Everything on Fuse 21 Circuit Dead, Fuse Not Blown

[mattnsb]

I removed my OBC and used an OBC delete panel to create a magsafe cell phone charging mount and USB port, connected to the "always on" power supply that previously went to the OBC. Since it's always on, it has a power button. Everything worked great. Then, the other day, I went to power it up, and when I hit the button, the stereo and everything else on that circuit (fuse 21) died. It acted like a blown fuse, but the fuse was intact. I swapped it anyway, but that did not resolve the issue.

Today, I swapped the "always on" power into my little cell phone charger OBC delete panel with switched power, and it worked without a hitch. No shorts there, and now it's not connected to the Fuse 21 circuit. But still the stereo, cabin light, and trunk light are dead. Further troubleshooting, I had a closer look at the fuse box. I did notice that Fuse 21, while seated, feels looser than the other fuses. It seats, but it's wobbly. I tested power there, and got 2-3v on the outer socket and 12v on the inner socket. Is my problem there? Seems I could tighten up the fuse connection, but I'm afraid to start digging around unless the experts think I should.

Any ideas?

[It's Soda Not Pop]

https://www.r3vlimited.com/board/forum/e30-technical-forums/car-audio-electronics/351037-the-infamous-fuse-21-problem

[It's Soda Not Pop]

This is what AI says:

When BMW E30 fuse 21 is intact but the connected systems are not working, the most likely issues are a loose connection at the fuse box socket or a break in the red/green power wire leading from the fuse box to the various components.
Systems on Fuse 21 Circuit
Fuse 21 (7.5A) provides constant power to several interior components:

• Interior (dome) lights
• Trunk (luggage compartment) light
• Glove box light (on early models)
• Auto-charging flashlight (in the glove box)
• Radio/antenna (memory function)
• Multifunction clock or On-Board Computer (OBC)
• Service Interval (SI) indicator on the instrument cluster
• Ignition key/seatbelt warning system

Troubleshooting Steps
Since the fuse itself isn't blown, the problem is likely a loss of power upstream or a shared failure point in the wiring or a component itself.

1. Verify the Fuse with a Multimeter: Visually inspecting a fuse is not always enough. Use a multimeter or test light to check for 12V power on both test points (or metal ends) of the fuse while it is installed in the fuse box. This confirms power is reaching and passing through the fuse.
2. Check the Fuse Box Socket: A common issue is that the fuse socket itself is loose or corroded and not making a solid connection with the fuse prongs. If the fuse feels wobbly, this is likely the problem.
3. Inspect Common Wiring Failure Points: The red/green wire from fuse 21 splits into multiple branches. Inspect known problem areas where wires can fray or short:
   • Trunk Hinge Wiring: The harness in the flexible boot near the trunk hinge is a very common point of failure due to repeated opening and closing.
   • Glove Box Light Assembly: The glove box light switch and wiring can become loose or short out, especially if the bulb is missing or installed incorrectly. Pull the glove box flashlight out of its cradle and check the contacts for any shorting objects.
   • Behind the Radio: Aftermarket or poorly installed radio wiring is a frequent cause of shorts on this circuit.
   • Under the Shifter Boot/Center Console: Wires running through this area can wear and short to the chassis.
4. Test for Power at a Component: If the fuse is good, use your multimeter to check for 12V power at one of the affected components, such as the dome light or the radio harness. This helps narrow down whether the power loss is between the fuse box and the component, or within the component itself.
5. Look for a Corroded Junction: The red/green wire branches at a junction in the loom near the passenger-side sill/kick panel area. Corrosion in this junction can cause intermittent power loss or dim lights.