Dodgy earth

[kdevitt]

Am having a few issues with the headunit in the car. Well its completely dead - so I guess its one issue!

I tested all the wires in the loom - and the earth had some power coming through it. Chopped the wire, and connected it direct to the negative terminal of the battery and the stereo started working fine.

So just wondering where the earthing point should be for the headunit and if its a quick enough fix?

[mrLEE30]

firstly how did you test all the wires in the loom?? What did you use? how did you acertain there was ''power'' coming through the earth?

you should have two live feeds to the stereo and one earth lead. one live is a permanant live and the other comes on with the ignition (its this latter one that carries most of the load) the earth can be connected to anywhere on the car body, but note painted areas can insulate the ground wire from ground, so best bet is to find a good non-rusted metal screwed fixing and attach to that. as for which lives are which you should use a multimeter set to read DC volts and see which is permanent live and which is live switched by the ignition.

if that is not clear then answer my first questions and i can begin to understand what you have done!

mrlee

[Morat]

Isn't there a third cable which tells the HU when the sidelights are switched on?

[mrLEE30]

depends entirely on what sort of head unit he has, some can conect to the dimmer circuit and dim with the cabin lights, some can be plugged into the cars phone system etc etc.

but almost all will have at a minimum:

1. a permanent live (low power designed to keep supply on the circuit board that stores radio station settings/speaker set up/memory etc etc)
2. a switched live (normally higher power whch will run the amp/CD/tape/lights etc etc
3. Ground/or Earth (actually its neither ground nor earth its technically the negative pole of the battery as ground or earth means exactly that, the potential of the earth!!)
4. a signal wire (switches to a 12V when required, used to tell the electric ariel/Amplifier/ect that the stereo is on and thus the ariel should be up/amp on/phone on etc - this is only a voltage signal and cannot actually run anything, it will send a voltage signal to a relay/internal circuitry telling it to switch on or off)
5. speaker wires, normally two front and two rear

then depending on the stereo you may have one or several RCA outputs, options to dim, phone connection, ipod connection, CD changer connection, RCA Input and probably a million other things i dont know about!!

if your car (and the E30 does!) not have a switched live (item 2) then it is not a problem to connect item 2 and 3 together, but realise the stereo will not turn off with the cars ingition, so your battery may not be too happy about this!

hope that helps,

mrlee

[kdevitt]

firstly how did you test all the wires in the loom?? What did you use? how did you acertain there was ''power'' coming through the earth?

you should have two live feeds to the stereo and one earth lead. one live is a permanant live and the other comes on with the ignition (its this latter one that carries most of the load) the earth can be connected to anywhere on the car body, but note painted areas can insulate the ground wire from ground, so best bet is to find a good non-rusted metal screwed fixing and attach to that. as for which lives are which you should use a multimeter set to read DC volts and see which is permanent live and which is live switched by the ignition.

if that is not clear then answer my first questions and i can begin to understand what you have done!

mrlee

I used a test light - both 12v switched and constant are fine. The earth lights the test light up dimly, nowhere near as bright as the other two.

[mrLEE30]

well this is your problem!! connecting the earth should not light anything!! but to be sure, when you say you used a light i assume you had a bulb with two pieces of wire?

where did you connect between? best bet is to connect one end to the negative of the battery. as you have already found the two lives assume they are ok. but if you connect your test light between negative and the earth wire and the bulb lights then you have a problem, either its not earth or its touching something live (in which case you would blow a fuse or be putting out a fire)

explain exactly how you tested the earth, and what the earth you refer to is.

does the stereo work with the two lives connected and the STEREO earth wire connected to and metal screw or bare metal part?

mrlee

[kdevitt]

Sorry - should have made my first post a bit clearer a I think! I figured the earth was the problem already - tested it out at the iso loom. I chopped the cable and earthed it at the neg terminal of the battery to test it, and the head unit started working perfectly.

Was just wondering where that wire leading into the ISO loom is actually earthed - so I can sort it out, or am I as well off just re-earthing it off the chassis?

[Alex]

you earth it of the top of the inner glove box

[mrLEE30]

ah ha!! what colour are you assuming is earth? black/brown if i remember correcly, but its been a while since i fitted a stereo in an E30 (and co-incedentally enough fitting one this weekend is a planned job as got a new one for Xmas!!)

personally i would ground the stereo directly to the chassis for now, and if you can wait until the weekend i will see what wires are behind my stereo, but if i was to guess it would go to a common earth behind the steering wheel area where your ECU lives (but my car is a LHD so may be slightly different. Problem tends to be with a 20 year old car many different people of many different abilities wire in many different stereos, so sometimes it may be best to leave any dodgy wires or i fear you may disturb something else more important then the stereo!! although it is a bit worrying that you have enough volts in the ground cable to light a bulb albeit dimly. for now i suggest you tape it up.

mrlee