Headlight / Foglight interaction prob with LED bulbs
 

On my USA spec 88 S4 with H4 headlights, I replaced my H4 halogen bulbs with LED bulbs. The bulls are the Philips ZES style (3500 lumen each) I really like the coverage of the bulbs and have used them in a variety of night and rainy conditions. The High/Low beams work correctly.

However, I have a problem with my fog light operation.
1. Fog light switch on + Headlight switch in parking light position = Fog lights work and there is a faint glow in headlights in down position.
https://cimg6.ibsrv.net/gimg/rennlis...b9c8ccd59c.jpg

2. Fog light switch on + Headlight switch in low beam position = Fog lights are not on and High beams are on.

There must be some interaction problem with the bright function that would normally disable fog lights when the high beams are activated. It's seems to be working backwards.

I tried grounding the headlights and fog lights separately and that had no affect.
I stared at the wiring diagram for about 4-days and that just gave me a headache.

Someone suggested that this issue was covered before and the solve was to replace the double headlight relay with two standard 53 relays. I researched the archives but was not able to find anything on this.

I am posting to see if anyone has information on this mod. Would there be any harm if I just try the swap of the relays?

Also, I'd be curious to know if you have similar replacement LED bulbs and stock wiring and you don't have these issues.

Thanks in advance.

I think that proposed solution was misstated. I think they mean that you need to reconfigure the two relays that control the foglights. This itself only applies to some years, other years have a single more complex foglight relay.

Anyway this will potentially solve the interaction of the low-beams turning off the foglights, not sure it will totally solve your issues. Let me take another look at it later. I think many folks have found issues like this when re-configuring the headlights (LEDs, HIDs, local relays, HID fogs etc).

Porsche tends to use the un-powered lamps natural grounding through the filament to act as a sensor for the relay. Without an actual bulb in the circuit this doesn't work the same.

There may be some simple fixes.

Alan

Yep that was me, I went back and corrected my post in that thread, thank you Alan !

As I thought your year only uses a single more complicated relay in XIII. This makes the solution more complicated. What happens is that the fog light switch feeds the foglight relay coil positive through a diode, the coil negative is connected to the headlight high beams supply from the double relay. The foglight relay is supplied by relay IV which is only active in accessory mode.

What is supposed to happen (and does with bulbs) is that the fog lights relay turns on only when the fog light switch is on and the high beams are off (because the coil grounds through the very low impedance (off) high beam filaments. The current and voltage across the filament is so low the filaments don't even glow.

If the fog switch is on and high beams are also on the relays coil sees the same potential on both sides so stays off. If the foglight switch is off and the highbeams are on the diode prevents the relay coil from activating.

You didn't actually mention what happens with the high beams on - but I suspect that then the fog lights do go off and the high beams stay on normally (although this wouldn't look different from what you describe for low beams it is actually very significant what happens in this mode).

With headlights off and without the low impedance bulb but with the LED bulb instead - the LED bulb impedance is not so much different than the relay coil so you get a little bit of high beam illumination, but not quite enough voltage across the coil to activate the foglamp relay - so fog lamps are off.

What happens to the LED bulbs then depends a bit on how they work in detail. I suspect that these particular bulbs use either of the low or high beam inputs as a power source and sense the high beam to switch modes. There will be enough voltage from the foglight coil to cause it to think it's actually in high beam mode. If what I said about high beam active mode above is correct for your LED bulbs this is probably the case, also borne out by the fact you say they work OK with the fog lights off.

So what can you do?

1) You could put a low impedance to ground on the high beam signal. This replicates the bulb but is undesirable since you lose efficiency in this mode (however it's only active when you have high-beams on, so NOT most of the time even at night). It's a bit brute force - but you could try it. The resistors (one per side) will dissipate a lot of power . Testing this out would prove the functional hypothesis however. I think you would need an approx 50W ~3ohm resistor or easiest to test it out with an actual headlight bulb temporarily (you have some redundant ones...)

2) You could break the connection from the foglight relay coil 85 terminal to the high beams and instead ground the coil directly. This will allow the foglights to be on regardless of the high beam (This is the ROW configuration anyway - and how mine are configured - its required if you have HID fogs).

You could test this configuration before making permanent changes on the CE panel: get a relay socket with pigtails and install either the stock foglight relay or a standard Bosch SPST relay in it.

