Help Needed with Amp Gauge Wiring!

I have a problem that you guys may be able to help me with.......

I was out in the car late the other night (just done my first 180miles since finishing it, so still de-bugging). I stopped for petrol....everything was working fine. Paid for my fuel (topped both tanks right up to go for a good run!), switched it on and noticed that it didn't seem to be charging? My amps gauge was sitting at zero, my volt gauge was under 12v, but the ignition/charge light was not on?

I gave up on the idea of a run and went home. I thought my alternator was shot as no ignition light at all when I switched on. Put the meter on the system and sure enough....no charging. Replaced alternator today....got an ignition light now but still not charging?

Now, my Amp gauge is wired in the positive line from my alternator, is this correct? Suspecting the amp gauge might be the problem, I by-passed the gauge and the system is charging great. I've now decided that the Amp gauge is obviously the problem and was not allowing current to pass from my charging circuit. Does this make sense? I'm now thinking that if the Amp gauge has died, then perhaps it has then taken the alternator with it as it has not been able to discharge (not too sure about this theory though as I know just anough about electrics to be dangerous!).

So, if I replace my amp gauge with a new one, how should it be wired? Currently the positive wire from my charging output on the alternator goes up to one side of the amp gauge in the dash, out the other and back down to my positive terminal on the starter. Is this right? I don't want to replace the amp gauge and have it blow again, taking another alternator with it!

Also, is there any way I can test the amp gauge? I checked for continuity but nothing....

The unit is a TIM 60-60

If the ammeter is wired correctly, all the current (both from the charging circuit and the loads) flows through it. If it doesn't pass current, nothing (really, nothing) will work.

Why do you have both an ammeter and a voltmeter? The circuit necessary for the ammeter is a potential source of trouble and unnecessary complexity, especially with a high-output alternator. Speaking of which, is your voltage regulator external or internal?

Mark McCaughran said:

Now, my Amp gauge is wired in the positive line from my alternator, is this correct?

....I'm now thinking that if the Amp gauge has died, then perhaps it has then taken the alternator with it

.... Currently the positive wire from my charging output on the alternator goes up to one side of the amp gauge in the dash, out the other and back down to my positive terminal on the starter.

....Also, is there any way I can test the amp gauge? I checked for continuity but nothing....

Click to expand...

No, that ammeter wiring is not correct. As wired it indicates only the current flowing from the alternator, which is either positive or zero (in other words it will use only half of the ammeter's needle travel), since current cannot flow into to alternator.

The intention of the ammeter is to measure current flow into (+) or out of (-) the battery. To get that, you need to connect the ammeter between the battery and the fuse panel that supplies the rest of the car. The alternator output should go somewhere on the load side of the ammeter.

Usually the sequence (current flow) starting at the battery + terminal is:

B+ --> short heavy wire (eg 10 gauge) --> big fuse (eg 60- 80 A) as close to battery as possible --> long heavy wire --> in one side of ammeter --> out other side of ammeter--> fuse panel and alternator output --> other fuses --> all the other small stuff (i.e. NOT the starter).

Note the starter terminal does not appear in this sequence. It is supplied all by itself by a really big and short wire (like 4 gauge or even 2 or 0 gauge) and current into it is not measured by the ammeter.

It is conceivable that the alternator took out the ammeter, and that the ammeter failure then took out the alternator. Seems strange, huh?

As you had it wired the ammeter must be able to handle the full maximum output the alternator is capable of. Conceivably the ammeter was not able to, and the ammeter opened up, by definition while the alternator was at its maximum output. This would cause the output voltage of the ammeter to rise until it found a current path to ground, possibly by punching through either the rectifier diodes or the regulator. Or maybe the ammeter just plain failed for no good reason with the same result.

So, double check the output current rating of the new alternator and make sure your new ammeter will at least survive that high a current (regardless of what it indicates when that is happening). Then wire it as described.

You can test the ammeter in two ways:

1. It should read no more than a very small fraction of an ohm from terminal-terminal with a good ohmmeter or digital voltmeter in low ohms range. IOW from teminal to terminal it should look like a dead short.

2. More directly: put the ammeter between the battery and some predictable 12V load like a radiator fan motor or headlight. It should read a value that makes sense for that load (fan: 10-40A; one 75W headlight: ~6 A). This method has the advantage of telling you for sure which side of the ammeter is which when you go to plug it in.

