project 1980 Austin Princess

[Des]

 

Problem:  too much power going through this circuit is blowing the fuse.

 

Investigation:  I've checked for loose wires, bare wires and errant connectors.  Everything is present and correct.  I've disconnected the items listed in Powers to eliminate them from the system.  Fuse sparks when fitting, will blow if fitted.

 

Conclusion: Earth problem?   It's a bit strange really, I can't see anything that's changed to cause the problem and I can't find anything that looks faulty so I'm not sure what to do next.

 

Solution: Leave the fuse out.

 

You could try replacing the fuse with a strong bulb, how brightly it glows will give an idea of the current flowing, while it's there wriggle and poke at various auxiliaries see if you get a flicker. If you get stumped I reckon a flasher unit across the fuse holder might just give a nice pulsing magnetic field along the errant circuit that could be tracked with a compass, or by listening to clicks from a transistor radio like a geiger counter, all the way to the bastard self tapper that's intruding the loom somewhere.

[PhilA]

Does the fuse blow if you have the hazard switch engaged? Might have shorted to ground through the bulb inside it

 

Phil

[vulgalour]

it blows if it's engaged or not, connected or not. It's really odd and not the first time the car has done this and while I did think it was the dodgy cigarette lighter that we replaced, now I'm not so sure.

[PhilA]

Hmm, fair enough. Sounds like it's time to start tearing it apart again.

[vulgalour]

Great joy.

 

I've been having a think about this and I'm a bit perplexed by it.  Mike has suggested we have a look at the unit where there's more tools, later this week.  The only things not working are cigarette lighters, interior light and hazards, nothing else is affected.  I've no radio fitted at the moment so if I had one in, it may have taken that out too, I can fit a spare to find out.

 

What's really confusing me is with everything that it should power disconnected, something is still drawing power through that circuit.  If there was still all the bodge in the wiring I'd expect this, but we've cleaned it all up and everything should be good now.  The other annoying thing is that this circuit was working perfectly fine and I can't think of anything I've done or moved that might have disturbed or broken something.  The car was working fine so I basically left well alone when it came to the MoT.

 

The only trigger I can think of is the sat nav.  Back in 2012/3 I had two occasions I can recall of the same sat nav blowing the same fuse when it was plugged in. A new fuse on both occasions resolved the issue.  Later, in  2017, I found that the dashboard cigarette lighter was faulty when it turned out to be the cause of repeated fuse blowing and replacing it with a brand new one cured the issue, until now.  The wiring behind the cigarette lighter is quite cramped since it shares a tight space with the heater controls and the radio wiring so it's possible something back there is touching something it shouldn't and plugging a sat nav in is what's causing things to move about where they shouldn't.  In theory, whatever has moved somewhere it shouldn't hasn't moved back and that's the root cause of my fuse blowing issue.

 

Does that sound at all plausible?  It's all I've got to go on without pulling everything apart (which I really don't want to have to do AGAIN), I'm not really sure what else it could be.

[PhilA]

Yes

 

Been there done that with flexible plastic and cigarette lighter sockets.

 

Either that or the sellotape wrapped around the + wire for the not-fitted radio has fallen off and is resting against the back of something metal...?

[SiC]

I assume no crud hasn't fallen down inside the cigarette lighter socket?

[plasticvandan]

put a low ampage fuse in with the various items that run the circuit disconnected and plug each one in and see which one blows the fuse?

[vulgalour]

Phil:  I checked the tape on the radio wires and it's still present (masking tape, in this instance) and nothing is touching anything else.

 

SiC:  Not that I can see, unless something has fallen on the back and is bridging contacts.  It's horizontally mounted so tricky for stuff to fall in it.

 

PVD:  That's just it, with everything disconnected it still blows the fuse, and I'm not sure what can cause that.

[vulgalour]

It gets stranger.  Off out on errands today so heated rear screen, headlights, wipers and fan on out of necessity since it's tipping it down.  All is well until we're idling at traffic lights and the wipers start slowing down, check the gauges and sure enough the ammeter needle is falling.  Wipers off, it picks back up, wipers back on and it stays there.  I've been unable to replicate the escaping electricity problem.  I can't find any loose connections, dirty earths, damaged wires or similar so I'm really perplexed by this.  I even sat with the car at idle and as many electrical items on as I could and the escaping electricity issue did not remanifest.

