Electrical Gremlins - Fuse 7

[Jhonno]

I've just recently started having issues with fuse 7 popping. I have an early Cerb, so fuse 7 is interior lights, boot switch, radio, and cigarette lighter. When it pops, my wipers start going, hazards come on and headlights (only) come on also (when the ignition is on). Sometimes it will pop as soon as you put a fuse in, others it will last for a while. The original fuse blow came I think whilst sat I think.. Parked up fine, came back to it 3 days later and driver's door/boot wouldn't open was the first signs something was up. Window drops on the door but no release. Passenger's door works fine. Windows work fine on the buttons. Alarm works fine when locking.

I am seeing a resistance from fuse 7 (in the fuse box) to earth. (~5ohms)

All the items above disconnected still shows a circuit.

If I unplug J31/J34 connection (front harness to rear harness) the circuit goes away.

I noticed I had a resistance between the output from fuse 7 (red 12v) in the main harness and a thick green wire (my wiring doesn't match the diagrams so wasn't sure which one it was). Turns out it is a 12v to the injectors (Pin 7 on J10).

If I unplug J10 the resistance on the fuse 7 goes away, and the resistance on the rear harness end of the main loom goes away, however if you check from a 12v feed under the dash you have a resistance to the J10 end of the loom.

I'm more a mechanical person, so I'm muddling my way through the Cerb's eccentric systems a bit, and trying not to get drawn by eccentric red herrings of the system's design. If i had another Cerb here I could test and compare!

Anyone have any light bulbs moments over this? Am I looking in the wrong direction?

[Jhonno]

O mage said:

Hi, I would say if fuse 7 blows then because its tvr maybe it is feeding something else and it seems there is a back feed effect going on with the hazards and headlights when circuit 7 loses feed but if everything returns to normal when 7 is replaced i would try and rule out radio etc first.

Edited by O mage on Tuesday 30th June 15:11

Edited by O mage on Tuesday 30th June 15:13

I removed the stereo unit and it still blows.

I thought it may have been the reverse light switch loom, as I found it touching the chassis (worked it's way off the box), but I was getting a circuit to earth from that when I checked the wires. Which, tbh, seemed a bit odd as the wiring runs back to the fuse box for it..

[Jhonno]

camel_landy said:

O mage said:

Hi, I would say if fuse 7 blows then because its tvr maybe it is feeding something else and it seems there is a back feed effect going on with the hazards and headlights when circuit 7 loses feed but if everything returns to normal when 7 is replaced i would try and rule out radio etc first.

The back feed is what sprung to mind for me too... I suspect the fuse and that circuit could be an innocent 'victim'!!

Have you checked continuity at all the earth points (don't forget the insulating properties of grease on connectors )? Also check nothing on either of the circuits in question is shorting to earth.

M

Yeah, I suspect the same tbh, hence trying to work out what is causing it!

The only grease on earth points is Contralube.

[Jhonno]

RUSSELLM said:

A point of note, if you haven’t already realised.

If you find resistance to ground at a fuse, be conscious of the fact that you couple be reading through something, like a bulb or a motor. I’ve never tried reading through a cigarette lighter, but I suspect that’s another item you’d read a ground through.

Edited by RUSSELLM on Tuesday 30th June 17:17

Yeah, I have come to realise this.. Hence perhaps this is a mute point to test, it is more cross systems that may be the issue.

I can't see how I'm getting resistance across the Fuse 7 live to the injectors atm.

I'm going to refit the reverse wiring with some new connectors. Replace the Ignition key of death as that has failed (started getting random ignition dying too, fiddling with the switch would bring it back). Also new window switches as the plastic has failed on them and clips broken.

Then go from there.

[Jhonno]

Is the reverse switch a 12v circuit or earth?

I was getting continuity to ground off 1 wire, but it feeds back to the fuse box?

[Jhonno]

RUSSELLM said:

Jhonno said:

RUSSELLM said:

A point of note, if you haven’t already realised.

If you find resistance to ground at a fuse, be conscious of the fact that you couple be reading through something, like a bulb or a motor. I’ve never tried reading through a cigarette lighter, but I suspect that’s another item you’d read a ground through.

Edited by RUSSELLM on Tuesday 30th June 17:17

Yeah, I have come to realise this.. Hence perhaps this is a mute point to test, it is more cross systems that may be the issue.

I can't see how I'm getting resistance across the Fuse 7 live to the injectors atm.

I'm going to refit the reverse wiring with some new connectors. Replace the Ignition key of death as that has failed (started getting random ignition dying too, fiddling with the switch would bring it back). Also new window switches as the plastic has failed on them and clips broken.

Then go from there.

Are your injectors supposed to be on fuse 8 ?

Yes they are.

[Jhonno]

RUSSELLM said:

Might be worth removing fuse 8, and see if the injectors are still getting power.

Will try that in the morning..

I am wondering if I am missing something obvious.

I put continuity tester on, and am getting it between the Fuse 7 12v and earth. None of the other systems are doing this.

Are the cigarette lighters known for having an issue?

[Jhonno]

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.

This is what I am currently thinking.. I think I got too caught up checking resistances to earth, rather than continuity. I'll pull it out and check it again.

[Jhonno]

Right.. It's not cigarette lighter.

I've cleared down old wiring out and started to tidy up in there to make some more sense in there, and my 12v+ on the fuse 7 circuit to ground has disappeared. Still getting a red/white dash bulb to earth continuity.

I realise I still need to find the first issue, but I can't get it to come back atm.

[Jhonno]

RUSSELLM said:

Dash bulb ?

Is that the illumination for the speedo etc ?

As in the red/white wire that illuminates the dash bulbs. I have continuity between that and +12v battery (Fuse 7), and earth.

[Jhonno]

O mage said:

If you lost feed now then i would take the fuse box out and check the wiring at the back for being rotten sometimes they go green.

Sadly the the 12v to earth continuity is back.

[Jhonno]

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.

[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.

[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.

[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.

[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.

[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.

[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..