Ammeter question

[bigjones]

Hi folks,

Please excuse my ignorance in electrical things, but...

Would an ammeter that only reads up to 30 amps be OK in a Midget? The car has an alternator and I don't have anything other than the standard electrical equipment (apart from a back-up electrical fuel pump).

Also, does it need to read negative amps as well as positive amps? I'm thinking that if, for some reason, the alternator quit charging, the ammeter would read negative amps, is that right. And if it only read positive amps, it would be damaged.

Cheers!

(Glad my old Physics teacher is not reading this)

 

[Nelson]

I had my ammeter installed at Donald Healey Motors before I brought my Sprite back to the States. It is a 30-0-30 amps meter. My car has a generator and not an alternator but I don't think that would make a huge difference. It will read positive amps when charging and negative if there is a current drain.

 

[spritenut]

I have a 30-0-30 amp gauge in one of my Sprites with a modern jap alternator.
If it hits 10 amps, that's because it I drained the battery trying to start it in the dead of winter.
Also if you hook i9t up backwards, it will read + charge when it is discharging.
When my brother was a kid, he hooked up the amp backwards in his TR4 and just had to show me how the car was charging when the engine was off and the headlights one, it charged even more when he blew the horn!
He really thought he found a cure to the energy crisis by putting in an amp gauge backwards.

 

[Sarastro]

In short, a 30 amp ammeter should be fine. Actually, any instrument's range shouldn't be much more than the expected, or you lose precision. Not that you need a lot, in this case.

My ammeter is -20 - 0 - +20, and I haven't pinned it yet.

Yes, you're right that you want both positive and negative directions to show charge and discharge current.

 

[bigjones]

Many thanks for the clarification folks!

Based on that I just got this one from ebay:

nice looking ammeter

Looks like he has a bunch.

Cheers!

 

[bigjones]

The ebay ammeter arrived:

What would be a good way of testing it out? I've been a bit wary of electricity ever since I set my multimeter on fire. :laugh:

Also, any idea on the best way of hooking it up to the car's wiring (do I need to put in a fuse before it, for example?)

Cheers!

 

[Sarastro]

I don't know what year your car is, and I'm primarily familiar with the earlier ones. In those, there should be one heavy brown wire coming off the battery's ungrounded terminal. Disconnect this, and connect it to one terminal of the ammeter. A new wire from the battery goes to the other terminal of the ammeter.

In later cars, two brown wires connect to a terminal of the starter solenoid. In this case, those two wires go to one terminal of the ammeter and a new wire from the solenoid terminal goes to the other ammeter terminal.

This is all unfused wiring, which carries the full system current, so be sure it is solidly connected, doesn't chafe on anything, and connections are well insulated. Use terminals at the meter and other connections; don't just wrap wires around the connection posts. If you add any wire, be sure it is as heavy as the existing wire. I like to solder any splices in wires and cover with heat-shrink tubing, but insulated crimp terminals are OK. Try to avoid electrical tape; the adhesive on electrical tape deteriorates, especially with heat, and it eventually unwraps. Where the wires go through the firewall, be sure there is a grommet to protect it from chafing.

Be sure to disconnect the battery while working on the electrical system. Disconnect the ground cable at the battery to do this.

You really can't test the meter very easily before installing it, but if it's new, almost certainly it will be OK.

As for fuses--any fuse you add will affect the whole electrical system. Some people put a 60A fuse between the battery and the rest of the electrical system, but if it goes, you lose everything.

When you're done, the meter needle goes in one direction for charge, the other for discharge. There is really no standard for which direction means what--I've seen them work both ways. If you don't like the original setup, though, just reverse the connections at the meter.

If you're uncomfortable with any of this, get some help. You're working with a lot of raw power, and it can be dangerous.

 

[bigjones]

Steve,
Many thanks indeed for explaining all that to me.
I'm now pretty confident of doing a good job - I'll report back later when I've installed the ammeter.
Cheers!