Relay question

[grannyknot]

On a basic automotive relay where #30 is power in, #87 is power out and #85 and #86 are the two sides of the coil with one side usually being the signal and the other side is ground; why does a layout like this work as well? No ground going to the relay? This diagram is from Pete Paraska site, Pete's V8 Datsun 240Z Pages

Thanks,

Chris

[beermanpete]

The ground path is on the red/white wire and red/black wire. The original headlight circuit for the 240Z has contant +12 to the bulbs and selects hte high or low beam by grounding on or the other. With the relay circuit the ground for the relays is through the high/low beam switch which in not shown in the diagram you have.

Edited February 22, 2014 by beermanpete

[SteveJ]

You aren't following it back to the high/low beam switch. That switch will connect either the RB or RW wire to ground.

I have three comments on that diagram.

1. I would reverse the 30 and 87 pins. Bring the 12 VDC from the battery to the 87 pin. Most relays you find today are SPDT. That means there is a normally closed contact designated as 87a. When the coil is de-energized, 30 and 87a are connected. If the 87a comes into contact with ground, you'll blow the fuse. If you bring in 12 VDC to the 87 pin, it is just an open circuit when the coil is de-energized.

2. If you use this design, I would suggest a 12AWG wire for the wires going to the 87 and 30 pins. Match up the 12 AWG wire with a 20A fuse.

3. This is the way to wire it if you are considering LED headlights. Otherwise, it may be easier to go with Dave Irwin's design that you can purchase from MSA.

[Captain_Zeros]

It helps to know where the rest of the harness in the car goes. Here's my own scribblings to keep it sorted in my mind.

The headlight on/off switch receives power from the alternator, then feeds to the fuses in the fusebox, which feeds to the headlights themselves.

The highbeam switch selects which element in the bulbs to ground out to give you either the high beam (M for main, pin 15) or low beam (D for dimmed, pin 16)

The knee bone's connect to the leg bone, etc

So long story short the relay signal pins do have a ground, it's just controlled by the highbeam switch in the cabin!

Also, this is why if you have a failing highbeam switch it can go into a "neither" contact position and your headlights don't work and you panic a lot. The more you know!

Edited February 22, 2014 by Captain_Zeros

[grannyknot]

Thank you Gentleman. That answers my question and few I hadn't thought about.

[SteveJ]

It helps to know where the rest of the harness in the car goes. Here's my own scribblings to keep it sorted in my mind.

The headlight on/off switch receives power from the alternator, then feeds to the fuses in the fusebox, which feeds to the headlights themselves.

The highbeam switch selects which element in the bulbs to ground out to give you either the high beam (M for main, pin 15) or low beam (D for dimmed, pin 16)

The knee bone's connect to the leg bone, etc

[ATTACH=CONFIG]69883[/ATTACH]

So long story short the relay signal pins do have a ground, it's just controlled by the highbeam switch in the cabin!

Also, this is why if you have a failing highbeam switch it can go into a "neither" contact position and your headlights don't work and you panic a lot. The more you know!

One point of clarification: That description applies to 72 through 78 Z cars. The 70 and 71 are a little different. The headlight switch in those is not on the positive side of the circuit. It completes the ground between the high/low beam switch and chassis. There is no switch on the positive side of the circuit in those cars.

[ktm]

Yet another correction. The 1972 switches ground as well.

[SteveJ]

Yet another correction. The 1972 switches ground as well.

Actually, according to the FSM, the standard 72 combo switch does switch the positive side on the headlight. The White/Red and Red wires going to the headlight side of the switch are positive.

Also, the parts manual shows a change in part number starting in September 71.

Now from what I understand, it wouldn't be unusual for Nissan to have put 1971 wiring harnesses and switches into 1972 models to use up stock. Do you have an early 72?

[Captain_Zeros]

Undocumented midyear changes are the norm in my '73 for sure, it wouldn't surprise me in other years.

[ktm]

Steve, I misread my wiring diagram years ago and was under the false impression. This thread caused me to re-read the wiring diagram......I stand corrected.

Thanks,

Bo

[SteveJ]

Steve, I misread my wiring diagram years ago and was under the false impression. This thread caused me to re-read the wiring diagram......I stand corrected.

Thanks,

Bo

That's easy to understand. It took me a while to realize that there were several different switches for the headlights.