Headlamp Relay

[Cyclobob]

I was looking for a diagram to add a headlamp relay upgrade to my 86 Carrera as I wanted to use OEM parts. In the end I did a lot of research and put the diagram below together. It would work for non OEM relays as well as 5 pin relays have the same pin numbers.

Bob

Pelican reference for the holder is here

And the relay is here

[Porboynz]

Good work, what sort of fuse mount are you using. I need to fit the headlight relays and fuse the dash light feed among other preventative measures. Maybe add a fuse to the fuel pump feed as well. Rather than the standard plastic clamshell fuse holder for cylindrical glass fuses I think I will find a holder that takes standard auto blade fuses.

[Cyclobob]

I used an inline mini fuse for B and a standard blade fuse holder for B and C. I might change these to ceramic though. I am reluctant to add new mounting holes so using existing holes under the relay/fuse mounting plate right now.

Bob

[Porboynz]

I like the idea of using the spare relay mount holes for the headlight relays, keeps everything standard looking and there is just the horn relay there at the moment.

[McLaren-TAG]

180 additional dollars to achieve a 'clean' look is going to be a hard sell for the masses. It looks like it was important enough to you to do it, and I can appreciate that.

Still 180 additional dollars for something that would sit under the carpet in most applications is a tough swallow. Easier I guess f you spent $20 for the parts at a swap meet.

[donporfi]

Edit. Was going to comment regarding terminal 30 but saw your 4 note.

[Tidybuoy]

Thanks for the diagram. I recently added power windows to my '74 and I did a similar project and put in a factory relay and fuse setup (relay parts donated from a parts 914). I think I will attempt to do the same for the headlights, using your diagram. I have a relay setup now but it's aftermarket and I have several spots available in my fuse box for this type of relay.

Vern

[stlrj]

It might be a better idea to consult with any Haynes wiring diagram for the proper way to wire headlamp relays. All cars, domestic or foreign, use headlamp relays and seem to be wired very similarly as though the electrical engineers went to the same school, using almost the same universal pattern.

Ultimately, it might be safer to wire the relays the way they were designed to be wired.



Cheers,

Joe

[Tidybuoy]

Originally Posted by stlrj

Ultimately, it might be safer to wire the relays the way they were designed to be wired.

The problem is that in 1974, the factory wired my headlights without relays at all. As a result, all the current flows thru the switch. Adding a relay simpy re-routes the current to flow thru a relay and used the switch as a switching device. This is actually safer and all modern cars use a relay system today.

[stlrj]

Originally Posted by Tidybuoy

The problem is that in 1974, the factory wired my headlights without relays at all. As a result, all the current flows thru the switch. Adding a relay simpy re-routes the current to flow thru a relay and used the switch as a switching device. This is actually safer and all modern cars use a relay system today.

Re-routing the switch current through #85 and #86 effectively eliminates the current problem. Even the later Porsche's as well as everyone else does it that way. Also, the reverse bias diode in this (red)relay is not needed since the load you are switching is not a high inductance load as in a fuel pump.

Terminal #30 is universally used as the supply voltage (batt) input and not the output (#87 is the output). Any tech, on seeing the above diagram would automatically scratch his head! WTF?

Sorry to be so obsessive about this, but as a trained Local 6 electrician/Local 39 stationary engineer, I have seen enough electrical wiring diagrams to know the correct way it is done.

Again, take a look at any Haynes or Chiltons wiring diagram ( no matter what make or model ) and you'll see what I mean.



Cheers,

Joe
74 911 with correctly wired headlamp relays.

[ZOA NOM]

and then there's this...

Headlight Relays Redux

[Scott R]

Timely, I just installed the kit that our host sells this afternoon. I'm just getting ready to install my H4's later this week. Kit was like $30 or something took about 10 mins to install. Probably something to consider.

[Cyclobob]

Originally Posted by stlrj

Also, the reverse bias diode in this (red)relay is not needed since the load you are switching is not a high inductance load as in a fuel pump.

