Help with electrical problem please

[Anglo-Frenchman]

I recently posted a query about an electricam problem which I have thus been unable to resolve and would apreciate any advice. There seems to be a permanent short somewhere in the circuit protected by fuse number 1 (20A) which blows each time the fuse in inserted. This circuit includes the horn, the interior light, the dash clock and also for some reason I seem to have lost the flash facility on the dip/main stalk but not main beam strangely. ?

I have disconnected the horn, the clock and the interior light but still the fuse blows (with a bright flash) each time the battery is connected. The only thing I can think of is that there is a fault with the indicator stalk but not sure if this is likely. I have had a look in Haynes to check out the wiring diagram and as Ralph has previously advised, there is a common purple wire that could be at fault but obviously I can only get limited access to parts of this wire. In the book they provide advice about how to check for a short circuit which includes connecting a meter across the terminals of the fuse and turning on the supply but surely this will blow the meter up?

I'm pretty rubbish at electrical problems and I'm sure that there is a very simple explanation but I cannot seem to islolate where the fault lies. The main issue is that the car is due from its equivanelt MOT next month and without the horn working its going to be a non-starter.

Thanks in anticipation,

Ged

[steve b]

Hi Ged ,

I think the H/L flash is powered from the same fuse as it normally works without lights or ignition on . If youcan't see any evidence of the purple wire melting/going to earth it may be easier to disconnect all the applications and run new wire from the fuse box to each item. Does it also supply the cig. lighter ?

Also were all the applications working before the fuse first blew? How many purple wires are connected to the O/P of the fuse box? you might be able to isolate the individual circuit by disconnecting each wire , if its not there , you will find a junction block behind the dash where the circuits seperate I'd think.

hope this makes some sense leccy prob's can be a bit daunting

cheers

Steveb

[Anglo-Frenchman]

Thanks for the advice and I will check it out. I hate these types of problem because its like looking for a needle in a haystack. The cigarette lighter still works by the way. Any thought on the Haynes advice about sticking the test meter across the fuse terminals?

Thanks,

Ged

[western]

putting a multimeter across the terminals will only tell you current is flowing, it won't tell you which circuit is at fault, you'll only find that out by disconnecting all then reconnecting each in turn.

Defender wiring diagram attached

and these of the wiring at the back off the indicator/flasher/horn stalk might help the permanent live is the darker purple wire in the middle of the photo.

Defender_wiring_P1.pdf

Key_for_wiring_P2.pdf

Edited November 11, 2007 by western

[101sean]

putting a multimeter across the terminals will only tell you current is flowing, it won't tell you which circuit is at fault, you'll only find that out by disconnecting all then reconnecting each in turn.

I'll have a look at the Defender wiring diagram.

Check behind the offside rear access panel that covers the back of the lights. There should be a purple wire there that comes off the same fuse to feed power to the back for interior light etc, it's not used in most and it's possible that the socket on the end may have come off or is shorting out some other way.

[western]

Check behind the offside rear access panel that covers the back of the lights. There should be a purple wire there that comes off the same fuse to feed power to the back for interior light etc, it's not used in most and it's possible that the socket on the end may have come off or is shorting out some other way.

Good idea, I forgot about that bit, the loom comes out of the rear right inner vertical face to the rear of the fuel tank, check for any damage on the loom at each side of the large grommet, mine got caught here after a x-member changed & caused me allsorts of problems, cured now by isloating it & running a new cable from the bulkhead to the rearend.

also check out the hazard switch as that gets power from a purple wire.

[Monster]

Hi.

you dont say what type of LR you have, or year! Blade or tube fuses..

It may be no consolation, but to be honest a dead short is the best type of fault!! At least you can track it down systematically.

If you suspect the indicator stalk, do you have a junction block feeding it? If so disconnect and try.

Could be hazard switch as western suggested. Try disconnecting that.

I seem to remember there is also a junction block in the engine bay somewhere feeding the back lights etc. Possibly disconnecting from here could remove short.

How are you tracing the short? If you are fault finding using fuses this will be quite costly, not to mention annoying!!

What you need to do is find the circuit side of the fuse. To find the supply side, put the black on something earthed. Then put the red on one side of the fuse (Volts dc selected on meter). The side which reads 12v is the supply. You need the other side.

