92 400E Battery drain
 

I recently purchased a 1992 400E
and its having a battery drain issue. I have pulled every fuse one by one along with other relays located behind the fuse block and tested the negative battery cable and my tester light lights up. I also pulled the fuel pump relay and unplugged the power antenna with the same result. there's a silver box on the passenger side underneath the the hood up against the windshield with a few what looked to be control modules inside of it and then one located outside the box behind it.
I didn't touch the ones in the box but I unplugged the one outside of the box and still got a power reading on the negative cable.
I read these cars have a biodegradable engine wiring harness, could that be to blame? and if so is there a way to test.
I also wanted to add that it had this problem with the battery I purchased it with so I bought a new battery, still has the same problem

Check to make sure all the grounding wires on the chassis have good connection. Also check the wiring to the alternator.

Okay. Thanks

Take a screwdriver or something steel and , with the engine off , touch it to the back of the alternator and see it is magnetic. ( the center of the back ) Also do the same with the pulley side. Sometimes alternator diodes fail in an conductive state causing the battery to drain. This conductive state energizes the alternator windings causing the drain.

How much is your draw?

Couple years ago i had a 92 400E at our shop, i spent hours and narrowed the draw down to something with the Diagnostic Module. Customer declined to approve anymore diagnostic time so i stopped. However the draw went completely away when i removed the fuse supplying the DM with power.

Thank you all.
I ended up taking to a local indy shop that's always been fair to me. I'll keep you all posted

Battery drain...
 

My 92 400E had that problem. I finally traced it down to one of the radio amps in th trunk on the passenger side. What you have to do is hold your hand on each of the 2 amps after letting the car cool off for a while. Then, if one on them is still warm after a while, then that the culprit. To stop the drain you simply unplug the electric going to it.

a practical solution
 

Well chosen solution.

All due respect to all the knowledgeable folks on this forum, but I'm often amazed by the great lengths and hours some will spend trying to diagnose their problem rather than spend for 1 hour of trusted Indy time to get right to the cause of it. When I hit the wall on an issue, I'll bring my car in to have the oil changed and ask him to let me know what's going on.

I spend a couple of hundred bucks a year to refresh the relationship and then get loads of free advice throughout the year in return.

Works for me!

Do please check first , if the charging light bulb is iluminated as you turn the key ? no need to start the engine for this. If the bulb is not working in some cars it stops battery charging up