Electrical problem after fuel pump install

[neinnein_nein]

I think this will make sense to some of you guys who know more about electricity, it doesn’t to me although I just spent a whole day trying:

Engine off, fully charged battery with clean terminals reads 12.7v, kc hilites and headlights work.
if I turn ignition key to run or crank, it doesn’t crank and the voltage from - battery terminal to batt + side of starter relay drops to 6 or 7volts, after which nothing works, no lights nothing. Once I unscrew the + terminal of starter relay, move the wires out, put them back and retighten, I’m back to square one where all lights work.
If I turn the key again, same drop and nothing works until I unplug battery and + terminal and do it again.

that’s 5 volts coming back against the 12 because of a short somewhere right?

there’s a short somewhere that is triggered when in start or run, does that make sense?

Everything was working before I put fuel pump in. Disconnected and checked everything I did, the problem is there in all configurations..

 

[franklin2]

You have a poor connection,, or your battery is bad.

You can think of it like a kink in a garden hose. As long as you do not demand very much water from the hose, everything is fine. When you open the hose wide open and are demanding a lot of water, the kink becomes a restriction.

Your meter takes very little current to operate. Your lights don't take a lot either, maybe 10-20 amps. Your starter can take over 100 amps. A poor connection will supply enough current to run your meter or some lights, but not enough to turn the starter.

If you put your meter directly on the battery posts and you get the same voltage drop, your battery is drained or is bad. It's the same, with no or low load the voltage on the battery will float up to 12v. But when you put a big load on it, the voltage will drop very low if something is wrong with it. If the battery voltage stays high, ,but when you move the meter leads to the battery terminals and it has dropped, then you know there is a "kink" in one of the battery terminals. keep moving down the line till you find the "kink" in the electrical wire (the poor electrical connection).

 

[Uncle Gump]

Yes to what franklin said... you most likely have resistance in the battery cables.

Google voltage drop testing... then take that new found knowledge to the vehicle and test where the high resistance is coming from.

 

[neinnein_nein]

Thanks a lot guys, I’ll chew on that and look into voltage drop testing.

 

[Bruisedfruit]

I had this problem intermittently for the last year. My battery terminals were clean as well. Wound up being the connection between the ground cable eyelet and terminal eyelet was corroded. Never gave it much thought since I couldn't see the corrosion hidden between the two flat pieces of metal. Just food for thought. I felt like an idiot afterward, was a good time.