Electrical Gremlins - Fuse 7

[O mage]

A lot of electrical problems end up being bad connections have you inspected the back of the fuse board and the other loom parts for verdigris you need to look closely at the back of the wire down in the plastic to where the wire actually terminates this is where they can oxidize.

[Jhonno]

O mage said:

A lot of electrical problems end up being bad connections have you inspected the back of the fuse board and the other loom parts for verdigris you need to look closely at the back of the wire down in the plastic to where the wire actually terminates this is where they can oxidize.

Fuse box is out.. No obvious issues. Also.. If you disconnect the boot loom from the main loom, you loose the continuity of Fuse 7 to ground, but it remains under the dash. I will re-examine all connectors under the dash.

[Penelope Stopit]

Jhonno said:

I've got the whole dash apart and everything unplugged. Nothing makes a difference. I'm still getting the continuity between the red 12v battery/red/white bulb illumination 12v, and earth.

The only thing I can think, is there is a chaffing somehwere on the loom itself..

I can't see how with nothing plugged in I can get continuity between earth and 12v any other way.

Ok then

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A little advice that may help you now or in the future

When testing for a short (blowing fuse). Although resistance testing may seem to be a good idea, the results can easily send you on a wild goose chase

Even though replacing a blown fuse with a good fuse may seem to be the best method for checking if the short is still on the circuit, this method is by no means fool proof and can burn the short off

It's good to be able to see the short appearing or vanishing as components are disconnected or cables intentionally disturbed

Connecting a 21 Watt bulb across the fusebox terminals will work a treat, the bulb will illuminate brightly when there is a short circuit and dim or extinguish when all is good (all circuits need to be switched off as they will also illuminate the bulb)

Something like this (throw the lens and bracket away)



Using 2 meters of twin 14/030 cable with these type of terminals at one end and above holder at the other



Gives you a test lamp that will connect to the fusebox terminals instead of a fuse, make sure the spade terminals can't touch each other and short out, insulate the terminals with tape or heat-shrink

Don't let the hot bulb come into contact and melt anything, put the holder on a piece of wood or hang it in mid air

Now you can wiggle away at the loom and will possibly see the short come and go

---

About the fault

I'm of similar opinion to

gruffalo said:

I would start at the cigarette lighter and disconnect that first. Your symptoms are exactly the same as I have seen before and it was caused by the cigarette lighter socket having a partial short circuit in it.

But

Have a hunch that there is either a short between the cigarette lighter supply cable, illumination cable and earth. When the short vanishes there is still a short between the cigarette lighter supply cable and illumination cable (rubbing loom or bolt through it)

The above short will illuminate the instrument cluster

But

What I've posted above could be elsewhere and will possibly be found at the radio wiring harness

If the short isn't at the lighter or radio looms then you need to think what other looms have a fuse 7 supply with an illumination supply and a possible earth cable in them

The bulb will definitely show the short by fully illuminating

Good luck in finding the problem

[Jhonno]

Penelope Stopit said:

But

Have a hunch that there is either a short between the cigarette lighter supply cable, illumination cable and earth. When the short vanishes there is still a short between the cigarette lighter supply cable and illumination cable (rubbing loom or bolt through it)

The above short will illuminate the instrument cluster

But

What I've posted above could be elsewhere and will possibly be found at the radio wiring harness

If the short isn't at the lighter or radio looms then you need to think what other looms have a fuse 7 supply with an illumination supply and a possible earth cable in them

The bulb will definitely show the short by fully illuminating

Good luck in finding the problem

Thank you. I shall be picking this back up tomorrow, weather and other jobs have stopped play for a couple of days.

[Penelope Stopit]

Jhonno said:

Penelope Stopit said:

But

Have a hunch that there is either a short between the cigarette lighter supply cable, illumination cable and earth. When the short vanishes there is still a short between the cigarette lighter supply cable and illumination cable (rubbing loom or bolt through it)

The above short will illuminate the instrument cluster

But

What I've posted above could be elsewhere and will possibly be found at the radio wiring harness

If the short isn't at the lighter or radio looms then you need to think what other looms have a fuse 7 supply with an illumination supply and a possible earth cable in them

The bulb will definitely show the short by fully illuminating

Good luck in finding the problem

Thank you. I shall be picking this back up tomorrow, weather and other jobs have stopped play for a couple of days.

Glad to possibly be of some help, keeping motivated is the hard part with a pain of a short like this one, sometimes they're solved within an hour or so, other times they can take days to find

Perhaps fortunately, you've been able to remove the short and witnessed it returning

[RUSSELLM]

If you’ve narrowed the fault down to the dashboard, what items do you think are on that circuit ?

I know you mentioned radio.

And did you say cigarette lighter supply or was it the cigarette lighter illumination ?

Edited by RUSSELLM on Friday 3rd July 12:30

[Jhonno]

RUSSELLM said:

If you’ve narrowed the fault down to the dashboard, what items do you think are on that circuit ?

I know you mentioned radio.

And did you say cigarette lighter supply or was it the cigarette lighter illumination ?

