Electricians - running 2 lamps in parallel and getting different brightness

measure the resistance of both headlights. Wire a resistor inline (rated high enough) that matches the resistance of the big headlight when combines with the small

Wire them independently to the battery each with their own relay. Use the existing feed from the switch to activate (all 4) relays. Alternatively use one relay for both 7" and another for both 5 3/4"

I feel a trip to as speciality auto-electrician coming on. Probably quicker and cheaper than burning the car out.

What are they wired to? I mean the power source? My hunch is that can’t deliver enough current

Does this mean the bodywork’s done? 🤞 

Haven't you already done a thread about this, or am I misremembering?

What happens if you wire the 'dimmer' one on its own?

Edit: yes, there was lots of good advice on this thread https://singletrackmag.com/forum/off-topic/dim-car-light-or-dim-me-electricians-to-the-forum-please/

The different light units aren't drawing the current as each needs. Simples 

Wire them in matched pairs

Consider heavier cable, parallel circuits can be more resistant 

The body is almost finished. 

I read the pegboard thread repeatedly and used it to do more tests. 

With both big lamps everything is correct which suggests there is difficult current. 

I will be taking it to a proper auto electrician  

My first trip to him suggested a bad earth which o have now proved wrong. 

I now think it is a difference in resistance between the units. 

I now need to know if a separate write for each light or a resistor between them is the best solution.

They are both powered from the original Porsche loom which i don't want to cut up if i can avoid it. 

Your thoughts please

Does the car run on Canbus? Possible the circuit has set limits to what it can provide based on the OEM lighting? 

To me it sounds like they aren't wired in parallel correctly.  I would make up my own wires first just to make sure they are both the same brightness when connected either individually or in parallel and then connect them back up to the Porsche loom and check that.   Something doesn't sound correct so in those cases I always start by trying to exclude things that we assume are ok, in this case the Porsche wiring

Edit: just read the other thread and it sounds like you have done this already.  My only guess now is that there is some additional circuitry in series for the old xenon headlights that is causing a problem but I have no idea what that is, sorry

Have you got a voltage reading against across each lamp in both configurations you can share? Ideally a current reading for each unit in each configuration as well but that’s more of a PITA to do.

With both big lamps everything is correct which suggests there is difficult current

It's the difference between units. The unit with the lowest resistance will flow more current, with a smaller current flowing to the other.

A pair is more likely to work because of the similarities in spec. 

Even manufacturers tolerances will eventually cause problems, hence matched pairs.

I think I agree with Timba - No just need to solve it.

Both circuits running about 12.8V. I have 2 little lamp units and 2 big ones. It doesn't matter which I switch, they all behave the same size wise. I can switch the connectors between the big and little and the litle stays gright.

I can only think ity is the resistance. I thin I need to know if it is better to stick a resister in. This can happen after the Porsche wiring loom so no great change.

or

Run a new line with relay etc and cut that into the existing Porsche loom.

Don't think you need too cut the Porsche loom to use relays. The control wire for the relay would need to attach to a plug that would connect to the Porsche looms lights connector. All the rest is new wires.

Perhaps I should have said 'patch in' rather than cut it. Connect the new wire into the fuse box for power and the switch circuit for control.

Nixie had it with his first post and has now repeated it, Worldclassaccident. Listen up. 😉

Adding lights to rally cars it was always a case of a new feed from the fuse box to the lights with a relay actuated by a feed from the loom (thin cable scotchlocked to the headlight wire). The extra lights required a switch in the relay actuator line but you won't.

Posted by: nixie

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Don't think you need too cut the Porsche loom to use relays. The control wire for the relay would need to attach to a plug that would connect to the Porsche looms lights connector. All the rest is new wires.

This is one solution and one that was raised by submarined and seconded by at least two of us on the previous thread. It's probably not an elegant solution, but it will work.

That said, a single relay for main beam (for example) will probably involve a parallel circuit to maintain voltage at the 10V minimum. That's when you get real world inequalities in resistance, just through manufacturing tolerances (yours and the LED unit).

I think that you're in DIY one relay per lamp territory or a shedload of electronics googling, unless you have any Uni of Southampton contacts where they have a good electronic engineering dept. IANAEE

I think we have the solution but I have managed top get the number for a local auto electrician who isn't scared of the unusual so will be contacting him for help. Each lamp unit is headlight, main beam, sidelight and indicator and the problem is mirrored on the other side. I want a solution that is neat, safe and unlikely to catch fire, none of which I can be sure of with my own wiring skills.

thanks all

Today 'I' fixed the problem.

https://photos.app.goo.gl/2qpDAF3PRhQZovnR6

There are 2 wires for indicators on each side. One for the main indicators and one for the side repeaters. I had them the wrong way around. The 'other me' in the photo also checked the rest of what I had done and declared it good and safe so wrapped in his special wiring loom tape to make it look pretty. He was even kind enough to ay that while he would have left the Porsche alone, he thought what I have done so far looks great and wants to see it when it is on the road.