 

I'm calling Old Car Stuff on it today because it self-healed and it's never been keen on very wet weather.  Conversely, it drove and ran quite nicely today, much more so than usual in wet weather.

[Squire_Dawson]

Even though it has an alternator these old ones can still struggle when you have a lot of electrical equipment in use, so I wouldn't worry too much about that. At idling speed the ammeter can show a discharge and as long as it charges once the car is moving all is well.

[vulgalour]

Physically unplugged the lighter sockets, unplugged the hazard relay (this is separate to the indicator relay, since its on a different circuit to those), removed the interior light bulb.  That should disable those items, I'd expect.

[SiC]

Likely the electrons are escaping through a cable that's been chaffed or cut on that circuit. Given its blowing fuses instantly it must be a hard short to the body.

[PhilA]

Yeah. Problem is in the wiring between fuse and one of those things.

[vulgalour]

Horn is on a different circuit (and works as it should) and the boot light is effectively removed since the switch I've got is wrong for the boot rams I have fitted, so that's not been working for some time now.  Damaged wire shorting on something does seem plausible given the way this is behaving.

[JeeExEll]

^^ Fair enough if the horn is still working as it should with the problem-fuse out.

Another possibility is a short to earth at the fusebox right behind the fusebox itself - tiny scrap of metal, remnants of blown fuses, fuse terminal touching bare metal or a fixing screw, etc.

 

Good luck anyway.

[vulgalour]

That's a problem we've had before so yes, also likely.  I can still see me building a custom dash and loom for this car you know.  Not because it'll look better, just because it'll actually sodding work properly.

[PhilA]

No, live goes to the bulb, then from there to the switches. Ground the switches to bring the light on.

 

So, with the bulb pulled you're looking at the wiring all the way to the lamps.

 

Phil

[vulgalour]

er... okay?  Maybe I should clarify precisely what I did and you can educate me on how to disconnect the actual wires.

 

cigarette lighters - the wiring loom terminates in several plugs/connectors per lighter.  I disconnected these plugs from the lighters.

Interior light -  the circuit is completed with a festoon bulb.  I removed the bulb.  I could have unplugged the connecters that go into the housing but that seems a bit pointless if removing the bulb achieves the same goal.

Hazards - removed the hazard relay.

 

As I understand it, that's how you disconnect these items so I'm not sure how I'm supposed to disconnect the actual wires without, I dunno, chopping the loom?  Which I'm not about to do, obviously, unless that's part of the repair.

[vulgalour]

Let's do a bit of problem shooting on this then.  Disconnected the speedo cable at the gearbox end so that the cable could be drawn through into the cabin and prevent risking damage to the clip that locks it to the binnacle.

20180405-01 by Angyl Roper, on Flickr

 

Then disconnect the hazard relay, all the wiring to the dashboard itself, the rear cigarette lighter and the interior light wires.  Test the fuse holder and something is still drawing power.

20180405-02 by Angyl Roper, on Flickr

 

The only things it can be so far as I can figure out are:

Door switches - forgot to disconnect these

Fusebox - could be bad connections in the back of it, or something of that sort

Wiring loom - broken or chafed wire somewhere unseen

 

Next job is multimeter and disconnect the door switches to see if that solves the issue.  There's not really much wiring the problem can be in at this point, it's just a nuisance that the wiring it's likely to be in is difficult to get to without dismantling the interior.

[Des]

Is it possible there's been a historic fault, like someone stuffed a spoon in a faggy socket, and somewhere inside the loom the insulation took a melting and now some unrelated strands are rubbing shoulders. What about connecting a horn unit across the fuse and give anything you can get to a good prodding, flexing, a ripping and a tearing whilst listening for changes in tone?

[SiC]

Is the short still there with the dash off like that? I wouldn't have thought it was the door switches, as most almost always short to ground when the door is opened. I.e. they have one terminal to the light circuit and the other is the body. Easiest, simplest and cheapest way of doing it.