Edited by RUSSELLM on Friday 3rd July 12:30

Interior lights, clock, radio, cigarette lighter, boot lock and it seems driver's door release.

[RUSSELLM]

Have you disconnected all those items ?

[Byker28i]

Radio? Most likely source of dodgy wiring. I had to redo all mine with solder joints replacing scotchlocks and twisted wires etc

[Jhonno]

RUSSELLM said:

Have you disconnected all those items ?

Yes.

[Jhonno]

Byker28i said:

Radio? Most likely source of dodgy wiring. I had to redo all mine with solder joints replacing scotchlocks and twisted wires etc

Radio disconnected and still pops the fuse. Can't see any issue with wiring from plugs to where it rejoins the main loom.

[Byker28i]

Cigarette lighter? When I first got the car and used it as a power source it popped that fuse, which was also boot lock, door lock etc. I nice introduction to ownership not being able to get in...

[RUSSELLM]

Jhonno said:

I've got the whole dash apart and everything unplugged. Nothing makes a difference. I'm still getting the continuity between the red 12v battery/red/white bulb illumination 12v, and earth.

The only thing I can think, is there is a chaffing somehwere on the loom itself..

I can't see how with nothing plugged in I can get continuity between earth and 12v any other way.

The bulb illumination wire.... What connector and pin number, is that ?

[camel_landy]

Byker28i said:

Cigarette lighter? When I first got the car and used it as a power source it popped that fuse, which was also boot lock, door lock etc. I nice introduction to ownership not being able to get in...

It was just showing you who's boss...

M

[RUSSELLM]

Just having a look on here.... http://tvr-cerbera.co.uk/files/WorkshopManual/LowR...

The Red/White wire that supplies illumination for the Radio, Fag lighter, door buttons, window switch illumination, speedo and various dials bulbs etc, all appear to go back to the Lights ECU.

The lights ECU, is fed by fuse 7.

I guess that’s why you’ve continuity between illumination and fuse 7 ?

If any of those bulbs, including the headlights, side lights, various dash lights, door lights are still connected, you’ll obviously read a ground through their bulbs. Not so on the LED’s, but more so on the filament lamps.

Might be an idea to disconnect the headlamps, in case a bulb has welded itself or the terminals have rotted

Edit.... in fact, disconnect the lights ECU all together, and see how your fuse 7 gets on.

Edited by RUSSELLM on Sunday 5th July 13:47

[Penelope Stopit]

If fuse 7 definitely supplies the lighting ECU's

Adding to the above good help

There is also the dim dip ECU but am finding it difficult reading the diagrams to see everything on the fuse

For some reason I thought that disturbing the loom removed the short

Dim dip below









All from this topic https://www.pistonheads.com/gassing/topic.asp?h=0&...

[Jhonno]

RUSSELLM said:

Just having a look on here.... http://tvr-cerbera.co.uk/files/WorkshopManual/LowR...

The Red/White wire that supplies illumination for the Radio, Fag lighter, door buttons, window switch illumination, speedo and various dials bulbs etc, all appear to go back to the Lights ECU.

The lights ECU, is fed by fuse 7.

I guess that’s why you’ve continuity between illumination and fuse 7 ?

If any of those bulbs, including the headlights, side lights, various dash lights, door lights are still connected, you’ll obviously read a ground through their bulbs. Not so on the LED’s, but more so on the filament lamps.

Might be an idea to disconnect the headlamps, in case a bulb has welded itself or the terminals have rotted

Edit.... in fact, disconnect the lights ECU all together, and see how your fuse 7 gets on.

Edited by RUSSELLM on Sunday 5th July 13:47

OK, so it seems continuity between the battery red and the illumination red/white would be to be expected.. An could then possibly explain the link between the fuse blowing and the headlight coming on, seeing as the Cerbera uses ground switches?

For clarity I'm not checking resistance anymore, I have a continuity setting on my multimeter, which looks for a completed circuit.

I've completely removed my cigarette lighter now.. I cant see a reason why it might be the issue, no loose bits, wiring looks OK, just some corrosion in the bottom of the socket itself..

Has anyone changed one of these out to a USB socket? Just having a quick look and most seem to be a larger diameter, so would involve cutting my leather/dash? Whilst it is out, I'll change it to something more useful.

[Jhonno]

Penelope Stopit said:

If fuse 7 definitely supplies the lighting ECU's

Adding to the above good help

There is also the dim dip ECU but am finding it difficult reading the diagrams to see everything on the fuse

For some reason I thought that disturbing the loom removed the short

Dim dip below









All from this topic https://www.pistonheads.com/gassing/topic.asp?h=0&...

I though disturbing the loom had as well, but it is back and I can't recreate it, so perhaps it was human error.

There is 2 lights boxes. Both of mine are hardwired earth and battery, as at some point they burned out in previous ownership of the car.

[RUSSELLM]

It is still blowing fuses with the light controller disconnected ?

[Jhonno]

RUSSELLM said:

It is still blowing fuses with the light controller disconnected ?

I'm having a play atm.. Just put a new Ignition kill switch in, so had the battery off.

Going to power it up and start slowly connecting things to see what happens..