Terminal #30 is universally used as the supply voltage (batt) input and not the output (#87 is the output). Any tech, on seeing the above diagram would automatically scratch his head! WTF?

To pick up on these and a few other points, firstly I wanted the change to look OEM so I wanted standard relay holders etc.

To answer stirj's points, when a relay coild is released, it generates a back EMF of several hundred volts, enough to cause sparking and degradation of the switch contacts used to energise it. This doesn't happen with a headlight filament. So by adding relay's you are are liable to cause damage to the switches you were trying to protect. The diodes cost a few cents so cheaper than new swicthes.

Secondly, yes you can use 30 as the feed if you wish, the problem is that on the porsche relay holder, 87a has a wire coming from it and when the relay is idle, 30 feeds power to this. You can cut the wire and insulate it, I just feel uncomfortable knowing that there is a loose wire with 12 volts being fed to it. By reversing the sustem and using 87 as the feed, only 30 is ever live and 87a is never used. Again, you can go with convention, but convention is flawed and this way is safer.

You could always buy the Jwest kit and live without diodes and the cost of the OEM parts, I am sure it would work well. I wanted to make sure my system was bulletproof.

Bob

[stlrj]

Originally Posted by Cyclobob

To answer stirj's points, when a relay coild is released, it generates a back EMF of several hundred volts, enough to cause sparking and degradation of the switch contacts used to energise it. This doesn't happen with a headlight filament. So by adding relay's you are are liable to cause damage to the switches you were trying to protect. The diodes cost a few cents so cheaper than new swicthes.

Hi Bob,

It's not the small relay coil that all relays have that generates the back emf but the the inductive load that does the damage when the relay opens and the inductance collapses. That's why fuel pump relays are red ( diode protected due to inductive load ) and headlamp relays are black ( without diodes, no inductance or back emf ).

Originally Posted by Cyclobob

Secondly, yes you can use 30 as the feed if you wish, the problem is that on the porsche relay holder, 87a has a wire coming from it and when the relay is idle, 30 feeds power to this. You can cut the wire and insulate it, I just feel uncomfortable knowing that there is a loose wire with 12 volts being fed to it. By reversing the sustem and using 87 as the feed, only 30 is ever live and 87a is never used. Again, you can go with convention, but convention is flawed and this way is safer.

Not my choice but #30 is the universal standard for the supply voltage, #87 is the normally open contact and #87a is the normally closed one.

I do give you credit for your highly creative solution. However it does make it nearly impossible to diagnose a headlamp problem should you ever have one or have someone else try to help. As far as safety is concerned, wiring it correctly is always safer.


Cheers,

Joe

[donporfi]

Is it OK to use the diode for any kind of load ?

[Cyclobob]

It's not the small relay coil that all relays have that generates the back emf but the the inductive load that does the damage when the relay opens and the inductance collapses. That's why fuel pump relays are red ( diode protected due to inductive load ) and headlamp relays are black ( without diodes, no inductance or back emf ).

I agree on the load that a relay is switching, equally the relay is a new inductive load for the headlight switches (previously they were connected to the headlights.

There are several articles on this, but this is really clear on the subject. Relays

Scroll down to the section on quenching diodes. to quote:

Anytime that a relay coil is driven by a circuit that is not specifically designed to drive a relay, you should use a quenching/suppression diode connected in parallel with the relay coil. The diagram below will show the connection of the diode. Initially, you may think the diode serves no purpose because the voltage applied to the relay cannot pass through the diode. This is true when the relay is energized. The diode comes into play when the power source is removed from the relay coil. When power is applied to the relay coil, a magnetic field is created and energy is stored in the coil. When power is removed, the magnetic field collapses causing a reverse voltage to be generated (it's called inductive kickback or back EMF). The back EMF can easily reach 200 volts. The diode will absorb the reverse voltage spike. This voltage, if not absorbed by the diode, will cause premature failure of switch contacts and may cause the failure of power switching transistors.

Anyway, I am really happy with my mods and all is working well.

Cheers

Bob