Dodgy relay or dodgy wiring?

Anyone experienced anything like..?....

Recently replaced and re-wired some auxiliary driving lights.

Previous owner (or at least one of the 12) left behind mystery wiring with:-
- No relay in the circuit.
- A heavy-duty old Lucas switch on the steering column
- Most mysterious of all - no live feed to that switch. So the lights wouldn't have worked even if they hadn't been corroded to death.

I used the existing positive feed wires routed from dash-to-lights but replaced everything else. Fitted basic 12v 55w lights, new earth straps, an illuminated-when-'on' switch and standard relay.
- The load feed is delivered through an in-line fused splice from cigarette lighter circuit.
- The relay actuator and switch-light positive feed from the ignition-switched fog light circuit.
- Relay and switch illumination earth are shared.

All was well on fitting. Worked first time. Continued to work for several weeks. Until last weekend. After two or three on/off cycles, failed to come back on.

Usual check-list ensued:-
- Bulbs not blown.
- Fuses not blown (corrected the 35A that someone had put in the main fuse board for the cigarette lighter)
- Power to cigarette lighter proven by mobile charger.
- Voltmeter probe shows current is present in the main load circuit after both the splice-point to lighter and the in-line fuse before the relay.
- Fog lamp working and switch still illuminating at "on" position.

On that basis, I'm suspecting the relay is defective. I think it's still clicking at power on - but it was always extremely (suspiciously?) quiet in that respect and it's tough to hear over the switch itself.

I've only not proved it's the relay on the basis that I didn't have a spare. So all that taking the dashboard apart (I placed it behind the instruments) to probe the connections would likely tell me was I'd need to take the dashboard apart again when I do have one.

Spare relays are on order (see what I did there, one for the fix - I hope, one for the cupboard) - so expecting to resolve shortly.

I'm a fairly neat electrician (if not massively experienced - hence the thread) so I'm not expecting to find any of my insulated/crimped connections have caused a dodgy earth. And the relay is in a block holder.

However, given how new the install is I wondered if there's anything else I should be on the lookout for that might promote a relay failure?

I'd rather avoid having to go back and do another replacement.

Thanks for any pointers/suggestions....

From your diagnosis, as long as no earth faults dodgy relay or dodgy switch does sound favourite

Probably just bad luck

I think I've followed most of that. As station house says it probably is just bad luck on a dud relay, or one of you connections has dropped off it.

A couple of questions spring to mind though.

How many lights have you connected on the circuit?

And please describe again how the relay coil is connected. Is the illumination in the switch in parallel or series with the relay coil? And is your relay one of the ones with a diode built in?

I know you said the fuses are ok, but are you feeding the lights from the cigar lighter fuse?

And is your intention that you can only use the new lights if the fog lights are on?

And is the switch illumination a build or LED?

Thanks for the Q&A assist.
A's below. Hopefully clarifies for any pointers...

Q. How many lights have you connected on the circuit?
A. Two. Simple 55w driving lights

Q. And please describe again how the relay coil is connected. Is the illumination in the switch in parallel or series with the relay coil? And is your relay one of the ones with a diode built in?
A. Switch illumination and relay coil live are in parallel.
A. Not sure if relay has diode. It came from an accessory specific light accessory install pack. I did read some stuff about the possible flow-back of current that can arise without one. However also never seen any differentiation for accessory relays available with/without diodes.

Q. I know you said the fuses are ok, but are you feeding the lights from the cigar lighter fuse?
A. Not sure exactly what you mean by "from the fuse". Probably my description isn't 100% clear either. No - I'm not running the load wire from the fuseboard direct. It's effectively wired in parallel to the cigar lighter. I've taken a spliced feed from the back of the positive feed to the lighter. (i.e. like I've plugged the lights in using the lighter socket, except the feed is relay switched)

Q. And is your intention that you can only use the new lights if the fog lights are on?
A. No. The switch light/relay coil feed comes from the fog-light live feed before it reaches the on-column switch. So this is live-with-ignition regardless of if the fog-light is on or not. So on original install, lights would happily switch on/off without the fog-light being on.

