Using the two blank fuse spots on my Prizm for accessories?

Question

After months of driving with the cord of my dashcam hanging over the rearview mirror, I finally decided that it was time to hard-wire my dashcam. At the same time, I decided that it was also time to eliminate the nasty ground-loop engine noise that comes from plugging in my phone or tablet into power and AUX-in (replaced the original audio unit) by installing a Bluetooth receiver.

I bought an add-in accessory socket, which I installed behind the knee-bolster using an Add-a-Circuit to make the electrical connection. I routed the dashcam power and the extension microphone for the Bluetooth down through the A-pillar and into the space behind the knee bolster, as well as the power line for the Bluetooth receiver which is double-sided-taped near the radio. I used the spare USB port on the Bluetooth receiver's power plug to power the dashcam.

The final task was to find a good fuse location to use for the Add-a-Circuit. The only two requirements are:

• Switched: Powered only when in Acc./On position.
• Max 10A original fuse: Limitation printed on the Add-a-Circuit instructions.

I noticed that the two fuse spaces shown above in red contain no fuses and are unlabeled in the Owner's Manual and also in the Service Manual, so I decided to try to see if one of them would work and I wouldn't have to split an existing fuse.

(Interestingly, I noticed that while my '95 has two empty spaces, this picture of the '96 uses the top-right space for the rear defogger.)

With my multimeter, I determined (I thought) that the top-right space is on when the key is on, and off when the key is off. I neglected to test the spot in "Acc" position.

I decided to put my accessory plug on the top-right fuse spot, tested that everything worked, zip-tied the wires in place, and reassembled everything.

When I later got to my destination, I had about 30 seconds remaining on a podcast, so I turned the key to "Acc" to stop the engine and let the last few seconds run with just the radio on. Imagine my surprise when the radio stayed on but the podcast didn't!

Of course, given that the '96 uses this fuse location for the rear defogger, it seems reasonable to me to assume that it is supposed to be on only when the key is "On" (like the rest of the HVAC system.)

All of that would be fine, except that it made me notice an additional issue: While it seems that the car cuts power to accessories such as the radio while cranking the starter, it (unsurprisingly) does not cut power to this fuse location. That means that every time I start the engine, I see the dashcam and Bluetooth receiver status lights flicker in time with the cranking noise.

This leaves me with a two-part question:

1. Will I do any damage to the car by leaving accessories connected to a place that isn't designed for them? Even if I don't actually do damage, could this make the car harder to start becausde it is diverting extra current away from the starter?

2. Might this result in damage to my dashcam and Bluetooth receiver caused by the current fluctuation while the starter is cranking?

I know that I can (should?) move the Add-a-Circuit to a different location, such as the radio fuse, but I'm still curious about the answer here.

Answer 1

1A) You will not do any damage to the car as long as the device(s) AND the fuse you're using do not exceed the capacity of the wire circuit for that fuse position. Assuming it's the same wire as the '96, the accessories you're attaching are unlikely to exceed the capacity of a wire intended for a defogger.

1B) Any difference in the difficulty in starting is going to vary far more with battery temperature and engine temperature than it will with a relatively insignificant draw from the devices you're adding. Many vehicles have a relay (BMW calls it an "unloader") that cuts accessory circuits during starting, but given that you're getting fluctuations (and not a complete shutting off of the devices) during starting, that means that this circuit at minimum does not have an "unloader."

2) Most likely not -- the designers have likely taken this into consideration. If you're extremely concerned, you could always turn them off/on manually when you shut down/start the car, which would also alleviate your concern about starting in 1B.