Then connect male 0.25" quick disconnects plugs on the pigtails for relay terminals 30, 87 and 86 and then plug these into their equivalent connections on the CE panel relay (XIII socket) and then just directly ground the 85 connection. If this works then implementing that solution permanently is the better fix you need.

Before any CE panel work - disconnect the battery ground.

Alan

Melted Brain
 

Alan, thank you so very much for taking the time for the very thorough reply. I think I will need to reread this a few times before I fully understand, but I think I am getting an understanding if what you are saying

MB: Sounds right, i just could not make that out from staring at the diagrams.


MB: Yes, as you describe


MB: I like this option and it makes total sense to me. I think I have some spare Standard "53" relays. Is that the same as the SPST relay you refer to?

Here is a diagram of what I think you are telling me to do. If the test works should I use the Fog Relay or just a standard "53" relay?
https://cimg3.ibsrv.net/gimg/rennlis...f075083799.jpg

Thanks buttloads. This was just the insight I needed to take the next step.

Mike

Problem Solved
 

Update -

I pulled the Fog Light relay and compared it to a standard relay, the ones with "53" labeled on it. I'd be damned if I can tell the difference between the relays from the diagram on the sides. They look identical. Since the pin numbering looked identical, I decided to try swapping the 'special' fog light relay with a standard one. Nothing exploded when I powered up so I tested everything. The good news is that the fog lights now operate independently of the high beams! No longer does turning on the fog lights activate the high beams. I'll call it good with that.

Thanks for your help Alan and Dave!

https://cimg9.ibsrv.net/gimg/rennlis...f9cc6af9d6.jpg
Fog light relay on the left

https://cimg1.ibsrv.net/gimg/rennlis...e6d7cc9dc3.jpg
Fog light relay on the left

https://cimg2.ibsrv.net/gimg/rennlis...4ff2bd3036.jpg
Fog light relay on the left

Ahh thats interesting! This confirms something I have suspected.

If you look at the ROW lighting configuration page (Page1) you can see there are 2 relay configurations - one of which (for Japan M193) has the coil 85 terminal like the USA connection to the high beams and the other (for everywhere else) has the directly grounded coil 85 connection.

USA and Japan safety equipment are often aligned and different to the ROW.

Here these different connections are implemented by different relays with different pin placements. The USA and Japan versions use a (narrow) corner terminal 85 connection while the ROW versions use the normal/standard (large) Terminal 85 location (see the first picture above). These two terminals are both wired in every socket - so only the choice of relay controls the operation. I had long suspected that USA cars were also wired this way (with both terminals wired in the socket - because it would be easier to just wire all cars the same way) but could never confirm. I think your discovery says they are for '87 & '88 at least. Your standard Bosch type (or 53B) relay has the pinout with terminal 85 as a large side terminal - so connects direct to ground - exactly as in my proposed "2)" case - and exactly like ROW cars.

This then is the easiest of all fixes - and should be fully robust.

BTW this means my proposal 2) test case would not actually work with the stock USA relay: because the USA relay does indeed have the small corner pin terminal 85 that the standard socket would not connect to. Anyway mainly irrelevant because this fix is far easier!

Alan

This is so awesome...I have done LED's for fog lights and head lights and have the exact same problem on my early '86. Going to throw a regular 53 relay in there and report back.

Its not equivalent for all years - I will take a look for an '86

Alan

Looks like an '86 is likely effectively the same - though the ROW mode relay & wiring is a little different - I think for this it will work the same way on a USA car.

Alan

All I can add is that you are soooo lucky Alan stepped in to help!

His tagline is there for a reason.

:thumbup:

Alan thanks a lot for checking for me !

Randy V I'm always grateful for all the help I always receive here....so many of the ideas and questions I've had answered and changes to my car are from the folks at Rennlist...my car wouldn't be half the car it is today without you guys!

Matt.

+928

I try to read all of his replies to posts, even if they have no relevance to anything I'm working on, just for the sheer learning aspect.

It appears so, and I will be trying your fix in a few days- will report in. Thanks Alan, and Michael too for starting this thread.

I can confirm that swapping the Fog Lamp Relay with a standard type 53 also works for 'USA' '85 through '86.5 MY's.
I have all three MY's running with LED's and no issues with feed back.

Glad to hear you got this issue easily sorted Mike :)

Dave K