Q. And is the switch illumination a build or LED?
A. Integrated LED in the switch.

Think I've got it now, sounds mostly good, just not so sure about taking the feed from the cigar lighter though.

The cigar lighter is usually powered via a 10amp fuse. 2 x 55w lamps will be drawing just under that on their own (about 9 amps). So plugging almost anything in to that whilst the lights are on is bound to blow your fuse.

Regarding your switch and the fog lamp feed, I think I just misunderstood. The new lamps wont be fog light dependent, but they will be headlight dependent. I.e. the fogs can only be on when the headlights are on. So that will be ditto for your new lights.

The diode containing relays are better, in general, in my opinion, but I cant see that not having one will cause you any problems in your current set up (I guess time will tell).

None of this has anything to do with your current problem mind. For that, I'd still be looking at that relay and the connections to it.

Thanks mundungus. Good to get a 2nd opinion so I don't miss an obvious (or even obscure) issue fix. Focus will be on the relay and connections there as you conclude.

Yes - I got one of the online calculators too and got the 9amps answer. However, the wonder of Land Rover variations means the official rating on the cigar-lighter circuit (according to my fuse-panel cover) is 17amps, not 10. As above, I found that had been given a 35amp fuse at some point. So replaced that with a 25amp. It uses glass-barrel type and I've never seen a 17amp replacement. The mix pack I do have didn't contain a 20amp either, so 25 was the closest available. On the basis the factory circuit wiring must be good for at least 17amps - and charging a mobile is the biggest load I'd ever put through it at the same time as the lights, I'm not too worried about that just now. I put a 10amp in-line fuse on the feed to the relay/lights so that should pop well before any threat to the primary circuit in case of a short.

Apologies I can't post a diagram - causing the misunderstanding on the wiring. The set up is independent of the headlights. Same for the fog light - which is a single rear light only and doesn't need the headlight's powered to switch on. So neither of those have to be powered on for 'my' circuit to function.

Can you point me in the direction of a listing/seller for relays that have built-in diodes? I've never seen one that's explicit on saying what they sell does/doesn't have them.

Yeah its perfectly normal to have a fuse that's lower in rating than the circuit can handle, the fuse is there to blow in good time before the wire starts melting or the device goes bang.

Your 10amp inline fuse will only protect against over current in that additional lighting circuit, where you might have trouble is if the lights are on and you decide to plug in an electric heater or something else that's quite juicy. Your 10amp fuse will be fine, because the lights are still only drawing 9 amp through it, but the wire from the cigar lighter to the fusebox will be taking that 9 amp, in addtion to what the new device draws. So that wire might smolder before your 25amp fuse blows.

I guess your fog lights are wired different to mine. I thought mine was standard, in that the fog light can only be used if the headlights are on.

This relay would be a good one with a diode. Although I wouldn't worry too much if you dont use a diode one. It doesn't sound like you have anything in that circuit thats particularly sensitive to reverse polarity.

12V And 6V Make And Break Relays Relays, Solenoids And Flasher Units : 12V 30A Mini Make and Break Relay II 072714

Thanks on both counts.

Good point - yes, I might dumb-down the main fuse back to a 15amp in case a careless passenger plugs in something too juicy into the lighter socket.

Thanks for the parts supplier too. Another useful bookmark.

Gents (Mundungus & Station House)

Thanks for the notes - this one turned out to be a simple and self-inflicted problem.

Despite my boasting of neat electrics, this came down to a failed crimp connection......


Which I replaced with my my preferred weapon for this kind of job - the heat-shrink butt connector. Only didn't use one before as they're relatively expensive and didn't think I'd need it used inside the (relative) dry of the cab....


At least the relay base connections were all good...



Next up - power to rear demist absent. Switch working. Dash light illuminating. Changed the relay. Suspect the voltage sensor switch may have failed...

Suspect the voltage sensor switch may have failed

Click to expand...

Had a similar issue when I rebuilt mine - I'd fitted the diode the wrong way round!!!!