14 replies to this topic

[huw_jenks]

1979 Mini 1000, electronic ignition.

I'm having a real problem with my car at the moment, with the first fuse going all the time. That's the 35amp fuse between loads 1 and 2 on the fuse box. I have a voltmeter but I'm not really sure what to do with it to find a short circuit (which is what must be causing the fuse to blow). Could someone give me a bit of advice for finding the fault with this particular part of the wiring please? A bit more info, the fuse blows as soon asthe ignition light comes on, without having to use the indicators, brake lights whatever. There are two white wires connected to the left side of the fuse (1) and two green wires (in one connector) on the right side of the fuse (2). One of the white wirees is going to the coil, not sure about the other. Both green wires go to a round box in the right hand side of the engine bay (a relay or something?)

Any help would be very much appreciated, as I can't drive my Mini without brake lights or indicators!

Thanks

edit - ive just had a better look and one of the green wires on the 2 side is going to the brake light switch, and only one is going to the round load on the right of the engine bay.

Edited by huw_jenks, 27 April 2010 - 03:48 PM.

[taffy1967]

I know the reverse light switch wiring can sometimes short against the exhaust, but that only usually causes the fuse to blow when you select reverse gear.

[huw_jenks]

I know the reverse light switch wiring can sometimes short against the exhaust, but that only usually causes the fuse to blow when you select reverse gear.

Well the car is up on ramps so I checked the reverse switch wiring and it looks to be fine. Of course, there could be a short on the wire before the switch, but I really don't know how to find it.

I really don't know how I should be going about trying to find a short circuit. Won't I need a fuse in place to let the current flow, and then I measure the voltage across components and wires to find where the earth is? If the fuse blows as soon as I turn on the ignition how can I do this? I really need some help, looking at wiring diagrams is driving me crrrraaaaazay...

[taffy1967]

How old is the fuse box and could the electronic ignition be causing a problem here?

[huw_jenks]

I'm not sure how old the fuse box is, could be original, but it appears to be upside down. I have the 35amp fuse at the top and the 15amp at the bottom but there are some tiny numbers on the plastic (1 to 8) which are in the opposite order. I shouldn't think this would make a difference as the correct wires are on each fuse connection.

I'm looking at the wiring diagram now and it seems that for the fuse to be blowing immediately after the ignition is turned would mean that the short circuit is occuring between connection 2 and the first switch in each parallel circuit. The brake light swith and reversing light switch seem fairly simple to check, but putting a voltmeter across the direction indicator/ hazards circuit looks a nightmare.

Could anyone please explain how I'm actuallly meant to do it?

[Yoda]

According to original wiring, the larger white wire is the ignition switch live feed to the fuse and the smaller white is direct feed to the coil. the two green wires are for the brake light switch feed and the indicater /hazard circuit.


Pull the two green wires off the fuse box and see if a fuse blows when you turn the ignition on, if it does then the fault is in the coil or feed wire, next pull the feed wire off the coil (white) if the fuse blows its a wire problem, if it doesnt, its a shorted coil. If the fuse does not blow, then put the green wires back on the box one at a time and see which one blows the fuse. this will help you eliminate certain parts of the wiring and locate which circuit you are looking at to find the short.

My suspicions are with a shorted ignition coil.

Over the years there have been some changes to this wiring setup but this is the most common one. and is listed for the 79 models.

Edited by customcart, 28 April 2010 - 06:59 AM.

[Wil_h]

I would disconnect the connectors at the fuse box seperate then if more than one wire is crimped together then connectet them one at a time. (i.e. connecte one, turn ignition on see what happens, remove connection and try another......).

This should atleast isolate the circuit that is causing the problem.

Whe n you know the circuit at fault, go through it bit by bit. for instance, if it's the brake circut, leave it connected at the fuse and disconnct the switch, then try the ignition, if it still goes it must be in the loom. at this point I'd try running a new wire to see if that solves it.

If you want to do it without spending lots on fuses, but the meter on ohms and do the above tests with the fuse removed, measureing between each individual circuit and earth. The faulty circuit will be 0 ohms.

[Yoda]

If you want to do it without spending lots on fuses, but the meter on ohms and do the above tests with the fuse removed, measureing between each individual circuit and earth. The faulty circuit will be 0 ohms.

