78 SC Hazard lights and turn indicator problem

[snbush67]

Thread update. For whatever reason some people never post the fix. Problem resolved, I had single filament bulbs in the turn signals, they need double filament bulbs one filament for the parking lights and one that blinks. Don’t think it can’t happen to you.. But don’t stop reading, there is a lot of great troubleshooting and revelations from Dennis the wire guru in this thread that should help anyone with turn signal or flasher problems. Perseverance, don’t give up and the problem is almost always never what you think it is.


Please help. I’ve done a bit of troubleshooting and not sure what to check next.

The car is a 1978 911 SC USA.

I just rebuilt the pedal cluster, replaced the master cylinder brake switch, changed out the shifter and coupler, resurfaced the flywheel and replaced the clutch, and the car is running like a beast, but now I’m having light issues, I did replace the transmission and alternator grounds.

I do not know when the lights last worked right. I have checked the grounds and I have tried three new hazard relays. I have cleaned and inspected the turn signal switch (not really sure how to test it, or if it could be the problem).

The cars brake lights and headlights, horn and dash lighting are working normal but the hazards and turn signals are not.

1. Hazard light issues;

When the hazard button is on -

Hazard’s light solid with key off, lights off or on.

Hazards light solid with key on, lights on.

Hazards blink (normal) with key on or run, lights off.

2. Parking light issues;

When the light switch is on - and key is off, on or run - right side parking lights and license plate lights come on. The left side does not light up.

3. Turn signal issues;

When the light switch is off and key is off, on, or run the turn signals don’t come on at all regardless of the turn signal selector lever position.

When the light switch is on both dash indicators light up, when I signal right, the indicators and lights goes from solid bright to brighter. When I signal left the indicators go out, but the left side external lights come on solid (not flashing).

I have verified the connections of my light switch they match;




I have tested the function of my light switch at the switch terminals-see chart;




The only thing I have found questionable is an empty piggyback jumper on the headlight wire, I think it’s a left over from the alarm system that was removed decades ago, see picture;



What next?

[Dpmulvan]

Did you check relay?

[Dpmulvan]

Check all your bulbs and connections, rear can get pretty crusty,

[snbush67]

Yes, I checked the relay I have four, the original and two new, they all work the same. Yes, all the bulbs and connections are clean and work when they get power.

[Snitzler]

I had a similar problem with weird turn signals. It was the wiring going into the 4 way flasher, my mechanic went over the connections and some were loose.

[timmy2]

Use the correct drawing for your year and confirm wiring perhaps?
https://www.dropbox.com/s/yjfoxfvwlci6lys/911_electrical_78SC_USA.pdf?dl=0

[snbush67]

Dennis, thanks for the diagram. I pulled everything and there was a black white wire disconnected, I connected it and the hazards flash now. All the wiring looks right.

Still have one issue remaining.

I’ve narrowed down the issue to the grey red dashed wire connected to 58R at the light switch. Everything works as normal when the light switch is off.


When the light switch is on, the right parking lights come on solid and both turn indicators are lit. If I disconnect the grey red dashed wire all the parking lights and indicators go out.

I’m getting closer. I can at least drive during the day with turn signals now.




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K&N filter Measurements

[timmy2]

You may want to go up front to the trunk and verify the fuse box wiring as well. Grey/red and grey/black are the running light circuits. Black white and black green are signal lamps.
Are both front and back lights acting the same? If just the tail light, could be internal to it.

[snbush67]

Yes both the front and back are acting the same, the problem is isolated to left and right, and only when the headlight switch is on, not front and back. I’ll double check the fuse box.

[timmy2]

Remember to count the fuse position (S) from the headlight back when using the Porsche factory drawing. (Closest to headlight is one)
The 78 color schematic is nice as each fuse is colored for the fuse size as well.

Any bumper changes recently? Easy to cross wiring there as wire colors change from headlight bucket to bumper lights.

[snbush67]

Yes, the front right fender and headlight was replaced. I’ll look there again.

[snbush67]

Update; Still haven’t got it.

Found and fixed two issues, a short at the rear left reverse light wire pinched between the light housing and cover. And bulbs in the wrong sockets. So a couple of problems went away.

Still have this problem; When the light switch is off everything works as normal. However, when the light switch is on, the turn signals light up bright solid but don’t blink with the hazard button or left/right turn signal, and the dash turn indicators don’t light up or blink.

[timmy2]

Have you pulled the relay behind the fuel gauge and tested it?
But then again, it works normally (flashes) with light switch off, but not with light switch on.
Maybe check wiring from 58a of light switch to N6 and then to
58 on the hazard switch is all correct. Also check the ground is electrically good on the relay base.
Could be a voltage loading problem (voltage drop that stops relay from operating) due to poor grounding in the circuits for the lighting and flasher.

[snbush67]

Yes, I pulled the relay. I have the original, plus three new relays they all do the same thing.

Yes, 58a goes to the resistor (N6) and then to 58 of E3 hazard switch.

I have checked the relay functions at the base, but not with the light switch on. I’ll try that, thanks.

[snbush67]

Thanks for staying with me, As you describe it voltage loading and poor grounding problem sounds likely. I’ve checked the grounds you’ve mentioned.

The relay base 31 goes to a solid ground.

On the hazard switch 31 goes to a solid ground.

I don’t see a ground from the light switch.

I did find a cut wire near the front fuse holder (see picture below) but don’t see the other end, is the ground wire shown original or an aftermarket hack?

[timmy2]

Doesn’t look original. Brown is ground on original wiring.

[timmy2]

Some suggestions:
Check grounds at each lamp fixture and the Tach gauge along with the turn signal harness.
Confirm every wire lands where it should on the hazard and light switch.
Does the dimmer function for dash lights on the light switch work?
Maybe try removing bulbs one at a time with lights on and flasher activated to see if it starts working?
So strange that it works otherwise.

[snbush67]

Yes, the dimmer function on the light switch works and all the bulbs dim including the hazard switch. I’ll double check the wires at the hazard and the light switch. I’m not sure what I’m looking for on the tach ground. Is it a brown wire from the tach to ground? I’ll look it up.

[timmy2]

Never mind the Tach ground, just recalled those bulbs are isolated from the housing.

[timmy2]

It really seems like a wire is in the wrong spot, somehow removing voltage to the flasher unit when initializing the headlights.