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I was helping out a neighbor with their car. 2008 Sonata four cylinder, I think that’s a 2.4? I have been working on cars for 15yrs so not much stumps me. This one was very odd though so I figured I would share online in case anyone else runs into a similar issue.

I could not find a wiring diagram for the car so diag was a little bit harder than it should have been. Car had no power to the interior. Power seats would not move, dome lights, no dash or gauge lights and security all dead. The dash clock was still working. Tested the battery, it was reading 12.4v as were most of the fuses in the under hood fuse box. Some of them read around 10v I think it was 10.8v. Interior fuse box in the drivers side dash read 2.2 volts on some of the fuses. Nothing on others. I was going to throw an ignition switch at it (no wiring diagram remember), but when I tested the plug I was getting the same 2v up there. The battery looked to be oe so I threw a charger on it just for kicks. When I opened the door power was back to the cabin, turned the key to accessory mode, gauges work, radio comes on etc. I didn’t try to start it, I just went straight to the parts store to get a new battery. It was Christmas Eve so I figured it would be best to get one reguardless. I put in the new battery, and go to start the car, all the dash lights and cabin power was working turn the key to start, I hear a click of a relay from under the hood, then every thing goes dead just like before I started working on it. I’m getting fairly frustrated at this point because the neighbor is asking me if they should get it towed which makes me feel like an idiot.

I’m thinking I have a power distribution or random ground fault issue. First thing I do is take out the new battery and pull off the ground cable. It’s held in place with two 10mm bolts. The car is kind of a sandy greyish color so I thought I could just wire brush the grounding plate and body. Nope! They primed and base coated right over the bodies grounding point. I had to Dremel it off it was so thick. Only the threads of the two ground screws were getting bare metal. The screws themselfs are not raw metal, I think they are blued so I have no clue how the car had been running for almost 10yrs or why the problem randomly presented itself now.

Hopefully this saves someone else the pita.
 

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You say 12.4 across battery ?? How about red on battery positive clamp,, and touch black to body or engine somewhere??

Follow negative cable from battery to body........ unbolt the lug from body,, grind paint off panel.. install lug with a lick of lube to slow rust growth..

Do same for cable lug at left frame rail adjacent to battery tray

We all should know that paint is an INSULATOR,,, not a CONDUCTOR...
 

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I have had a similar problem.
While in a supermarket parking lot and ice cream in the trunk, got into the car and turned the key to start
My 2001 Elantra, went from having dash lights on to dark.
No power to headlights, interior lights, nothing.
There was no click or anything.
It had happened once before but after one or more tries, it started up.
This time nothing. Took out test light.
Going from battery pos to neg, the test light bulb glowed bright
Everywhere else was dark. No tools.
Getting ready to call tow truck.
Tried one more time and it finally started.

Next day, cleaned up battery terminals, grounding points, engine grounds, chassis grounds
And just because it all started going wrong when I turned the ignition key to crank, replaced the ignition switch.
Next six months, no problems starting. No light problems.
 
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