Put your meter to resistance or ohms. Touch red and black leads of meter together and you should read low ohms like 1 or 10 approx. Place one end again on earth and the other on the circuit side of the fuse. This will probably read low ohms again. (If there is no fault then you are looking to see Meg or Kilo ohms).

Disconnect suspect items or plugs one by one and hopefully the meter will go from low ohms to high ohms. when this happens it is the area of your fault.

I have had two faults where the short disappears when the rear connection in engine bay is disconnected.

(I dont even think it was used, there possibly for the trailer lights).

Apologies if i sound patronising but i do not have any idea of your experience.

Hope this helps.

Let us know how you get on.

[western]

Ged's truck is a 110CSW 200Tdi 1992 IIRC

[FridgeFreezer]

Wot he said ^^^ a solid short is far better than intermittent. At least you can trace it through, just chase the wires through the loom until you find something wrong.

[Anglo-Frenchman]

Many thanks to all. Sorry I forgot to say about the 110. Its a 1993 200tdi Defender CSW with blade fuses. I did not take your comments as patronising Monster, you have pitched the info at just about the right level for my grasp of matters electrical. I will attmpt to track it down over the next few days trying the process you have outlined.

BTW, when trying to find the circuit side of the fuse, will I not get a 12v reading on both sides while there is a short?

Thanks very much,

Ged

[steve b]

the circuit side will be the one with loads of wires coming off it, also the one showing to earth - meter on ohms one probe into fuse blade grip one probe to a known earth point meter will show dead short/ buzz. Easier way is to check for 12v with fuse out . 12v = supply side, therefore the other side is circuit

cheers

Steveb

[Eightpot]

Have a look under the plastic dash tray - the loom comes up from the fuse board and across under here into the back of the dash binnacle.

There are a couple of multi connectors under the tray, in the centre, which feed things like the interior light, cigarette lighter, radio etc, and it's not dificult for one of the live wires to come loose and earth on the bulkhead, particularly if you fitted a radio/cb and possibly been tugging at wires for any reason.

[Anglo-Frenchman]

Thanks for the advice. Unfortunately (?) I have factory fitted airco so no dash tray and all wiring annoyingly hidden behind seemingly immovable grey fasçia. Me and the 110 have been on a marathon 600km trip today so no chance to look but will try tomorrow.

Ged

[Anglo-Frenchman]

Further to this query - still no joy locating the dead short despite lengthy search and much help from Monster. The thing that is puzzling me (and when it comes to electrics it really doesn't take much) is this;

I set my meter to volts and take a reading across the fuse terminals of the offending circuit I get a reading of 12.4v noting that this circuit is MEANT to be permanently live because it supplies the clock and interor lights etc which have to operate even when the ignition is off. My question is if/when I locate the source of the dead short, will the volts reading drop to zero and if so how then are the clock/interor lights etc powered under normal circumstances. Surely some power must run through this circuit at all times. I apologise in advance for the bone question.

Ged

[FridgeFreezer]

I set my meter to volts and take a reading across the fuse terminals of the offending circuit I get a reading of 12.4v noting that this circuit is MEANT to be permanently live because it supplies the clock and interor lights etc which have to operate even when the ignition is off. My question is if/when I locate the source of the dead short, will the volts reading drop to zero and if so how then are the clock/interor lights etc powered under normal circumstances. Surely some power must run through this circuit at all times. I apologise in advance for the bone question.

You say 12v across the fuse terminals - does that mean you're putting one lead in each fuse terminal? if so that's not really going to tell you much except the load side is 12.4v lower than the supply side, but you knew that anyway because the load side has a short to ground (0v) somewhere.

What you need to do is measure the resistance (Ohms / ) from the load side connection to ground (one lead to ground, one lead to the load side fuse connector pin, polarity isn't important). This will be a very low number, probably zero, because of the short (no resistance, see?). When you find (and cure) the short this number should increase.

[Anglo-Frenchman]

Thanks. Current ohms reading is about .9 with dial set to 200ohms on meter. I have been going through the circuit this PM but no joy and about to set fire to the 110!! Have disconnected the flash/horn connector block in the dash, horns, fog lamp, interior light in fact all the loads that are meant to be supplied by this circuit but still getting the same reading. I suppose this means that the short must be in wiring but with so little of it accessible I am frankly a bit stumped. I cannot understand it as no wiring has been touched for ages and as luck would have it, its the MOT next week to so no working horn means no ticket. B*llocks!!

