Need help tracing short. Fuse keeps blowing within minutes.

Is it two minutes of OK, then all the sudden, full-on short circuit, then fuse pop?

Or a more gradual, "fuzzy" short circuit where you can see the fuse lead slowly disintegrating (it and the wires near it would probably be hot)?


--Mike

 

Dear 308,

Having had a british car, (mga), I am familiar with the evils of lucas wiring, still have a scar across three fingers from a wire turning white hot as i tried to remove it --- any way, another story.

The big news is that british electronic equipment, specifically the stuff wired by lucas is in its own seperate reality - It is powered by smoke, yessiree, folks if you cut the wire and smoke comes out ---guess what, nothing works.

Lucas wasn't called the prince of darkenss for nothing.

The germans invented intermittant wipers, Lucas invented intermittant lights and ignition. pretty cool dude eh??

so, If you don't detect a warm wire going to items on the afflicted circuit, try unplugging or disconnecting items one at a time. have lots of patience and lots of fuses.

You have my deepest sympathies,
chris

 

...that the reason Brits drink their beer warm is Lucas used to make their refrigerators.

 

Buy a bunch of fuses! You could always put a bullet in there, the guys on Myth Busters proved that it won't go off on you...
Or, unplug all those items off the circuit. Then you'll know if it's in the wiring leading from the fuse to the accessories or related to an accessory. Re-install each, one at a time until you find the culprit.

 

Why don't you remove all loads from the circuit and place an ammeter in series after removing the fuze. See which one is taking all the juice. My ammeter will cope with 10A and has a delay fuze so you can get a quick reading before the internal fuze blows. Or if you don't have an ammeter, see which load makes the biggest spark when you reattach the wire for each. 10A will give you a pretty good spark. The spark test is crude but revealing in the absence of a good ammeter.

If you remove all the loads and still have high current draw, it's a wiring harness problem.

 

The best way to diagnose this type of fault without having to change counltess fuses in the car and ammeters is as follows:

(I'm assuming you have spade fuses)

1. take a blown fuse and carefully cut away some plastic from the top of the fuse. Carefully solder in two wires and connect to a HIGH wattage 12v bulb. 55W bulb is ideal. This tester can be reused again. If you have bullet or glass fuses then you can connect with crocodile clips instead.

2. plug in the fuse body with test lamp into the fuse box.

3. when voltage is flowing in the circuit the lamp will light, but the overall current in the circuit will be limited by the bulb (you MUST use a high wattage bulb). The bulb will burn, DO NOT touch it. A 55W bulb will draw 4.6A. If the current in the circuit is limited by anything else of higher resistance, i.e. the radio, then the bulb will not light since there will be insufficient current in the circuit but the bulb filament will allow electricity to flow in the circuit (current will be governed by the single highest resistance in the circuit).

4. fully diconnect the radio, see if the bulb goes out.

5. fully disconnect the volt meter, see if the bulb goes out.

6. If the bulb goes out then you found your problem

7. if the bulb is still on after disconnecting the two items then there are 2 further possibilities; either more consumers on the circuit than you think, in which case you need to identify them and isolate, checking the bulb each time. Or there is a sort circuit or bad connection in the wiring.

Fixing a wiring problem could be a nightmare, best solution is to run direct wires to the consumers, disconnecting the loom wire at each end.

From your description, my guess is a failiure in your radio, probably the amplier transistors, but on the other hand, if it's a moving coil volt meter and it's old then that could be the problem too.

One last thing, not to scare you, BUT, take your time, take the dash out and find the fault. This is exactly how most car fires start. If you find the problem then things will be fine. IF you don't find it, or are not convinced get to an auto electrician. If you like your car then don't risk it !

Hope this helps,

Rob.

 

Danger Will Robinson!

In general, our radio-shacks style DMMs may not be useful in tracing this kind of short. The currents we're dealing with are just too much (say, > 10A if you're blowing a 10a fuse, right!).

1. If we have similar DMMs, in the low amp modes you'll blow an internal fuse (no biggie, but you may have to go to radio shack to get a new one).

2. In the 10A mode, which might be unfused, if you exceed it, you'll nuke your DMM. (Ask me how I know.)

 

Just checked my reference books and 0.1A is spot on for these consumers, although you must have had the radio volume set low. If you have a big amplifier you can pull significantly more during loud playback. I can calculate the approx. current draw for a given amp if you need.

Regarding the use of a DMM. Here's my 2 cents; Most professional auto electricians will use only a test lamp to isolate a fault to a particular part of the wiring before reaching for the DMM. The first thing they're taught in tech school is NEVER use an in-line DMM to check current flow (inductive ammeters are OK). Car batteries can deliver unbelievable energy. My father nearly lost a finger one time when he was reconnecting a car battery and his wedding ring touched the spanner which in turn touched the starter solenoid terminal and BANG. I was about 10 years old at the time, but I can remember that day very vividly (especially the exciting ride to the hospital). The wedding ring literally dissolved.

If you are disconnecting/reconnecting a battery always disconnect the earth FIRST and reconnect the earth LAST, this reduces the chances of arcing to the body.

Anyway, glad you isolated the problem. Let us know what it was in the end.

Re-reading my prev. post I've just scared myself -- I'm off to buy a fire extinguisher for my 355 !

Good luck.

Rob.