SWB Relays Analyzed

[304065]

It looks like there are SIX relays in an early 911

These carry part numbers
901 615 102 00, for horn, heating blower, ignition coil (quantity 4)
PCG 615 101 10 for headlight flasher (quantity 1)
PCG 615 101 10 for fog headlamp and warning light (quantity 2)

That totals seven relays. . . (somebody needs to help the PET editors count. . . )

Yellow Relay- Control Light Relay. This is a specialized relay with only terminals 30, 86 85, and 87a. Note that it is missing the normally open contact 87 that modern relays have. 30 is a black wire from fused side of Fuse #1, direct connection to battery. 86 is the hot side of the relay coil, this is wired in parallel with terminal 30 of the flasher can, when the coil is energized, the flasher is powered up and the relay contact opens, removing power from the normally closed terminal 87a. 87a, in normal operation carries power from 30 across the relay contact to the blink light unit. 85, of course, is the ground, in parallel with the S terminal on the flasher. Interestingly, this relay, and therefore the flasher coil, are switched by the ground side of the circuit- when you pull the hazard knob out, you are completing the path between 85 and ground.

Gray Relay- Relay for Light Signal 30/56, 57, 85, 56b, 56a.

57 on the light switch is the low beam contact- when the switch is pulled out current flows to the ignition switch. If the ignition switch is in the "run" position, then current flows to the stalk switch. With the stalk switch in the normal position, current flows to the unfused side of relays 9 and 10, which does three things- it illuminates the low beam bulbs, illuminates the warning lamp in the speedometer, and flows through terminal 57 of the relay through the coil to ground 85, energizing the coil and diverting current from 30/56 to the 56b (low beam) contact.

Fog Light Relay -Photo Copyright Tom Bridgers- the fog lamp harness is described here

[http://www.early911sregistry.org/for...g+wire+harness

This must be for TTG fog lamps, because in the regular 911/911s diagram, the fog lamps did not use a power relay. Again, this is a yellow relay with only 30, 86, 85 and 87a. We know that when you flip on the high beams, the fogs go out, right? That makes sense-- this relay isn't switching the power for the fogs, it's switching them OFF when the high beams come on. That is the only explanation for using a yellow relay-- the power to the fogs is carried between 30 and 87a, and when the relay coil is energized from the high beams, the wiper breaks the circuit. If you look in the photo below it looks like the terminals of the yellow relay are: 85 (ground) at top right, 86 (coil hot) at bottom right, 30 in the middle and 87a at lower left. That means that when power from the fuse box is present on the big yellow wire, the relay opens. Looking at the diagram in the thread referenced, you can see that the wires make a triangle-- from fuse box #2 to switch contact #2, from switch contact #1 to terminal 87a, and from terminal 30 to fuse box #3. The lamps are wired in series, presumably from fused side of #3 to one lamp, to the other lamp and then back to fused side #2. I guess the ground is present at the switch to provide a ground path for illuminating the indicator.

Horn Relay -The horn relay is down by the battery -See Photo below from Bob Tilton- and usually is pretty rusty. In the photo below, you can see the harness connections-- two black/yellow to the multiple 30/51 terminal, 87 is a red from the UNFUSED side of fuse Nr. 4 to the normally open contact and a multiple to the hot side of the relay coil 86, 85 is switched ground brown/white through the horn button. (Instead of a pure brown ground, it has a white tracer). The stamp for the horn relay is described here: http://www.early911sregistry.org/for...ghlight=wehrle

Ballast Resistor Relay - - Photo Copyright Nate Y. System logic is in the diagram-- 30 is a red/white to the bus connector on the electrical console, 86 coil hot is a yellow to the starter, 85 is ground, 87 NO contact is a black wire that goes inside the ballast resistor case, then out to the coil. The photo below (with the black background) is the ballast resistor relay, it appears identical to the horn relay, even though one of the terminals is not used in this application.



Webasto Heater Relay -A "conventional" relay with 85 ground, 86 switched power from ignition contact 75, 30 power from the battery positive through a 25A fuse, and 87 the normally OPEN contact connected to pole 1 of the Webasto switch. In this respect it is the same as the Ballast Relay and Horn Relay. All this does is enable the heater only when the ignition is on.

[304065]

Here's the location of the underdash relays and the wiring diagram. Also the diagram from the Engine Electrical. Horn relay location.

[304065]

Raj,

Looking at the factory 911S diagram, it appears that the bumper-mounted Hella 128s are a VERY simple circuit- grey wire from the light switch to the hot side of fuse #6, and from the same terminal, a white/yellow to the switch in the dash. From the other switch contact, a white/yellow to the hot side of fuse #5. From the FUSED side of fuse #5, a white/green down to the filament connection of each fog lamp-- return to ground through the brown wire riveted to the lamp.

With the headlamp switch in the first or second position the gray wire gets power and the lamps come on. I don't think they switch off with the high beams. Can anyone confirm?

The wire layout for the TTGs is in the other thread, I give the wire connections to the relay above. I don't have factory TTG lights, but perhaps someone here who does could confirm the connections to the relays? And a photo of the connections inside the fuse box would be helpful.

[304065]

But see here. . . this is an LWB diagram (two fuse boxes) but instructive. . .

http://www.early911sregistry.org/for...fog+light+wire

[Allen Henderson]

Written above:

"Fog Light Relay .............must be for TTG fog lamps, because in the regular 911/911s diagram, the fog lamps are unfused. ...l."

No, I believe this is for under the bumper fog lights.

Fog lights, driving lights...were all fused. But as stated..the relay was used to turn off the fogs when the high beam comes on.

-Allen-

[304065]

Allen my total bad, instead of "Unfused" I meant "did not use a relay."

[Allen Henderson]

For lights requiring high power, say over 50 watts, a relay was often (always?) used so as to keep the high current from going through the switch itself, especially for example the stalk switch which had small contacts.

I think fog lights are nominally about 30 watts, which at 12 volts is less than 3 amps. The pull-out light switch could handle 3 amps without much problem I believe. So that is why the lower power lights didn't necessarily need a relay and the power could go directly through the switch.

I think ALL factory installations of DRIVING lights did use a relay for light power; the switch was only used to activate the relay.

-Allen-

[304065]

Just to keep everything in one place, here is the fuse box diagram against the real fuse box.

[Allen Henderson]

Allen,john et al

I was wondering if you guys can help mke with this as i see it these are the possible senarios

-SWB without bumper fog with TTG fogs/driving light- wiring would be different one would come on in low beam other on high beam
-SWB with fog lights below bumper with TTG fog or TTG driving-i have seen photos of what seems to be fogs above and below bumper.

-The same senarios for LWB their wiring was different Also,i am wondering if the electrical system upgrade in 69 had any impact on how they were wired.

Raj

Raj, there are a lot of combinations. What specifically do you want to do and on what car?

The 1972 dated diagram applies from 1969 to 1973 inclusive I believe. Up until recently, at least, you could get from Porsche the switch harness. The "extra" harness for a pair of lights that is shown on the silver car..Well, I never looked for that harness. It too might be available from Porsche. Those LWB harnesses could be made to work for a SWB car but would not be correct and the relays had changed for the LWB to the round ones.

For SWB very very likely you will have to make up a harness but can use early relays to get the job done. First, we have to know exactly what you want to do. Thanks.

-Allen-

[304065]

And now for the Piece de Resistance. . .the Factory Hazard Circuit Logic!