Ged

[western]

does the horn & other bits work if you apply a 12v supply straight from the battery to each in turn ?

[Anglo-Frenchman]

i have not tried this approach but if they do work then what does this tell me? Presumably I still have a short somewhere in the loom but which I can't trace. Sorry, don't mean to be an arse.

Ged

[western]

just thinking if the horn or it's switch were duff that could be your direct power short, same goes for the other items, worth a try even as just a check to make sure all of them work correctly.

& your NOT being a a*re

[101sean]

just thinking if the horn or it's switch were duff that could be your direct power short, same goes for the other items, worth a try even as just a check to make sure all of them work correctly.

& your NOT being a a*re

Another favourite for a dead short but not on the circuit you are looking at is the rear number plate light. Had one burn out the light switch on my 101 and seen it on 2 other vehicles. The defender uses the same carp fitting.

[Anglo-Frenchman]

One of the main problems is that I don't really understand how electricity flows or the various implications if it doesn't! For example, I have tried disconnecting the flasher stalk from the loom at the multi-plug to the lhs of the steering column. This appeared to make no difference to the ohms reading on the meter so I assume then that this piece of kit must therefore be OK. Is this thought process correct?

Another oddity is that the affected circuit also controls the rear fog lamp. But strangely even with the fuse out, when you turn the fog lamp on, the indicator light is illuminated on the instrument panel & I get a reading across the bulb holder with the meter set to volts although the bulb does not light. Additionally the fog light only comes on normally when the iginition is switched on so why is it part of a circuit that is permanently live (ie; clock; interior light, horn etc.)

Ged

[western]

I'll have another look at the wiring diagram & see if I can find a answer.

had a look at the wiring & can't see any reason why your rear fog light isn't working as it should.

[FridgeFreezer]

One of the main problems is that I don't really understand how electricity flows or the various implications if it doesn't! For example, I have tried disconnecting the flasher stalk from the loom at the multi-plug to the lhs of the steering column. This appeared to make no difference to the ohms reading on the meter so I assume then that this piece of kit must therefore be OK. Is this thought process correct?

Not sure if this will help but I'll try to explain electricity to you... (this could go very wrong! )

Basically it's like water and wires are like pipes. A battery is like a tank of water on top of a tall thing. Ground is like the drain the water runs away to, completing the circuit - if there's nowhere for the water to go, nothing can flow. Volts are the pressure of the water, amps are the volume of flow of the water. A load or resistance (EG a bulb) is like a restriction in the pipe, it takes a bit of pressure to push the water through it. Watts are work done, and are Amps x Volts (or flow x pressure). A switch is like a valve. Your alternator is like a pump, keeping the tank topped up.

A short circuit is like a break in the pipe - all the water flows out to the ground (electrically, something that is connected to the -ve terminal of the battery) so there is nothing left after the break for the load, 'cos it's easier for the water to fall out through the break than to get through the restriction that is the load.

Your thought process is correct - if you keep disconnecting things, you are proving that that thing is not at fault.

In the example above, a short circuit could be where a wire has rubbed against something or become damaged so the insulation has rubbed off and can then touch a part of the chassis or body - this means the electricity has no resistance between the wire (which wants to carry 12v of electricity to your lights and things) and the ground. Since it's far easier to flow straight to ground like this, all the electricity goes this way. As there's nothing to obstruct the flow (unlike a bulb, which has a filament with resistance), the only thing dictating the number of amps that will flow is how many your battery can provide (EG how big is the pipe from the water tank - answer: f'in big!). As the little wire can't handle this many amps, they stick a fuse on it. This gets hot and blows when it's limit is reached, preventing the wire getting hot and setting itself on fire.

This is why putting a bigger fuse in is bad - the wire may melt before the fuse does.

[Anglo-Frenchman]

Fantastic! Ever considered a career in teaching? Your explanation knocks spots of that which my physics teacher peddled ****y years ago.

After spending about 2 days on this i have conceded defeat and have booked it into a local garage. Thanks to all who have offered advice freely - long may this forum survive!

Ged

[western]

Here's hoping they can find the problem.