Addendum to high beam wiring upgrade, potential nasty.

Nick Gustas · 07-30-2002, 04:08 PM

All relays have some inductance, meaning that when power is removed from the relay coil circuit, and the magnetic field collapses, a reverse voltage spike is sent out to the trigger circuit. On a sensitive circuit, a diode across the terminals to damp the spike is a good idea, it can potentially shorten relay life unless used in conjunction with a zener diode.

relay spike info page

Scroll to damper diode info

The high beam circuit on the S4 is sensitive to reverse spikes!

I haven't looked at a wiring diagram, but I believe the high beam tell-tale on the dash is directly connected to the high beam wiring. With one relay connected, or lowish battery voltages, the reverse spike isn't high enough to cause a problem. With both relays connected, and full engine voltage, the instrument cluster may intermittantly "reboot" when the high beams are turned off. A double flash or more would consistantly do it. I believe a high voltage spike is going back up the wiring and into the cluster. Oops

I fixed it using a 1N4004 diode soldered across the high beam bulb socket so the polarity would not get mixed up down the road. One diode fixed the problem, but I put one on each relay to be safe.

The striped end _must_ be connected to the positive terminal, purple in my case.

My cluster seems to have survived, and other relays may cause the problem, but I wanted to forewarn of this quirk.

James R. · 07-30-2002, 04:17 PM

coil if undamped can produce 1000's of volts...the circuits on the sensing end can be destroyed if they're not properly designed to handle such events.

The diode will fix this for you...it will slightly slow the opening of the relay...but in the context of this application you could care less.

I'd put those diodes on ASAP :-) Cathode end towards the postive supply...Anode end towards the negative supply...then you're golden.

Regards,

James R.

Nick Gustas · 07-30-2002, 04:30 PM

in the morning, my first thought was OH ****. Got lucky and didn't fry it.

I didn't add a diode initially because the high beam circuit didn't have a burned out bulb detector, and the test run with a single relay didn't cause any trouble. Basically it slipped my mind.

Live and learn. I learned that audi's can be touchy, isolate, isolate, isolate.

Audiboy · 07-30-2002, 04:43 PM

the headlight stalk is just a switch that applies power to the fuse to the high beams. There is no relay in between.

In fact, the headlight stalk switch handles so much current that it tends to burn out - or at least the connector to it. If one wants to fix the headlight, he should hook up a load relay that will assist or bypass the stalk switch. The fog lights have a relay powered by the stalk switch. The headlights don't.

James R. · 07-30-2002, 04:47 PM

the voltage drop across the wires leading to the lights. So he effectively makes the wire shorter by bypassing everything with a relay that I assume is driven by the old headlight stalk switch. So he acheives higher terminal voltage at the light bulb (aka brighter) and reduces the load on the lamp switch thus extending it's service life.

Regards,

James R.

Audiboy · 07-30-2002, 04:49 PM

07-30-2002, 04:08 PM	#1
Nick Gustas Senior Member Account #: 3094 Join Date: Mar 2000 Posts: 7,470	Addendum to high beam wiring upgrade, potential nasty. All relays have some inductance, meaning that when power is removed from the relay coil circuit, and the magnetic field collapses, a reverse voltage spike is sent out to the trigger circuit. On a sensitive circuit, a diode across the terminals to damp the spike is a good idea, it can potentially shorten relay life unless used in conjunction with a zener diode. relay spike info page Scroll to damper diode info The high beam circuit on the S4 is sensitive to reverse spikes! I haven't looked at a wiring diagram, but I believe the high beam tell-tale on the dash is directly connected to the high beam wiring. With one relay connected, or lowish battery voltages, the reverse spike isn't high enough to cause a problem. With both relays connected, and full engine voltage, the instrument cluster may intermittantly "reboot" when the high beams are turned off. A double flash or more would consistantly do it. I believe a high voltage spike is going back up the wiring and into the cluster. Oops I fixed it using a 1N4004 diode soldered across the high beam bulb socket so the polarity would not get mixed up down the road. One diode fixed the problem, but I put one on each relay to be safe. The striped end _must_ be connected to the positive terminal, purple in my case. My cluster seems to have survived, and other relays may cause the problem, but I wanted to forewarn of this quirk.

07-30-2002, 04:17 PM	#2
James R. Elder Member Account #: 227 Join Date: Mar 2000 Posts: 17,814	Ehh it can be damaging...the back EMF from a large relay coil if undamped can produce 1000's of volts...the circuits on the sensing end can be destroyed if they're not properly designed to handle such events. The diode will fix this for you...it will slightly slow the opening of the relay...but in the context of this application you could care less. I'd put those diodes on ASAP :-) Cathode end towards the postive supply...Anode end towards the negative supply...then you're golden. Regards, James R.

07-30-2002, 04:30 PM	#3
Nick Gustas Senior Member Account #: 3094 Join Date: Mar 2000 Posts: 7,470	Already done, it clicked in my head after I rebooted it a few times on the way to work in the morning, my first thought was OH ****. Got lucky and didn't fry it. I didn't add a diode initially because the high beam circuit didn't have a burned out bulb detector, and the test run with a single relay didn't cause any trouble. Basically it slipped my mind. Live and learn. I learned that audi's can be touchy, isolate, isolate, isolate.

07-30-2002, 04:43 PM	#4
Audiboy AudiWorld Super User Account #: 9 Join Date: Mar 2000 Posts: 13,205	[puzzled] wait, what are you fixing? what's broken? what high beam relays are you talking about? the headlight stalk is just a switch that applies power to the fuse to the high beams. There is no relay in between. In fact, the headlight stalk switch handles so much current that it tends to burn out - or at least the connector to it. If one wants to fix the headlight, he should hook up a load relay that will assist or bypass the stalk switch. The fog lights have a relay powered by the stalk switch. The headlights don't. __________________ --- no sig --- FAQ Mk1 A4 - by BSK Mk1 TT - by QuietsTTorm

07-30-2002, 04:47 PM	#5
James R. Elder Member Account #: 227 Join Date: Mar 2000 Posts: 17,814	He has higher wattage bulbs....which draw more current, this increases the voltage drop across the wires leading to the lights. So he effectively makes the wire shorter by bypassing everything with a relay that I assume is driven by the old headlight stalk switch. So he acheives higher terminal voltage at the light bulb (aka brighter) and reduces the load on the lamp switch thus extending it's service life. Regards, James R.

07-30-2002, 04:49 PM	#6
Audiboy AudiWorld Super User Account #: 9 Join Date: Mar 2000 Posts: 13,205	I see he wasn't stock ;P __________________ --- no sig --- FAQ Mk1 A4 - by BSK Mk1 TT - by QuietsTTorm

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