ammeter

[jond]

hi just spent a few hours working on the rocco one job on the list was to fit an ammeter i did ask about this in the projects section so following those instructions i wired up my second hand ammeter and instantly it blew a 15 amp fuse so i tried a digital one i got for a couple of quid off fleabay and it worked fine so my second hand one dont work i just want to make sure it was wired up correctly before i totally condemn it earth is easy to find next to the fuse box its just the positive i need help with looking at the back of the fuse box the red side is live all the time(about 6 spare blades) at the other side of the rear fuse box there are about the same number of spare blades switching the ignition on and only one of these became live there could be possibly another blown fuse as i did try a few of these connections with my dodgy ammeter does this sound like the right place to connect my ammeter to will try to post a picture over the next few days any info would be great thanks jon

[marcus]

Sounds like you're trying to connect it up as a voltmeter ie in parallel across positive and negative whereas what's needed with an ammeter is a series connection.

Positive on the ammeter is connected to your positive supply (ie battery if you want it to work all the time or ignition positive if not) where the ammeter is the only thing connected to the supply (other than the starter motor) and everything else is connected to ammeter negative.

In other words, for the ammeter to work, all the current flowing from the supply (other than the starter) has to pass through the meter.

The reason the fuse blew is that an ammeter has a very low internal resistance and when you connected it in parallel, that low resistance allowed a very high current to flow.

Not sure why your digital one didn't blow a fuse though.

[jond]

Thanks for the reply plenty of information there i think the key here is i didnt know the difference between an ammeter and a voltmeter i thought they were all the same the first gauge is for amps any info on wiring this up so it wont blow a fuse the second gauge is for volts and thanks to the info i know why it works ok and wont blow a fuse which one is best to have fitted does the volts one give an indication of the alternator condition and the amps one let you know how the battery is working thanks

[jond]

Just done some research on the net it seems that fitting an ammeter can be dangerous if fitted incorectly and not really any use fitted to a car with an alternator the low resistance in the unit can mean high current flowing through thin wires which could have meant a fire glad i asked the questions on here the voltmeter is a lot safer to fit apparently as it has a high resistance . I will check everything over tommorow to make sue everything is ok and fit that instead any input on this issue would be great as it could stop someone else making the same mistake as me , cheers ps ammeter now for sale

[Nate]

Most ammeters that I've seen have 2 huge connections on the back of the gauge. As said above, one side connects to battery +ve, the other connects to everything that used to be connected to battery +ve. This leaves 2 huge, unfused cables coming in to the cockpit. Even when wired correctly, if something damages the insulation on the cable, you've got a fire waiting to happen.

You can get some that have a sender that fits on the battery, with small, low current and most importantly fused wires running to the gauge. this is a much better option, and generally the gauge would only work when ign is on

[jond]

Thanks Now fitted the volt meter and hopefully all sorted While working on the electrics i decided to sort out the radio wires Found positive and negative plus all 8 speaker wires There are now a few left that i am not sure about 2 more brown wires one yellow/white and two blue/grey These seem to be bunched together in the loom has any one got any idea what they could be for i tested them for power with ignition on/off but nothing there thanks jonathan

[marcus]

If you can wait until this evening (might have to be tomorrow), I'll scan the relevant page from Haynes and send it to you.

From memory, the white/yellow is for an electric aerial and the blue/grey for illumination.

[ianaudia4]

Yes, the blue/grey cables are for illumination, put the headlights on and test them with a multimeter.

[jond]

ahh thats it seem to remember i may have asked the same question with my last rocco thanks for the info ill tape them up as i wont need them wonder why there are 3 earths maybe one wire has been cut so it looks like 2 also now ive got a voltmeter fitted i can run a spur off it for my switched live to the radio

[marcus]

radio.jpg

Scan of the Haynes manual radio page, the pencil annotations are mine and relate to my car.