Weird electrical problem

[MaxB]

Any ideas about this?

Start up (K series 1.4 ; 1995 S3) and ticks over BUT none of the dials register (rev counter, fuel gauge, oil pressure, temp)..rev the engine and ignition light illuminates and gauges click into action, but showing incorrect reading eg. half a tank of fuel when full, low oil pressure when fine etc). All this suddenly happened when on mini-blat. Engine performing normally.

Alternator blown?

ECU?

Grateful for any advice

 

Edited by - maxB on 25 Jun 2013 23:52:14

[Stationary M25 Traveller]

Connect a voltmeter across the battery to see what the voltage is at various revs. Should be around 14V when running and alternator charging.

 

Very possibly an earth cable damaged or disconnected under the dash.

[Jonathan Kay]

*arrowup* *arrowup*Agreed.

 

It might also be worth repeating the rev test a few more times: is it a randomly intermittent fault rather than being linked to the engine speed? SM25T's approach should reveal that anyway.

 

Jonathan

 

Edited by - Jonathan Kay on 26 Jun 2013 08:06:24

[MaxB]

Many thx.

Now no gauges or ignition light working! But running ok.

Haven't got voltmeter.

 

[Stationary M25 Traveller]

They are pretty cheap ..... try Maplin.

 

Essential bit of kit.

 

Edit ...... if you update your profile with an idea of your location, there may be someone nearby that has one !

 

Edited by - Stationary M25 Traveller on 26 Jun 2013 09:03:05

[revilla]

You can get digital a multimeter (amps, volts, resistance etc.) on eBay at the moment for £2.97 (Buy It Now, Free Postage!) here.

 

Thats got to be the bargain of the century!

 

If you want to get involved in diagnosing any electrical problems on your car you will find that you really do need one - without it you are just guessing in the dark as you cannot "see" electricity so cannot see what is going on. With a simple multimeter (doesn't need to be anything special or super-accurate) you can see straight away where there is and isn't the voltage you would expect, and where current is flowing.

 

If you keep fiddling with it without getting one of these, somebody on here will tell you off sooner or later (and rightly so 😬).

[Paul Deslandes]

I would guess an earth has come off behind the dash or a fuse has blown. Do all your lights work, i.e. side (front and rear), headlights, brake and reverse, indicators? Have you been doing anything to the car recently where you might have disturbed or loosened a wire connection somewhere?

[Paul Deslandes]

Just checked the wiring diagram and all the items you mention are, as far as I can tell, fed from the same fuse, so that would be my first port of call.

 

Top row, second from left is the instrument panel fuse, 10A rated. It also supplies the heater fan so, if you have one and it doesn't work, that would increase the odds further.

 

here

 

 

 

Edited by - Paul Deslandes on 26 Jun 2013 10:36:33

[Klunk]

Hi Max,

 

Paul's beaten me to it. The positive feed to the gauges and the ignition light are all connected. It's the plain green wire to the back of each of the gauges/ignition light.

 

Regards,

 

Giles

 

Edited by - Klunk on 26 Jun 2013 12:11:21

[Paul Deslandes]

Having read the original post again, I suspect there's a partial short that made everything look odd before it eventually blew the fuse.

Switch everything off, particularly the heater fan, replace the fuse (assuming that it has blown) and see if it all behaves normally when you turn on the ignition. If its working properly, switch on the heater fan and see what happens. Its one of those faults that you have to work through progressively until you find it unfortunately.

[Jonathan Kay]

Multimeters:

 

1 It's worth buying one that has audible continuity testing.

 

2 Probes with a small spring clip make life easier.

 

Jonathan

[Paul Deslandes]

certainly agree re audible continuity tester. I have one with and one without and the former is the favourite. A selection of spring-clip and pointed probes is ideal. For auto use, a home made 1metre long negative lead, with a suitable banana or wander plug for the meter on one end and a crock clip on the other, is extremely useful.

[revilla]

Re: Jonathan Kay's point about audible continuity testing, a word of caution (which I will also post as a thread in it's own right to make it easier for people to find it) ...

 

I recently noticed that my brake lights weren't working. I suspected the switch as the most likely candidate. Tonight I whipped off the lid of the pedal box, stuck my multimeter continuity tester across the switch (all switched off first), pressed the brake pedal and ... "beeeeeep" ... So switch looked fine. I then switched to voltage mode and checked for 12V on one side of the switch, then pressed the brake pedal expecting to register 12V and ... about 0.5V! A check in resistance mode showed about 150 ohms resistance through the switch in the "on" position, which as far as a 21W brake light bulb is concerned might as well be "off", but the continuity tester beeped as though all was fine.

 

A replacement switch from spares fixed my problems.

 

The moral of the story is that continuity testers beep when the resistance is below a certain threshold level, which may be surprisingly high. Just because a switch or earth connection passes the audible continuity test doesn't mean it's resistance is low enough to mean that it will perform well when asked to carry a more realistic current load. In my case 0.5V was appearing across the bulb, with the remainder of the 12V or so being dropped across the switch.

[MaxB]

Many thanks everyone for your great help here.

You were right--fuse blown linking instrumentation, ignition and blower. Suspect there was a short as you suggest that led to odd symptoms and later blew fuse.

Giles, I see you are Shropshire based. Just moved here; is there an active group?

Max