Blowing no.3 fuse

[amazon69]

Evening.

My number 3 fuse (8 amp) keeps going, leaving me with no rear lights, front side lights and no dash lights. The fuse does not go straight away, but after about 2 minutes of driving. The reason for it going does not seem related to me doing anything else, ie using indicators, fan, overdrive etc. It also does not go if I leave the sidelights on but do not drive the car, which seems odd to me, but then I know nothing about electrics. I left it on the drive for at least 20 minutes, engine off, side lights on, fuse stayed good.

The above suggests it's not a hard short to ground or similar, but that's about all I can think of.

Any ideas/tips for what I should be looking for? I looked at the wiring diagram but I didn't find it much help.

Thanks.

[dingov70]

Evening , my suggestion would most likely be due to corrosion causing a high resistance , the increased resistance increases the load on the circuit & eventually the fuse pops . It would be an idea to replace fuse & get someone to switch on your lights briefly and see if any are dimmer than the rest . If you find one light dimmer , check the bulbs /bulb holder & connections and then work along the loom breaking & making each connection after looking for corrosion .

Spraying electrical cleaner on anything you separate is worthwhile , you might not actually notice the crud causing your problem . Also look at the wires carefully to see if any have been pinched / crushed , check connectors to ensure that all of the wire is secure , broken strands are a potential resistance issue . Be methodical , you may amaze yourself with what you find

Almost forgot , check your numberplate lights as well , never forget the glaringly obvious ( I did once , never again ! )
Hope it helps you

[swedishandgerman]

That's the simplest fuse in the box!

It just feeds the side lights, rear lights and dash light dimmer.

Just from experience, I would head to the black connector block in the nearside inner wing. It goes manky quickly and I recommend a regular clean out. There's also a connector block for the rear lights, but I've never had a problem with that one. It's just the front sides, so you might care to open them up and check for deterioration in both of those too. The connectors on the back lights deteriorate badly too, so just stick your nose in around those too.

Sadly, the other thing is the dashboard light dimmer. Have a fiddle about with that one and see if it blows the fuse. With age, they only work in the fully clockwise position. I've never had one blow fuses, but they are a weak point.

All else, take it for a drive up to Exeter and we'll check it out together!

[simonvolvo]

Is your light switch still okay?

There is a coiled resistance wire inside (rheostat for dimming the instrument lights). A known problem: with age the compound in which the resistance wire is bedded down disintegrates (feels like there is sand in the switch when you turn the knob to dim the instrument lights). The resistance wire then becomes dislodged, gets tangled up and can cause an internal short in the switch.

[amazon69]

Thanks all.

I will have a better look around later today hopefully. The rear lights seem good, as does their connector. I will look at the front block connector.

The dash rheostat has always been great (ie working..) but it could be that of course. I did not manage to blow the fuse whilst fiddling with it however.

As with most things on these cars, it will be something simple, so I just need to look a bit harder.

[Ron Kwas]

A69;

Your symptoms are somewhat unusual...in review, a blown fuse is caused by an over-current, and if circuit is original and unmodified, with no additional loads having been added, it is unlikely that corrosion in the cirtcuit is an explanation (sorry Dingo, it does not work like that in simple circuits as we have here...see explanation: https://www.sw-em.com/Fuses,%20Alloc...d_a_Blown_Fuse )...BUT, there IS a place where corrosion might contribute to a blown fuse...and that is explained by the manner in which a fuse senses the current...it has a slight resistance which (intentionally!) generates heat as a function of the current being passed [governed by a derivative of Ohms Law: P(heat)= I2(current, squared) X R (Resistance) ], and if this self-generated heat has an additional factor added (caused by UNintentional resistance) in the form of corrosion AT THE FUSEHOLDER, or at the fuse itself, this can effectively decrease the current rating of the fuse to blow open at some lower level... See also: https://www.sw-em.com/gastight.htm#I2R%20Heating

Also, the fact that fuse blows only when engine is running can also be explained by the slightly higher system voltage (being pulled up by Chg Sys)...this increases the heat being generated at corroded fuse slightly to possibly put it over the edge of being sufficient to "blow the element open".

...so much for the long-winded explanations...bottom line is: Visually inspect Fuse and Fuseblock for corrosion, or better yet, using an IR thermal sensor, inspect same (and if no high rise above ambient is observed) I'd place a Current Meter (DVM) in series with the circuit to measure actual current being passed in the circuit...I expect it would/should be about half the fuse rating (which is well within the ability of the fuse's 8A rating to allow and tolerate without "nuisance blowing")...if it is for instance 7A, you need to locate this additional user/load...if Fuse or FH are corroded, the effective rating may actually be way reduced to cause fuse to blow at a normal (non-fault) circuit current magnitude.

Good Hunting!

Additional: The Resistance wire in Lightswitch Simon speaks of used to dim Inst Lighting, does indeed develop surface corrosion, which makes the slider contact feel nasty and gritty as we turn and move it...but this again is a mechanical effect which, unless wire has completely corroded through, fell apart, and is contacting chassis somehow (short to chassis!, including possibly intermittently), cannot cause the increase of current necessary to blow the fuse...remedies for this do involve taking the Lightswitch to bits to get access (in an Amazon switch, certain vintages of Lucas 1800 Lightswitches re open with direct access to the resistance wire (see: https://www.sw-em.com/1800_Light_Swi...orrections.htm ) and lubing the Res Wire with (guess what?...yup ACZP).

[swedishandgerman]

Originally Posted by amazon69

The rear lights seem good, as does their connector.

Did you have a look at all the individual connectors at the back of each light cluster? Pretty much means taking the whole light out and checking for fray wires: as they build up with corrosion, the wires start failing and I wonder if there's something touching, causing a short.

Forgot about the number plate lights too! Worth checking the wire from the NS rear connector up to the plate lights hasn't twisted and broke where the boot lid opens and closes.

[blueosprey90]

Have you screwed anything into the body recently?

On my PV 444, I screwed the trim back onto the front windshield support and unbeknownst to me, screwed into a wire to the tail lights that were running behind there.

[swedishandgerman]

I've just nicked this from our Ron Kwas from a separate discussion about back lights on this forum:

http://https://www.sw-em.com/Amazon_...estoration.htm

It shows all the wire ends that corrode which could well lead to the wires fraying. I wonder if you have a frayed wore touching the bodywork back there

[amazon69]

Amazing..thanks for all the help, just for a blown fuse.

A tenner says I've found it...the note about number plate lights did it..one of the bulb holders was loose and shorting out on a nearby screw, but only with associated vehicle movement.

Thanks again