Radio shuts down when I hit the brake

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Philo
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Radio shuts down when I hit the brake

Post by Philo »

So yes, after a recent head unit install sometimes when I hit the brake pedal the head unit shuts down and then comes back up. Like a reboot. My initial thought is the head unit power is on the same circuit as the brake light switch, but why would that cause the head unit to shut off. It doesn't happen all the time so it's a bit weird. The head unit is powered up on the original wiring for the old radio.

Any thoughts ?
kojo96
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Re: Radio shuts down when I hit the brake

Post by kojo96 »

Check your ground connections. In my 84 the radio would get some static when I applied the brake pedal, this only on AM (weaker signals). Going through all my ground connections cured it.
Mike W.
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Re: Radio shuts down when I hit the brake

Post by Mike W. »

Philo wrote: Sep 22, 2020 8:24 PM the head unit power is on the same circuit as the brake light switch,
You answered your own question. The E28 fuse box, if you still have it, is marginal. As are the old bullet fuses. When you hit the brake lights, the initial inrush when the filaments are cold, is greater than when they are just on. Plus yours is an M5, which would have a 3rd brake light. So, 3 brake lights, a head unit that probably draws more than stock and may be more finicky than an 80s stereo, is enough to momentarily drop the voltage enough to shut it off.

Clean the fuse contacts, wire brush, scotchpad or the like, not sandpaper, and put in a new ceramic fuse, ideally a brass one, not aluminum. Then cross your fingers. If that doesn't work you either need to power it from a different fuse, or perhaps a diode and big capacitor might work by the radio. You are not the first with this problem.
vinceg101
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Re: Radio shuts down when I hit the brake

Post by vinceg101 »

^^^What Mike said.
Did the M5 come with the Auxiliary Fuse Holder (that little 2-fuse holder stuck on the side of the main fuse box)? If so, this is where you want to wire your head unit to. That main dash fuse is overloaded as it is even without an aftermarket HU, lightening its' load is always helpful.
cek
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Re: Radio shuts down when I hit the brake

Post by cek »

vinceg101 wrote: Sep 23, 2020 11:44 AM ^^^What Mike said.
Did the M5 come with the Auxiliary Fuse Holder (that little 2-fuse holder stuck on the side of the main fuse box)? If so, this is where you want to wire your head unit to. That main dash fuse is overloaded as it is even without an aftermarket HU, lightening its' load is always helpful.
There are two connectors on the Aux Fuse Box:

- The 2.5 RD connector - Input is always hot, un-fused. This is the one you want to use for the always on head unit power source.
- The 2.5 GN connector - Input is hot in RUN only (and is un-fused). Do NOT use this one for the 'accessory' input on a head unit. Because it wont' be on when ignition switch is turned to the accessory position.

I learned this the hard way.

However, that point is moot for this discussion because the real culprit is the always on input.

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cek
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Re: Radio shuts down when I hit the brake

Post by cek »

And, while we're at it, after doing it both ways, I now prefer to use the extra connections on the C302 Accessory Connector for adding always-on electronics. Specifically Pin 3 which comes off of Fuse 4 which is a nice healthy always on 25A.

I prefer this because I don't like pulling additional wires through the firewall. Pin 3 is unused and it's easy to find a connector from a spare harness.

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Philo
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Re: Radio shuts down when I hit the brake

Post by Philo »

God ! ... this is why I love this board. You guys rock. Thanks for the pictures Chuck !! Will run a new wire to that cool little two fuse mini box. I do have a wideband 02 sensor already wired to that fuse location so hope the head unit wont overload the fuse. I guess I can put the 02 power lead on the non-ignition auxiliary fuse location and the radio on the ignition auxiliary side. I really don't listed to the radio with the engine off, with two massive power amps I drain my battery in no time :)

Thanks again.
cek
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Re: Radio shuts down when I hit the brake

Post by cek »

Philo wrote: Sep 23, 2020 5:07 PM God ! ... this is why I love this board. You guys rock. Thanks for the pictures Chuck !! Will run a new wire to that cool little two fuse mini box. I do have a wideband 02 sensor already wired to that fuse location so hope the head unit wont overload the fuse. I guess I can put the 02 power lead on the non-ignition auxiliary fuse location and the radio on the ignition auxiliary side. I really don't listed to the radio with the engine off, with two massive power amps I drain my battery in no time :)

Thanks again.
Who's Chuck?

You should check your grounds too.
Philo
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Re: Radio shuts down when I hit the brake

Post by Philo »

Ahh, sorry, excuse me your greatness :) .. "Charles"
Mike W.
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Re: Radio shuts down when I hit the brake

Post by Mike W. »

Philo wrote: Sep 23, 2020 5:33 PM Ahh, sorry, excuse me your greatness :) .. "Charles"
I believe it's Charlie. At least that's what I call him. Mr Gray is the one who is emphatic about Charles.
Philo
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Re: Radio shuts down when I hit the brake

Post by Philo »

Thanks for all the help guys. I found the issue :) I pulled the radio and inspected the wires and found a bare wire end that was touching ground and switching off the radio. The wire was connected to the switched power line. I't was a left over switched power leg used for the satellite radio control box that I never terminated. Man I feel stupid, I could have started a fire.

Thx for the help !
Philo
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Re: Radio shuts down when I hit the brake

Post by Philo »

Dam, still happens but not as often. Back to square one, gonna run a new power line.
Mike W.
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Re: Radio shuts down when I hit the brake

Post by Mike W. »

Yeah, if it was shorting out enough to drop out the radio I would have thought it would blow the fuse.
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