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Good day to you all, I hope you & you're families are all keeping well.

I have a pair of Lucas 'Square 8' fog lamps fitted to TRoy, my 1970 TR6pi. These are wired through a relay, which takes one of it's live feeds from the sidelight circuit, meaning that they will only work if the sidelights are on (but they do have a separate switch). Recently I've noticed that if I turn the fog lamps are on, whilst the headlamps are on, the extra load applied to the fog lamps, causes the head lamps to dim. Do I need to consider fitting relay(s) for the headlights. If so, what's the best way of going about this: I seem to remember there being a feature on this, in TRaction 'Technicalities', a few years ago, but needless to say, I can't find it now!

As ever, your advice will be much appreciated,

Cheers,

Paul

Edited by TR Paul
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A relay may not be the answer to this Paul. Where do your foglamps get their power feed from?  They really should have a separate power feed , not from the headlamp circuit, in which case this isn't voltage drop on the wiring.  Does the dimming happen with the engine running or is this with ignition off?  Heads and fogs together will mean approx 20 Amp drain and there might be a slight drop in battery voltage or if you have a 'weedy' alternator that could also not be up to it at idling speeds - do the lamps brighten as you rev the engine? 

 

 

Edited by RobH
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Hi Paul

If you do go the relay route, Moss have posted some useful instructions on their website. Search for “117-515” and then click on instructions. 

Cheers

Steve

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Agree w RobH, might need more grunt from the alternator.

If the issue is the wiring though it's easy to insert a relay in a way that is reversible.  I put 2 relays in as below (high and low beam) but 1 for the regular lights could be ok if you're not on high beam often.  The screenshot image below connects to a dip switch from a CR so is slightly different from the AAW diagrams.

Run a new higher current (fatter) fused wire to where the bullet connectors connect for the headlamps behind the grille.  This will minimise voltage drop.  This connects to the 30 terminal on the relay which I mounted to the back of the sidelight fitting so as not to drill new holes in the metal.

Use the existing wiring to "switch/activate" the relay into the 86 connection.  This doesn't need much current to activate so it take a current load off the elderly wiring loom.  Ground the relay at the 85.  The 87 connects to the feed to the headlamps (blue/red tracer).  You'll need to make up some short lengths of cable connecting bullet terminals to spades.

If this doesn't improve things I suspect Alternator is not up to providing the goods for your additions.

 

 

image.thumb.png.ff02e56d9fbf276fd62637ae0f140975.png

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On 5/13/2020 at 1:47 PM, RobH said:

A relay may not be the answer to this Paul. Where do your foglamps get their power feed from?  They really should have a separate power feed , not from the headlamp circuit, in which case this isn't voltage drop on the wiring.  Does the dimming happen with the engine running or is this with ignition off?  Heads and fogs together will mean approx 20 Amp drain and there might be a slight drop in battery voltage or if you have a 'weedy' alternator that could also not be up to it at idling speeds - do the lamps brighten as you rev the engine? 

 

 

+! do as above but if you have a 15ACR alternator I doubt that would be up to the job, get a 18 ACR and I suspect that your problem would be solved. Have you got a Bosch pump type that draws 15 amps as that will compound the problem on your car? 

Bruce.

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Posted (edited)

Hi, Paul, Rob, Steve, Bruce & Matt... Thanks for taking time to responding to my questions. I especially like Matts' relay wiring diagram. TRoys' alternator  is a 70 amp (Metro?) unit, from Minispares . I had it tested a couple of years ago, as I was having charging issues (which was actually a Fawlty ignition light),  and it came back with a clean bill of health. I feel it is still working efficiently, as the ammeter always give a good reading,  & there have been no other 'charging' related problems since I cured the ignition light problem (with much good advice & guidance from the TRR forum). The fuel pump is a Lucas unit.  My fog lights take one live feed from the sidelight circuit, the other one coming from the fuse box.

Cheers,

Paul.

Edited by TR Paul
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Paul - Using relays is a good idea anyway for best efficiency and to save wear on the switch contacts but may not solve the dimming.

The fact that the fog lights are fed separately from the fusebox should mean they are not sharing wiring with the headlamps so should not be causing extra voltage drop. Is there any other feed from that fuse and if so what colour is the wire?

You haven't told us whether you see this dimming only with the engine idling nor whether the lights brighten again if you rev. The point is that an alternator does not provide its full rated output until it it reaches 6000 rpm ( alternator revs that is, not  engine revs).  At idling  a 70A unit may only be doing 2000 rpm and may only be capable of supplying 25A or so at that speed.  If the effect disappears at (say) 2000 engine rpm it would show that this is indeed the cause.

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Hi Rob, good point; I haven't had a chance to try the lights out (with the engine running, as you described). I will report back, when I've done this.

Cheers,

Paul.

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