How many volts should I be seeing here?

[gazchap]

Can't seem to find an Electrical section in the Technical part of the forums, so I don't know if this is the right place.

The head unit on my MR2 Turbo stopped has barely worked properly since I bought it a few weeks ago, so I set about replacing it on Sunday. After taking it out, I now find that the new one I plugged in just flat out won't power up, and neither will the old one.

They do, however, work fine when plugged into my Corolla, so that rules out the head units.

The fuse is still intact in the MR2, and indeed the cigarette lighter socket still functions properly (which runs off the same fuse) so it's not that.

With a multimeter, I've checked the wiring. The ignition wire is getting around 11.86V when the ignition's on, should that be above 12V?

Also, the constant/battery feed wire is only reading just under 9V - should this be 12V too?

If these figures are too low, what should I be looking at in the car to boost them? Not a rewiring job, presumably?

[orhan]

You do know there should be two lives one is ignition switched and the permanent live. Both should be at battery volts and from the way you describe there may be poor connection somewhere and i would say start from the fuse and work your way theough the wires to head unit

[mattmoxon]

Does sound like a dodgy cable/connection somewhere, what is the battery voltage?

[VxDuncan]

Engine running or not? Typical charging voltage is around 13.8V.

[SV8Predator]

orhan said:

You do know there should be two lives one is ignition switched and the permanent live. Both should be at battery volts and from the way you describe there may be poor connection somewhere and i would say start from the fuse and work your way theough the wires to head unit

The OP has just explained the voltages of the two 'lives'!

I'd say the battery is flat with the voltages that you are experiencing. If not, then start the car, rev the engine for a few minutes and then measure the voltages of the two wires while the engine is running.

If they are still low, then there is a wiring problem, particularly with the permanent live if it is only showing 9v. That would make me think it has been wired wrongly and this wire is not providing proper battery voltage. Try taking a wire direct from the battery and connecting to the head unit. If it switches on, then this 'permanent live' (9v) wire is the culprit.

[gazchap]

The head unit that came with the car powered up at least, until I set about replacing it, so I'm assuming I may have pulled a wire somewhere and broken a connection somehow, if they should both be at 12V+.

As far as I can tell the battery is healthy, not confirmed with a multimeter yet but a 140mi drive home with the car (and numerous short journeys since) with lights and other electrical systems on didn't cause any of the usual discharging symptoms.

Looks like I'll have to trace the wires back then. I was kinda hoping to avoid that - being a Jap import it's a bit of a mess behind the dash where previous owners have added things like car computers and turbo timers etc.

[GarryA]

Your battery will still show 12.1v if it is flat and around 12.7v if charged.

Are you measuring the voltage between the live wire and the chassis (direct) or between the live and the return wire which you presume is attached to the chassis?

You could be looking for a return through another live which is going through a component and then to the chassis.

[CoolHands]

check earth straps too - just as important. As a temp measure you could run a jump lead from the neg battery to a clean solid earth point on the chassis somewhere. Sometimes earth straps corrode through / break / get left undone by previous owners etc

[gazchap]

Between the live and the chassis - well, the metal behind the dash.

[GarryA]

Have you got a piece of wire you can run from the neg terminal of the battery so you can put the meter direct on that and the live you want to test?

[gazchap]

Nope, but I can get some. I'll give it a try, presumably if it reads 12V+ in that scenario my earthing is buggered somewhere in the car?

[gazchap]

Voltage reads 14.3V on the switched live when the engine is running, as I'd expect. 11.8V on the permanent live, so it's almost as if it's losing 2V somewhere along the line.