Or simply connect a bulb across the fuse clips, this will show load or short on the circuit connected. in theory, the circuits connected will only make the bulb glow, when a switch is operated or if short circuit.

[GraemeC]

Pull the two green wires off the fuse box and see if a fuse blows when you turn the ignition on, if it does then the fault is in the coil or feed wire, next pull the feed wire off the coil (white) if the fuse blows its a wire problem, if it doesnt, its a shorted coil. If the fuse does not blow, then put the green wires back on the box one at a time and see which one blows the fuse. this will help you eliminate certain parts of the wiring and locate which circuit you are looking at to find the short.

My suspicions are with a shorted ignition coil.

Over the years there have been some changes to this wiring setup but this is the most common one. and is listed for the 79 models.

The coil supply comes from the unfused side of the circuit therefore even if directly shorted it would not cause the fuse to blow

[GraemeC]

edit - ive just had a better look and one of the green wires on the 2 side is going to the brake light switch, and only one is going to the round load on the right of the engine bay.

For reference - this sounds like the flasher unit.

[Yoda]

The coil supply comes from the unfused side of the circuit therefore even if directly shorted it would not cause the fuse to blow

My Mistake, I was going from memory and have checked the diagram and i agree, most likely to be flashers. if not, suspect the brake light circuit.

[huw_jenks]

Thanks for the response guys, I've done a bit more checking this afternoon and there has been a bit of progress.

I diconnected the two green wires, cut and seperated them then put an oily bolt across the fuse connections which gave off a wisp of smoke when a current went through it. With the first of the green wires connected and the ignition turned on there was no smoke, so no current. With the second wire attached to the 2 terminal there was a wisp of smoke as soon as I turned the ignition.

So I traced the suspect wire through the loom to the indicator flasher unit. There didn't appear to be any holes in the insulation, but I put on new spade connectors and made sure it was safe as I taped it back into the loom. When I checked for a short circuit using the same method as earlier, there was no smoke coming from the fuse box so I think everything is okay... I think.

So tomorrow I'll buy some more 35amp fuses and find out for sure. I'm not totally convinced that everything is working though, so don't be surprised if I'm back on here tomorrow asking more questions!

[all4sreturns]

I had similar problem with my 1970 Morris Mini (re-engined with 1275). Though as well as blowing a fuse on start up, it occasionally happened mid-driving, though can't remember whether it happened when I was indicating, as I haven't used it through winter.
The stereo unit went off and on intermitently too. Could all this be a bad earth? I had some wiring replaced and I've had a couple of different garages say they've 'fixed it' though a week or two later it blew again. When the fuse blew, the whole ignition system was dead, absolutely no power at all!
Should I go through the same processes as described to elemenate the problem as I don't fancy replacing the whole wiring loom? Should I be checking all the connector/earths etc?
The car was excellently reliable apart from this damn fuse blowing. Any further ideas on top of what has been mentioned so far?
Cheers!

[huw_jenks]

I had similar problem with my 1970 Morris Mini (re-engined with 1275). Though as well as blowing a fuse on start up, it occasionally happened mid-driving, though can't remember whether it happened when I was indicating, as I haven't used it through winter.
The stereo unit went off and on intermitently too. Could all this be a bad earth? I had some wiring replaced and I've had a couple of different garages say they've 'fixed it' though a week or two later it blew again. When the fuse blew, the whole ignition system was dead, absolutely no power at all!
Should I go through the same processes as described to elemenate the problem as I don't fancy replacing the whole wiring loom? Should I be checking all the connector/earths etc?
The car was excellently reliable apart from this damn fuse blowing. Any further ideas on top of what has been mentioned so far?
Cheers!

Well your problem also sounds like a short circuit somewhere! If it's happening when you're driving it's more difficult to trace too, could be caused only when the wires are moving about.

But my problems appear to be over for the time being. I've only moved the car up and down the driveway so far, but the engine has been running and revving with no fuse going, so thanks for the help guys! I didn't find out for definate where the short was in the end, but taking the loom apart and reconnecting bits seems to have done the trick.

[all4sreturns]

I think you're right, it could be a bit more of a pain as it happens when in motion. I think it will be a case of going through each piece of wiring and checking for splits in the plastic insulation, and checking all connecting clips. Lots of time!