Fitting led lIght bar - Large Voltage Drop

[Lexlee]

Hi, posting for a friend,
Whilst fitting a LED Light bar ( believed to be 120w) to a Landrover Defender (1995) he tapped into the signal wire of a relay that supplies his roof spot lights and used that as his signal for the light bar relay..
He has used the horns earth for the relay and also connected the earth from the Light bar to the same point, as it's redundant (air horn fitted)
His problem is that the bar initially lit up on main beam, then when to try the flash ... Nothing. The relay for the light bar can be heard clicking on and off, and when the bar is disconnected has correct voltage at the light bar connection point. when the bar is reconnected the voltage drops to 3v at the same point. Thinking the Light bar was now defective he then tested it on another battery and it works fine..
As there's not a great deal there could it be
1 Problem with signal to light bar relay
2 the earth point for relay being same as ligh bar
3 the relay itself
The wire he has used is 17 rated at 17 amp
Hopefully I've explained well enough ... If anymore info is required only to glad to supply it . Friends can be a pain as I hate typing , any ideas on subject would be greatly appreciated.. Thank you

 

[Midwest]

There's a member called davesparks who messes around with Land Rovers. Hopefully, he'll be along to assist.

 

[Lexlee]

Thank you Midwest, hopefully be able to give some guidance which I can pass on meaning I will stop being pestered

 

[sean_ork]

Run a temp earth from the neg batt terminal directly to the new light earth/negative (bypassing the existing earth/s) - then try it.

 

[davesparks]

It could be volt drop on the cables used, which can be fixed by using a bigger cable. If the bake is rated at 17A I guess it's 1.5mm thin wall automotive cable.
When we fitted a pair of 50W spots to the back of our van we had to use 4mm to overcome the voltage drop in the cable.

Also you might well have corrosion in the earth path, whereabouts on the vehicle have you connected it to earth?

 

[Lexlee]

Hi,
Sean_ork... Have run a cable to Earth on battery.... No change
Connected earth back up then ran pos from light to battery it lights up. So I assume the earth is ok?
Davesparks ...... The cable (just used micrometer First time in years) is 3.5 mm and is rated at 17amp says he got it from H..fords. The earth for the relay he's has connected that to the original earth for the vehicle horn (air horn fitted drivers side) and also connected the earth from the light bar to the same point as the relay earth ( original vehicle horn)

 

[westward10]

Have you got the vehicle running when you try this.

 

[Lexlee]

Hi Westward,
Yes he was out last night and not working.

 

[rsgaz]

tapped into the signal wire of a relay that supplies his roof spot lights and used that as his signal for the light bar relay.

So the relay is switched by the spotlights, but what supplies the positive 12v feed to the relay (ie, pin 87 or 30, doesn't matter which way round), which it then outputs to the LED bar?

The cable (just used micrometer First time in years) is 3.5 mm and is rated at 17amp says he got it from H..fords.

You mean the outside diameter of the wire including the insulation? The Halfords 17A wire works out at 1.85mm².

 

[westward10]

Have you tried taking the relay out of the equation, for example connect the the output for the led bar direct to the incoming supply.

 

[Lexlee]

A direct feed from the betters and it works,,
There is an extra 6 holder fuse box on bulkhead under the bonnet supplied by battery as vehicle is ex utilities ( elec or water board or similar) the feed for spots and light bar are from there. The vehicle is outside would photos help, the wiring isn't pretty as just trying to get it to work then guess he'll idly it up.
I said before on initial operation the light bar worked it was only when the flash was used and now the spots work as before but no light bar.

 

[westward10]

Have you left the wiring as it is and linked out the wires at the relay which switch the bar.

 

[Lexlee]

Do you mean take the signal wire and connect it to the wire that supplies the bar , basically removing the relay from the equation?

 

[westward10]

No, there must be a feed for the switching part of the relay which then puts the feed out to the bar when the relay is activated. Link these together but disconnect the battery first.

 

[Lexlee]

Ok, I'll go and give it a try, so leave relay as is and link the input and output of relay.... Two mins

 

[Lexlee]

Hi,
Just tried it, light bar works, take it the relay is goosed?

 

[westward10]

Possibly, it either needs testing or replacing. Can you see the switch rating of it.

 

[Lexlee]

Yes it clicks when power applied, rated at 40 amps..

 

[davesparks]

Hi,
Sean_ork... Have run a cable to Earth on battery.... No change
Connected earth back up then ran pos from light to battery it lights up. So I assume the earth is ok?
Davesparks ...... The cable (just used micrometer First time in years) is 3.5 mm and is rated at 17amp says he got it from H..fords. The earth for the relay he's has connected that to the original earth for the vehicle horn (air horn fitted drivers side) and also connected the earth from the light bar to the same point as the relay earth ( original vehicle horn)

3.5mm, is that the diameter? Conductor size is measured in square millimetres, I don't add the squared part as I'm not sure how on the iPad and everyone here understands how cable size is measured. I've just looked up automotive cables an 1sqmm cable is rated around 17A.

The earth point near the horn seems quite a distance from the usual places to install a light bar, could you not get a connection to the bodywork anywhere closer to the light bar?
The two possibilities I can see for this are either a high resistance connection or voltage drop, check that all connections are clean and tight and measure voltage at every point of connection with reference to the battery -ve to see where the voltage drops in the circuit.

 

[Lexlee]

Hi Dave,
Sorry forgot to mention light bar is at front in A-Frame in front of where it says Defender on the slam panel.

 

[westward10]

You have confirmed it functions when you bypassed the relay so the issue must lay there.

 

[Lexlee]

All wire and connections are new. With the bar disconnected there is 12v at the supply to bar, with it connected without the link in mentioned above their is 3v at the same point, there's only 1foot of cable to the light bar from that point.

 

[davesparks]

Hi Dave,
Sorry forgot to mention light bar is at front in A-Frame in front of where it says Defender on the slam panel.

Ok, I've got a pair of spots mounted to the A bar on my defender, each one is earthed directly to each spot's mounting bolt onto the A bar.
If it's the new plastic A bar then I'd probably go for one of the bolts which hold the bumper to the chassis rail at the bottom of the A bar.

If I remember rightly the horn is earthed to the inside of the wheel arch that it is mounted to, I'd not be keen to put a lot of current through that path.

 

[davesparks]

Having read the rest of the thread then I'd say Westwood has your answer, if it works when the relay is bypassed then the relay is probably goosed