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montyjn

Member Since: 06 Sep 2016
Location: London
Posts: 41

Radio Voltage Issue

Hi,

I have a Pioneer MVH-X380BT fitted to my 1987 v8 110 which has stopped working. When disconnected both the switched and permanent live wires give 12 volts, but this then drops to about 2 volts when the head unit is connected. Is this likely to be a bad earth issue? It was previously working fine..

Thanks in advance.

Monty

24th Oct 2016 11:56am

custom90

Member Since: 21 Jan 2010
Location: South West, England.
Posts: 19613

It does sound a bit like earthing, continuity test the earth to a good chassis point and see what results you get. Diesel$ Live$ Matter. ⛽️🛢️👨‍🔧🧰⚙️ RED, WHITE & BOOST! 🇬🇧

24th Oct 2016 12:28pm

Cupboard

Member Since: 21 Mar 2014
Location: Suffolk
Posts: 2971

2011 Defender 110 Puma 2.4 HT Corris Grey

Assuming you're measuring 12V or 2V between the radio's positive supply and it's ground supply, can you measure from the "12V" to a decent bit of body and then ground to a decent bit of body, preferable when the "12V" is in its "2V" phase.

When it's reading 2V between the radio's supply wires, if it's a bad earth then it should read 10V from the radio's earth to a decent earth. It's possible that both of the 12V feeds are dodgy but much more likely they're OK.

24th Oct 2016 12:37pm

montyjn

Member Since: 06 Sep 2016
Location: London
Posts: 41

Thanks for the quick replies. Will have a play this week and see what the readings are.

24th Oct 2016 2:43pm

montyjn

Member Since: 06 Sep 2016
Location: London
Posts: 41

So, having tested the connections I managed to isolate the issue to the permanent live supply which drops to 2-3v when connected to the radio (it reads 12v when tested unconnected). As a temporary measure I have switched the two feeds to both be switched from ignition (the radio now works; and as I have a battery isolator, permanent live doesn't really seem too necessary..).

Could there be an issue with a relay? The earth seems fine when used with a direct feed from the battery...

31st Oct 2016 2:06pm

Cupboard

Member Since: 21 Mar 2014
Location: Suffolk
Posts: 2971

Unlikely to be a relay because it's a permanent supply so isn't fed through a relay. More likely a dodgy joint or fuse.

31st Oct 2016 3:18pm

montyjn

Member Since: 06 Sep 2016
Location: London
Posts: 41

Thanks Cupboard. I assume the only way to check is to run a bypass wire back either side of any connection/fuse to test?

1st Nov 2016 10:04am

Cupboard

Member Since: 21 Mar 2014
Location: Suffolk
Posts: 2971

Depends how long you want to spend diagnosing the issue. Have you looked at the fuse itself? It's possible that you've just got a damaged fuse. It would be useful to measure the voltage at various points with the radio connected. At some point you'll have a 12V feed and at some point that will drop to 2V. If you start at the fuse box because that should be easy then just trace the wires.

If that's going to be effort, then so long as it's OK at the fuse box then you can just run a new (fused) feed from there.

1st Nov 2016 1:44pm

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