I have a 1977 280z and recently the battery was draining for an unknown reason.  I do not drive the car, it sitting in my garage. I also do not leave the key in the ignition.  After a lot of trial and error I noticed when I disconnect the black and yellow wire from the starter the battery remains charged.  Once connected it starts a slow drain.  When the battery is charged the starter works well. 

Any ideas what to look for and see why the starter would drain the battery? 

 

 

The black/yellow wire should only be energized if the key is in the start position. If you have a voltmeter, see if there is voltage to ground at the black/yellow wire. Also check for voltage to ground at the terminal on the starter where the black/yellow wire lands.

One of two things may be happening:

1. The switch on the back of the ignition switch is going bad and supplying voltage (but with a very small amount of current) to the black/yellow wire.
2. The starter is going bad and somehow is backfeeding the solenoid, again with a very small amount of current.

I didn't see any obvious cross-connection possibilities in looking at the wiring diagram. I hope no one ever decided to re-wire things to "make them better".

Ok, so from the negative terminal on the battery to the black yellow wire I see .8mV is that enough to drain the battery over time? 

No, it is not. Did you take a measurement of voltage to ground at the solenoid terminal on the starter with the black/yellow wire removed?

From the solenoid terminal to the ground is .8 as well.  I left it disconnected for now to see if something else is draining the battery.  Should know in a few days

 

Make sure the dome light isn't staying on. That can cause a "mystery battery drain". The incandescent bulbs aren't that bright, and you might not notice it in the day. I have seen that as a current draw in someone's car before. It was caused by a wonky door switch that the plunger wasn't breaking the circuit like it should.

Good point, let me check that.  Thanks for the help, appreciate it.  Will update the thread in a couple of days

 

Are you sure you measured 0.8 milliVolts, or was it 0.8 Volts?

Just double checking... 

Let me check again and make sure the drain is coming from the starter.  I thought I narrowed it down to that but now I am second guessing myself

Well that sure looks like milli-Volts to me.

I just took the same measurement on my car and 0.0 Volts, but even if your reading is real (and not simply noise being picked up by the meter somehow), that voltage would still result in a current draw in about 3 milli-Amps worth of battery drain. And while that low of a drain would eventually deplete a battery, but would (should) take months.

It has to be something else.  I see the voltage drop in days and not months.  I am not sure where this is coming from.  It never did this before and I haven't touched the car in months.  Too cold to work on during the winter. Time to pull fuses

Less than 1 mV is the same as noise. If was an analog meter set on the lowest voltage scale, you probably wouldn't see any meter deflection.

Things that can drain the battery and you might not notice:

1. A bad door switch that leaves the light on (mentioned)
2. The clock, even if it's not running.  (That's on the cigarette lighter fuse.)
3. Voltage regulator going bad. 
4. Aftermarket (or previous owner) modification such as stereo or car alarm.

How do you detect what's causing a drain? Well, probably one of the most elegant ways is to construct a test light to replace a fusible link.

Parts:

1. 1156 bulb socket (https://www.amazon.com/Socket-Harness-Sockets-Adapter-Connector/dp/B09BCMWFP5) 
2. 1156 bulb (https://www.amazon.com/Bonlux-10-Pack-Dimmable-Landscape-Replacement/dp/B0928RWLG3) 
3. Clip leads (https://www.harborfreight.com/28-piece-electrical-clip-set-67589.html)

You want an incandescent bulb because you want low resistance in your test light. Attach a clip to each wire of the socket and install the bulb in the socket.

With the doors closed and key out of the ignition, pull a fusible link. Clip the test light to the fusible link block. If it doesn't light up, put the fusible link back and move to the next fusible link.

If the test light turns on, note the color of the fusible link and take a picture of the fusible link block so that we can tell which link it was. Once we know which link, we can let you know how to drill down better to the guilty circuit.