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MG Midget and Sprite Technical - Amps and such

You may remember I made up my own wiring loom for my Sprite. I am now arriving at the stage of fitting and connecting up all the electrical stuff like each of the lights, indicators etc. My plan is to use a 12v car battery with leads with crocodile clips so I can feed power into the relevant colour coded wires at various points in the loom. I can then test each component as it is installed and before going on to add the next item.
That way I hope to keep some logic to my progress rather than connecting everything up first and then testing, which would probably end up with random faults I would find difficult to trace.
Does this sound like the way to go?

I will put an inline fuse into one of the connection leads as a safeguard. Would a 10Amp fuse by appropriate? - or would a 12v bulb be a better idea?
GuyW

If you have fitted all the switches and wired them up, but not the consumers a run through the switches will test them (and the wiring isolations though as the loom is new it's pretty safe to think your isolations are good).
Then you could connect the loads one at a time and test them before moving on to the next.
If you find a consumer that doesn't work or blows the cars fuse(s) isolate or fix it before moving on.
That way you won't need to bother with wander leads, but having said that your idea is fine if you want to go that way. Nothing wrong with it.
The 10A fuse should be fine, except possibly for blower fans (or heated windscreen, beer cooler etc - just kidding). The 12V lamp won't tell you anything except that there's voltage present, which you already know. A voltmeter connected between the tested circuit and earth would tell you more because it would show voltage drop if any. The best place to connect the voltmeter initially would be at the consumer because if doesn't work but the meter says battery voltage is present it means the wiring and any joints, relays, fuses, etc along the way are good so the consumer (eg: headlight for example) is either a dud or not earthed. Simplifies the testing.
If you have one handy it's a good idea to have an isolator switch on the +ve battery pole where you can get to it quickly. Just in case....
PS: not a fan of Croc clips because they pop off or touch things they're not supposed to especially if you're working alone.
Greybeard

Thanks for your comments Grey, that's helpful.
I am starting from the components though as the switches aren't in yet. No switches until dash is in, no dash until windscteen, no windscreen until wipers ...!
But for a diversion I am fitting all the rest end stuff first and thought I would test as I went along. I made the loom as three sections, connected by multiplugs. So the bonnet loom has one plug and all of the behind the dash wiring goes through another.
My intention was to test the wiring, components and connections for the rear cockpit loom and back as one unit first.

If not crocodile clips when working alone, then what?
GuyW

Ah I see. Fair enough then. If you can find crimp connectors that are a good fit on the pins and in the sockets of your multiways (without being crimped obviously) that's one way.
Greybeard

I think your method is good Guy, something like I'd do myself. 10A fuse about right - if you have a short to earth at the component end or in the cable itself the fuse should blow. A bulb is not as good as Grey says. I'd probably use croc clips as that's what one often does with electronics type testing although you have to be careful at the battery end of course! I assume you don't have a 12v bench supply.
Bill Bretherton

No 12v bench supply though I could use an old battery charger. It charges at 2Amps, ut since I have a reliable 12v battery that just seems easier with less potential of messing up my vintage charger!
GuyW

Have you got a circuit tester on your multimeter----------lot safer and the bip noise gives a feeling of satisfaction of finding a circuit that works
If you have to use a battery ,use one of them little square 9v batteries, can't hurt anything with one of them
William Revit

Willie
Whilst I understand the safety factor, it can be useful, surely, to test a circuit with 12v to ensure there is no volt drop and to "stress" the circuit with the voltage and possibly the current it is designed to carry. Perhaps use a multimeter initially as a basic test, assuming you can get a test probe to both ends!
Bill Bretherton

Continuity check function is good up to a point Willy. But only really for continuity testing point to point. Not really man enough to use for isolation tests I think. A big battery and a voltmeter are better (and a megger better still, but I suspect few of us have one). But still a good tool for finding breaks.
Little 9v battery - hmm, not so much. Most decent multimeters use them anyway, so no real advantage over the continuity check function. And it won't impress a headlight or a horn for example, so still need the big 12v battery for consumer testing anyway.

Can I just mention the battery isolator switch again? (Nudge). Essential equipment IMO for our cars with their rather crude electrical systems. I have the red-key type, but I'm now pondering the best place for a double pole 300A "thump" switch to kill everything instantly. You know - the big red button "E stop" you have to twist to reset. They're not even expensive, although they are hellish ugly.
(Overkill? Moi?)
Edit: crossed with Bill. And you're right - don't use the 2A charger. It probably wouldn't end well.
Greybeard

Don't forget if you are using a 10A rated glass fuse, that's its instant failure rating and will probably fail rapidly at about 6amps. The later type fuses are sized by their rated capacity and will carry 10A without failing. That's why the glass fuses in the fuse holder seem excessive!
Graeme Williams

This thread was discussed between 30/09/2019 and 06/10/2019

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.