1976 280Z Wont Start

[Dave WM]

its fun to get a set of resistors and make up various P and S connections and the real fun bridge circuits, and see if you can calculate the V drop at each junction.

add that to the fact that this is really a kind of square wave , not even a sine wave AC and you have an inductor and a resistor and it gets real tough real fast.

Bottom line is either his battery is bad, wiring or contacts are bad ,injector(s) bad, or ECU is bad. No reason to keep trying to start until you get those voltages right.

I suppose its possible the dropping resistors are all nearly open  but not likely.

My bet is a bad battery or cable connections from battery to ground (chassis ground), remember he has dim light and low horn. I would not do another thing until that was resolved.

Edited August 25, 2017 by Dave WM

[Captain Obvious]

Other than the fact that your system voltage drops to 8 volts when you get to the ON position, everything sounds pretty much normal. When the key is ON and engine is not running, you should see battery voltage on both sides of the injectors. The 8V level may be so low the your ignition module or ECU won't work right, so we'll have to get to the bottom of that.

And the cross threaded stud on the starter could be part of the problem. You need a good tight connection between the starter cable and the smaller white wire which connects to the starter. If that cross threaded stud is preventing good connection between those two wires, then that could be part of the problem. Beyond that, some ideas?

Bad battery.
Bad connection(s).
Blown fuse or fusible link.
Something is drawing huge amounts of current and dragging the battery voltage down.

Don't burn anything up before I get there.    

[JSM]

The other thing you could do once you get the new starter innstalled, if the voltage drop continues, pull fuses one by one so you can possibly isolate which circuit is causing the drain. 

[NicholasKoenig]

Went through the fuse box on the inside of the passenger foot well. Put new correct amp fuses in and cleaned the connection tabs. 

Wiggled the injector wires while someone looked at the volts (key one @8.5 or so), no change. So I don't think it's a loose or crimped wire in the injector harness. 

Temporarily put a smaller nut on the starter to secure the power wires and its tight. (New starter will be here tomorrow with proper and new connection). 

Battery is reading 11.59 volts from the posts while car is cranking. The fuel pump is reading about 4 or so when cranking.

Applied power directly to the pump while cranking with a drill battery and she started right up and ran strong until power was removed 

So I guess the injectors aren't clogged haha

 

[Dave WM]

this is not rocket science, you need to trace the battery voltage starting at the battery post.

with key on and car voltmeter reading 8.5v

1.  check for voltage between the neg post of the battery and a clean bolt on the chassis, should be near 0 volts, if not then you have a poor ground connection.
2. check voltage from chassis (neg) to fuse links each side, voltage should be the same and should be near full battery voltage, if not you have a problem with the spade connectors or the fuse link itself.
3. beyond that you will need to start tracing things like fuse box voltage, combo switch voltages, etc..

I think the key is the headlights being weak. they do not go thru the ign switch so I think they should be able to be traced down. Once you find out the problem there I suspect the rest will fall into place.

again, no need to do anymore trouble shooting on why the engine is not starting until you are able to get a 12v reading on the voltmeter.

 

[Captain Obvious]

Yeah, that 8V thing is definitely a problem. Gotta be a bad connection somewhere.

I don't think it's a huge drain dragging the battery down. If it was, you would have seen smoke by now. It's got to be a connection issue.

I'll come take a look at it for you for beer. You got beer? 

[NicholasKoenig]

SHES ALIVE!! I want to thank everyone who replied and helped get my Z back under its own power. For any new onlookers with the same issue and as some closure to those you helped Ill go over what it ultimately took.

1) My red EFI wires were wrong  (I mistook these both as positive when putting the engine bay back in order after it being apart for months) but managed to get them sorted.

2) As a result or coincidentally my combo EFI relay under the dash went bad. Had pry marks from PO and the casing was fairly loose. Replaced with a new combo relay. 

3) Missing(?) and/or insufficient grounds. I say missing hesitantly because comparing my 76 280z to Captain Obvious' 77 280z, my engine bay lacks a handfull of grounds. Whether that was a design change or the process of many hands on the Z over the years. That said, despite me sanding and scuffing all my ground wires, it looks like the connection between the chassis harness and alternator wasnt clean enough. I had tested for ground on the block and it read ok, I had done the same on the chassis and it read ok as well but this reading wasnt accurate, I suppose the bolt I tested wasnt clean enough!  With those clean it literally started right up. 

Now Im turning my attention to vacuum leaks to get it idling tip top. If I forgot anything, Im sure Captain Obvious will fill it in at some point as he was the man who woke my Z from its coma!

 

[Captain Obvious]

My pleasure. Glad to help, and good seeing you again. For the record, when I left, his Z was not only running, but after we addressed a couple other issues, it was "running better than it ever had before". Haha!  I'm happy with that as an afternoon's work. Sometimes you get lucky and it's the first or second thing you check. Sometimes not. This time, we got lucky. Only thing that went wrong is we never got to the beer!  

About the ground connections... After I got home, I looked at the two wiring diagrams for 76 and 77, and according to the schematics they definitely added more ground ties in 77. And also according to the 76 diagram, the only tie between the block and the body is that connection pair we identified yesterday. There is, however, a connection on the diagram right from the battery terminal to the body (like you are going to do with that ground strap I left with you).

So the bottom line is that you might be missing a body ground connection that was built into the original factory battery cable, but other than that, yours is the way it's supposed to be for 76. And the inclusion of additional ties between body and block in 77 makes me think they weren't completely confident in the 76 design. And I'm not confident in that single Phillips screw on the back of the alternator either. I understand the purpose of single point grounding, but it seems questionable to count on that one single small connection to handle all the current from everything on the body. Headlights, brake lights, heater motor... everything. Through that one single Philips screw? I don't trust it.

Good luck moving ahead from here, and looking forward to seeing you on the road!

[Dave WM]

On my 75 there is a direct connection from the battery to the chassis up near the fire wall. so the neg term has 3 leads a big one to the starter mount bolt, one to the EFI harness and one to the chassis thru the firewall bolt. there are more of course but those are the ones directly connected to the battery. interesting how they kept changing that stuff around. I suspect the firewall bolt one may be missing if the battery cables are non OE, its a pretty short connection of about 6-8 gauge IIRC.

Edited August 28, 2017 by Dave WM
added pic

[Captain Obvious]

Yeah, the diagram for the 75 and 76 are the same, and what you suggested is exactly what I think happened. The original battery cables are long gone, and the replacement cables did not have provision for the pigtail firewall ground connection. That left only one tie point built into the body wiring harness that runs along the pass frame rail along the engine.

One end or the other of that wiring harness tie point wasn't making good connection, so the body (and everything on the entire car that uses a body connection as ground) was floating. Stuff don't work so good like that. 

[Dave WM]

added a pic. I found that battery connector at a local retail auto parts place, the thicker ground wire was not quite long enough to reach the fire wall, so I had to splice the OE lead. I was careful to get a good soldered joint there. Note the wire from the battery lead was actually a larger gauge than the OE lead to the firewall.

I considered getting the larger gauge to finish the connection but figured if the OE gauge was ok per design I would not need it any larger.

[siteunseen]

4 hours ago, Captain Obvious said:

Only thing that went wrong is we never got to the beer!  

So the bottom line is that you might be missing a...case of beer.

That's a direct violation of OSHA's required safety meetings.