is probably a simple thing, but with my limited knowledge, I can’t figure it out. I asked the question in another forum but got no responses that directly answered the question, so I am trying again in this more specialized forum.



If I use a voltmeter and check my battery while it’s in the truck (1987 F250, 300), it reads around 12.5 volts. If I check voltage from the negative battery post to the solenoid relay post attached to the positive battery post, I get the same. If I check voltage from the negative battery post to the starter side post on the relay, I get 0 volts. All so far is as expected. Here is where my befuddlement comes in. If I DISCONNECT the ground cable from the battery and have no other grounds to the battery, then I test voltage to the STARTER SIDE POST OF THE SOLENOID RELAY, I’m getting full battery voltage. How is this possible? What am I missing?

I should mention that I know the solenoid is good.



Thank you

 

If you are measuring from the negative post you will still get 12v. You shouldn't get anything if you measure from the the frame.

 

If you use a testlight doing the same test, it should not light or it will be very dim.

What's happening, you have many circuits in the vehicle. Some are tied to the battery at all times, like the memory for the radio, You didn't say what year vehicle you are messing with, but the later it gets, the more electronics and the more stuff is tied to the battery at all times, like the ECM which holds the memory in it up. These constant drains are very small. Small enough to where they will not drain the battery for several months, but on most modern vehicles after about 6 months the battery is going to be dead.

So back to your question, there is a path from the battery to the starter large cable. Who knows where it's finding it's path, but the power is leaving the battery post, running around through these electronic drains, and is coming out on the starter solenoid circuit. Since the starter is not grounded now, the voltage can go through the starter solenoid, go to the common ground of the starter, go back through the starter motor itself, through the brushes and back up the large starter wire and to the post where you are getting your reading. This current is very very small that you are reading, because your meter requires very little current to operate. That is why I said use a testlight, the old timey testlight with the regular bulb in it takes more current from the circuit to light up, so it most likely will not light or not light very bright. A testlight is a better tool to use in most circumstances troubleshooting vehicles, except troubleshooting the electronics of the vehicle.

If I didn't explain that well enough or if you have more questions, ask away.