As for whether you should have one at all, that's a matter of taste. I know there's been some heated argument about that on this forum and I don't want to start it up again, but simply to say that they measure different things so it depends on what it is you want to know about your electrical system. The ammeter tells you what your battery and alternator are doing moment by moment in terms of current flow. This is useful if (for example) you want to know what kinds of loads various things place on the system (which you can tell only with the alternator not running) and (with the alternator running) whether and to what extent it is keeping up with those loads.

With a voltmeter you can tell (crudely) the state of charge of the battery (with the alternator off) and whether the alternator is keeping up with the loads (with the alternator running) and whether the alternator is overcharging the battery.

Voltmeters do have the advantage that when they fail they do not keep the car from running, whereas ammeter failure, when wired the conventional way as above, shuts everything down. And having one does require running the alternator output (another heavy wire) all the way up to the dash whereas otherwise it could go straight to the battery. So the complexity and probability of failure is definitely higher with an ammeter in the system.

In a way you are lucky you miswired yours, because its failure still allowed you to get home.

Mark:
Seconding what Alan said and if I may add a few notes.
Ammeters measure flow, voltmeters measure what is essentially electrical pressure. Most ammeters will require a heavy gauge wire as they are in series with your electrical system, and need to "See" the flow either way to indicate direction of that flow. Also that connection needs to be able to handle everything in your vehicle as far as load excepting the starter as Al said...that would most certainly fry your ammeter.
Voltmeters run in parallel with your system and measure the level of voltage so they can virtually be mounted anywhere in the battery circuit and to ground and give a reading....voltage drop on the meter being the load(s) on your system.
IMHO ammeters are more trouble than they are worth, and if you really want one I would try and find an inductive type that uses a pickup on your accessory line and does not route the current thru the meter.
I would bet that a high resistance in that meter basically fried it, your alternator could not find a load and went beyond the limits of the diodes and requlator.
Good luck whichever way you go
Cheers
Phil

Guys,

Thanks for the detsailed and helpful answers. I know see where I went wrong and it is good luck that my mistake allowed me to get home!


The reason I had a voltmeter and an ammeter was purely because the gauges came with the car and were already inserted in the dash, it wasn't really a personal choice. I just tried to use what I had.

Having read your explanations of the operation and the 'usefulness' of the gauge I think I agree that it is perhaps just not worth the bother, especially considering the wiring requirements and the failure mode if it happens to go pop! I was bidding for another gauge I had found on ebay, but I think I'll just leave in the one that I have as it sits needle pointing straight up and the light still works so it doesn't look too bad.

Thankfuly the car is charging now, as I have an ambitious 400mile round trip to Applecross (north west of Scotland) with a Stratos rep, a Marlin, a V8 Dutton and a Cobra at the weekend. It is one of the nicest drives in Scotland, so I hope the car holds up as I'm really looking forward to it.

Thanks again guys.

M.

awatkins said:

It is conceivable that the alternator took out the ammeter, and that the ammeter failure then took out the alternator. Seems strange, huh?

As you had it wired the ammeter must be able to handle the full maximum output the alternator is capable of. Conceivably the ammeter was not able to, and the ammeter opened up, by definition while the alternator was at its maximum output. This would cause the output voltage of the ammeter to rise until it found a current path to ground, possibly by punching through either the rectifier diodes or the regulator. Or maybe the ammeter just plain failed for no good reason with the same result.

Click to expand...

In this case discribed it does seem that Ampmeter is in the wrong place. if the ampmeter was to become an open curcuit then there would be no connection of the cars electrical system to the alternator and there would be no current flowing from the alternator. the alternator would appear to be not working if you checked the voltage at the battery or anywhere else because there would be nothing connected to the alternator. The ampmeter only needs to be able to read the maximun amount of amps that all the electrical components can draw if they are all turned on at the same time. the alternator will never produce any more amps then are drawn from it. even if you have a 180 amp alternator and the cars total load is 120 amps the alternator will never need to produce any more then 120 amps. it is common to have an alternator and ampmeter that have the same rating or higher so if there is a high load on the alternator ( charging a dead batery , spot light connected in the cigarette lighter, shorted out component ) and the alternator is to reach its maximum rated output, the ampmeter will not burn out. you will not burnout an alternator by trying to drawing more then the amp rating. the maximum load rating is all you will get. you can over heat an alternator by drawing a high load with insufficient cooling
ampmeters are useful when using a Generator because you can hurt a Generator if you draw a higher load then they are rated for. with an alternator it is best to use a voltage meter.

Good luck with your trip, Mark. Don't forget to take photos of all the cars.

I've also heard it said that Ammeters are a fire risk. reading the helpful posts above, I can understand why that might be. For that reason alone I going for a volt meter instead of the (period correct) ammeter in my P4 build.