- My model of MX-5 is: 2019 ND RF
- I’m based near: Manchester
- I’m looking for technical help or recommendations on: hard-wiring electronic accessory
Dear All,
I purchased a new two-radio set for my ND a few weeks ago (amateur radio VHF, not CB, though that’s likely irrelevant). The radio body sits in the boot on the left hand side screwed to a cross-brace behind the carpet. From there, I have a 12V (positive) power wire running forward under the trim on the left of the car to the fuse box where I’ve tapped the “accessory” cigarette lighter circuit using a (fused!) Add-a-Circuit tap. There is also a control line that runs alongside the 12V wire to the radio head unit sat just forward of the gear stick. The 0V (negative) wire from the radio is attached to a bare metal bolt close to the mounting point in the boot. This is the way I’ve always wired radios in other vehicles in the past (notably previous Landies).
The two-way radio itself works well in this position. However, after installing it, the car would occasionally complain that I’d left the “acc” on when exiting the vehicle, even though I’d ensured I’d turned the electrics off with the starter button. Then, just a couple of weeks ago, while driving I suddenly had an amber “battery management malfunction” message pop up on the dashboard. This continued intermittently, until just the other day the dashboard went red with the battery warning light on, messages about the battery management system, and telling me to stop the car (I didn’t, and it was fine).
So obviously the electrickery in the vehicle doesn’t like me tapping the cigarette lighter fuse in the fusebox and then grounding the radio close to its mounting point in the boot. Can anyone suggest how I should be wiring this in? I can’t believe NDs are closed to adding similar electronic devices (e.g. audio power amps, etc).
[edit] Forgot to say, the problem goes away if I disconnect the radio!
Cheers,
Chris
I considered doing what you did when I fitted my dash cam but chose to plug it into the 12v socket instead.
I would disconnect it and see if the problem reoccurs as the issue may lie elsewhere.
Heh, thanks @Teleman. I should have said – the problem goes away if I disconnect the radio 
Have you tried plugging the unit into the accessory socket rather than using the circuit tap. It should then pick up the ground (0v) and 12v from the same source which helps with further diagnosis.
It could be that you need a separate negative from the fuse box rather than bolting it to the boot.
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That is extremely odd.
Earthing point is irrelevant so long as it is a good solid clean chassis connection, ideally short and close to the AR kit.
The accessory socket is isolated by the ACC relay, see the circuit in this link
It suggests something else is bypassing the ACC relay, possibly as a result of your addition IF you were the first person in there. But there are a lot of things connected, and all sorts of them could be acting up IF they have a different extra source of 12V.
Is that the only power feed to the AR kit?
Is the control wire to the AR powered from anywhere except the AR.
Those wiring diagrams are generally very good once you understand their style. There may be some LHD/RHD allowances to be made, but the basics are very useful.
EDIT.
Have you a nice big filter choke and caps on the 12V going into the AR kit to prevent it talking to the car electrics and security? I notice from the MX-5 circuit the Commander (start button module) shares some of that power wiring.
@RichardFX thanks for the link to the schematics! Re your comment on the decoupling cap, the radio already has a hefty filtering arrangement on the power supply (it’s not a cheap Chinese CB!), and actually I was wondering if there was a discharge current from it that’s causing the “don’t forget to turn the power off” message. As for RF getting into the vehicle’s circuitry (which could definitely be a problem close to 433MHz), the problems don’t at all correlated to transmit cycles, so it seems unlikely it’s that.
Will keep prodding and poking…
You need to connect the + and - from the radio to the battery directly, via a fuse in both lines - at the battery.
Make sure you use sufficiently heavy cable.
What is the radio make/model?
Where is the antenna mounted?
73
G6DU
@Troppo It’s a Yaesu FT500D, antenna mounted on the left of the boot hatch just out of the way of the transome when folding the roof up and down.
By the looks of it, connecting directly to the battery seems to be discouraged, and I can understand why if it bypasses any battery condition monitoring, which is why I’ve not done it. Cheers and 73.
OK on the antenna.
The Yaesu needs a direct connection to the battery - it pulls relatively high current on tx, and using the chassis as the negative is bound to cause problems with the car’s electronic systems - as you have found.
Standard practice with mobile radio systems is to connect directly to the battery with fuses at the battery. Recommend you do that.
73
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If anyone is wondering what 73 is about, think it refers to this.
“73” is a Morse code abbreviation that means "best regards
Chris and i are amateur radio operators.
His callsign is M0DWK and mine is G6DU.
Yes, 73 is an amateur radio abbreviation that means “best wishes/regards”
Kind of like a secret handshake…
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