[Pillock]

I think what Ben is alluding to, is that if the wiring has a cut and is touching earth then disconnecting it from the fag lighter won't have any effect.

 

Battery --- fuse --- switch --- OMGCUT --- fag lighter -- ground

 

If electric is escaping at OMGCUT where it's chafing on something metal then anything after that is irrelevant, you're still getting a dead short which is blowing the fuse.

 

I guess you could, with the battery disconnected, see if any positives have low resistance to ground? You should be able to isolate it to a single wire at least that way.

 

Or an exorsism.

[JeeExEll]

Seat belt warning light switches, either at the seat-buckle-things themselves / or the wiring to them?

 

Depending on cable-route, possibly accidentally nicked a wire when screwing the console back in and a little bit of movement in use has the live feed now shorting to earth?

 

They're on the same fused circuit, fuse 5.

 

And the carpet's been out recently for recolouring.

 

Maybe??

[vulgalour]

Des: an historic fault seems likely, one that's been ignored or worked around somewhere.

 

SiC: yep, still shorting with the dash out.  At least I feel moderately confident it's not the dash causing the fault, so that's something.

 

Pillock:  that makes sense, though if it is a cut it's going to be difficult to find.  I wonder if this car met Junkman's P6 in the past?  Would explain a lot.

 

Jee: Seat belt warning light is on a different circuit and working as it ought so I don't think that's the fault.

 

Probably won't get to investigate this further today, errands and such have rather got away with me today and I haven't much time between now and work starting so might have to poke it tomorrow instead.

[vulgalour]

Bolting the dashboard back together gave me chance to have a little think and a look at what's what.  The run of wire that goes to the rear cigarette lighter does go by so brackets and stuff that could pinch the wire and short it out, to find out for certain I need to remove the passenger seat so I can lift the carpet, which is a job for another day.  The other thought is going back to early in my ownership when wiggling the fusebox would make the wipers work when they decided not to so it could be a fusebox issue.  I'm planning to replace the fusebox with a modern blade type anyway and I'm thinking that this might be a sensible upgrade now.

 

The one thing I do feel comfortable about is that the problem isn't in the dashboard and at least it's not a problem that prevents me from using the car, it's just a niggle at this point really.  Oh, and the multimeter didn't tell us anything we didn't already know without pulling the passenger seat out, as mentioned, so we need to get the Princess to the unit for the better diagnostic kit Mike has which will help us see a bit more clearly what's going on, I'm told.

[JeeExEll]

Don't immediately dismiss seatbelt-warning-light switches / wiring yet Vulg. (See page 75 in this thread, MikeKnight @12.01 - I thought we'd been here before!!).

 

Maybe see also handbrake warning-light switch, wherever it is (I don't know exactly but it could well be powered from the same fuse / part of the wiring loom via the contacts of an "operation-while-ignition-on-only" relay).

 

 

* Does anyone have a copy of a wiring diagram for a Wedge? *

I realise there's not much wiring in a car of this age but it would make fault-finding so much easier.

 

There's a pdf for late 70s MGBs on the net which I would expect to be similar but may be misleading.

Wiring diagrams for a Wedge don't seem to exist on the web.

 

Maybe they've never existed, ever.

Even at BL.

[vulgalour]

I've got a copy of the diagram.  Do I need to scan it and post it here?  Seatbelt and handbrake warning lights are on a separate circuit, both only work with ignition on.

[JeeExEll]

That could be useful Vulg if / when you can, and it lets others see it. Didn't know if you had one.

 

Am guessing but wiring for seatbelt & handbrake lights on these could still be overcurrent-protected by the same fuse 5, but run through two separate pairs of contacts of a relay - an electrically operated switch which only closes it's contacts when the ignition is on. Other circuits protected by the same fuse like hazard flashers are not run through an ignition relay and would have power available all the time.

 

But that is a guess. Mike obviously had reason to believe at that point a year ago that the seatbelt lights were protected by this fuse so it's worth checking out. Could be something really quick and simple to fix.

 

More Wedge info required.

[vulgalour]

I'd rather not cut and rejoin anything if I can help it.  Not a big deal to remove the seat and lift the carpet to check the wire in situ and repair/remove if needed.