Headlight and Flasher mystery

[Nustad 510]

Hi all,

Ive got a 71 510, four door, RHD.  At the end of a long resto. I've got an electrical issue that has me stumped. Really hoping someone can help.

 

With ignition on, the flashers all work normally.

With headlights on, the front fender indicators partially illuminate.

If the ignition is on and the flashers working and I turn on the headlights the flashers stop blinking.

The horns also stopped working, despite operating fine on a bench test.

 

All the fuses are new and intact and ive cleaned all the connections I can find.

Seems to me like an earth or stray current issue.  Any suggestions would be much appreciated.

 

Cheers, Shaun.

 

[datzenmike]

Does the horn relay click when the horn is used?

 

Is the horn fuse blown or not making contact. Hold a quarter across the fuse contacts and try. If it then works the fuse is bad on bad contact. NEVER trust a fuse by looking at it. Power from the fuse to the relay is on the Green/Yellow wire. No power... no horn.  

 

The horn relay is turned on by the Black/Green wire being grounded at the steering column. You can ground the Black/Green wire at the relay to see if the relay is working. If it does then the horn button may be dirty or not working.

[Nustad 510]

Hi datzenmike,

Thanks for the reply. The horn relay does click when I push the horn button. The horn fuse is 20amps and I have full battery voltage to both sides of it in the fuse box. I will try grounding the Black/Green wire to test the relay.  Weirdly, the fender indicators illuminate when the horn is pressed. Appreciate your help.

Cheers, Shaun.

[datzenmike]

If the relay clicks the horns are not connected, or the wire off the relay, or possibly... the relay is bad corroded inside. I think this old style can be pried apart and fine emery cloth drawn through the contacts to polish them. You can also replace with one from an '80s vehicle.

[Nustad 510]

Thanks Mike,

 

I grounded the Black/Green wire to the negative battery terminal and got nothing. Using a multimeter I checked for continuity in the wiring between the two horn assemblies and from the horns to the relay.  That was OK. I have battery voltage at the relay and continuity in the wire between the relay and steering wheel.  Looking very much like the relay is to blame.  Thanks for your advice. I'll let you know what happens.

[Nustad 510]

Hi all,

 

Changed the relay for another stock one and tried a modern Nissan three pin horn relay as well and still no luck. Pressing the horn button only lights up the fender indicators. I did get a small electric shock with fingers on the old style relay casing and my hand against the strut tower, whilst pressing the horn button. Unfortunately to confirm this I had to press the horn a few times. ouch! Beginning to think there's an earth issue as the headlights make the fender indicators illuminate too.

[datzenmike]

Check the earth connection. There should be a black wire from the negative battery cable to the voltage regulator mounting bolt.

[Nustad 510]

Thanks again Mike.

 

I only have the negative battery cable to the engine block.   Ill be adding the cable you described. Cheers.

[datzenmike]

Usually there is a smaller Black wire molded into the negative battery cable that has a lug clamped to the external voltage regulator mount. Pretty much a length of 10 or12 gauge wire from anywhere on the block to the body sheet metal will do even from where the ground cable bolts to the head. I made a small one from the bolt on the engine bracket, around the rubber engine mount and to the bolt holding the rubber mount to the sub frame. This grounds the block directly to the body.

[BrandonS]

I apologize if I’m way off base, but this is what I understand from quickly looking at the wiring diagrams. I also checked on my car and the fender indicators only flash with the turn signals/hazards and do not come on at all with the headlights. Looking at the wiring diagrams I suspect yours to act the same, however my car is not US Spec.

 

The headlight system is ground switched meaning that 12V always sits on the circuit/bulbs and a ground is made at the switch to complete the circuit. The other lights (as Mike states below) are the opposite and switch 12V power with a constant ground.  If everything is working fine until you turn on the headlights and then your indicators light up, it sounds like you have a short between your headlight circuit and the indicator circuit. The side markers lighting up when your headlights are on is indication that your headlight circuit is providing 12V to your indicator circuit (aka short between the circuits) which in turn utilizes that circuits built in ground. The turn signals won’t work then because that circuit already has the 12V that the flasher would be providing and the lights are already on. 

 

I see you said you were doing a resto. I am not sure what all you have touched but at first narrow the scope of your inspection to the areas you touched. If you didn’t remove your dash or touch wiring in it do not start by ripping away at things in there looking for the problem. Only after elimination of the areas you worked in should you dig deeper.  In regards to areas you were working in, at first I would inspect connectors as these are the most handled items so they are more susceptible to damage from flexing, tugging and such.  Next look for exterior damage on your wiring bundle (chafing, pinch points, etc).  If you find any peel back the tape/wrap and check for damage to the wires’ insulation. Also, if there are any crimps in the system look those over for any wires that may have escaped them.  Then slowly widen your scope as you eliminate areas. 

 

Past visible inspection you’ll need some sort of device to test with, like a multimeter. 

 

 

Edited November 30, 2018 by BrandonS
Editted for the truth

[datzenmike]

To be clear the 12 volts is only on the headlamps (when the headlights are turned on) and the ground is selected by the dimmer switch and the headlight relay. All other bulbs..... marker, turn, 4 way, interior, tail lights, parking, license and dash lights are grounded on one side at all times and 12 volts comes from a fuse through a switch. 

[BrandonS]

11 minutes ago, datzenmike said:

To be clear the 12 volts is only on the headlamps (when the headlights are turned on) and the ground is selected by the dimmer switch and the headlight relay. All other bulbs..... marker, turn, 4 way, interior, tail lights, parking, license and dash lights are grounded on one side at all times and 12 volts comes from a fuse through a switch. 

 

This is true, I updated above to reflect this.

[Nustad 510]

Thanks people,

 

I am pleased to report the problem is fixed.  I ran a cable from the negative battery terminal to  the body (a captive nut on the strut tower) as Mike described above.  Everything now functions correctly. Upon reflection, my previous 510 had this exact cable, I somehow didn't realise it was missing on this car.

 

Thanks again, Cheers

Shaun.