So I installed a 280ZX alternator in my 240Z

[cnwayland]

I'm not sure what that last sentence since the starter grounds through the mounting bolts.

 

I attached an image of the ground strap:

 

 

Zed, I'm trying to wrap my head around the signal flow and the short-cut explanation helps.  I found that the '71 has the similar gauge setup with FUEL/AMMETER ~ and I'm praying it's the only thing that shorted.  I'm not 100% sure that it's happening only under starting.  I need some more experience with my multimeter to accurately know what I'm testing for.

 

Additionally, after reading through the link http://www.classiczc...ons-w-pictures/ Blue posted, I'm wondering if the issue is related to the electric fuel pump I installed utilizing the stock wiring harness...  

Edited March 19, 2015 by cnwayland

[Zed Head]

I see now.  That strap's purpose is to give a good path to ground for anything electrical attached to the body.  Body, engine, negative battery cable to battery or alternator.  Where is the negative cable attached?  It would normally be attached to that bolt and to the negative post.  That is the main path to ground for the engine block, but more importantly, the starter motor.  The starter motor draws a lot of current, that's the only reason the battery cables are so big.  If the negative cable doesn't have a good connection the current will flow through whatever path it can find.  Maybe the burning wire is providing that high current path for the starter motor.

[240260280z]

Yup! The neg battery cable usually goes to a bolt holding the starter to the transmission.

[cnwayland]

Okay guys, this is helping.  Hope I can help you help me.

 

A few answers:

 

The battery is grounded using the passenger seat belt bolt due to the battery relocation at the rear of the car.  I could relocate that ground up to the starter through the firewall, though the unibody s30 should ground at any point give the connection is good.  If there is a way to check for good ground connection, let me know.

 

Below you can see the blown ammeter ground which is in the molex connector with the fuel level sending wires (YELLOW, YELLOW/RED). 

 

 

I traced the ground back to where it connects with the rest of the grounds in the harness and it looks like it's not overly cooked at this point:

 

 

This is shot through the Ammeter Gauge housing in the dash looking toward the firewall. You can see the fried ground leading up to the rest of the harness.

 

I fully charged my battery and started looking for draws in the system and realized that I was showing 11.5 draw at 600 vDC.  I pulled fuzes with no avail until I pulled the 10A DOMELAMP LUZ INSTRU.  That dropped it to .15 @ 20 vDC.  Pulling the rest of the fuses didn't do anything.

 

I'm going to get a '77 Volt and Fuel gauge to swap in, but I want to find the source of this disaster first.

Edited March 19, 2015 by cnwayland

[cnwayland]

Oh, and additionally pulling the RED/WHITE connector in the harness for the dash power at the passenger kick drops the circuit draw out too, but this may be obvious.

It does seem logical that if the starter doesn't have a great ground that the ammeter would seek ground since huge amounts of current run to the back of the gauge through the dash harness.

Edited March 19, 2015 by cnwayland

[Zed Head]

Measure resistance from the starter body (where the negative cable would normally attach)  to the negative cable.  That will tell you something about the quality of that ground path.  You want a tiny number.

 

If you want to take a chance you could run a jumper cable (the big wire kind, used to jump dead batteries) from the end of the negative cable to the starter bolt and see if the wire gets hot again. 

Edited March 19, 2015 by Zed Head

[cnwayland]

Measure resistance from the starter body (where the negative cable would normally attach)  to the negative cable.  That will tell you something about the quality of that ground path.  You want a tiny number.

 

 

I measured resistance of less than 0.1 @ 200 ohms.

[Zed Head]

Probably not the problem then.

 

The fact that it only short circuits on Start is a giant clue though.  I would focus on that.  There's a only a couple of circuits that are different between Start and Run.  If you can leave it on Run with no problems, but the wire burns on Start, those changes are the source of the short.  Maybe the solenoid wire is shorting to the wire that burns.  When you hit Start, there's enough juice to power the solenoid and burn the wire.  I'd follow that solenoid power wire out and see what it's touching.  I have old charred insulation in one of my wire bundles, ugly but no shorts.  Someone's past mistake.  Maybe you have one of those, but yours went too far.

[cnwayland]

I'm showing continuity between the ignition start on the solenoid and GRND in the engine bay, so this doesn't seem right, yes?  

[Zed Head]

Continuity is correct.  It's  a matter of degree.  Actually, ohms.  The solenoid itself passes current to actuate.  Typically, resistance through a solenoid is about 60 ohms.  You need to measure resistance, not just continuity.

[cnwayland]

Zed, I wish I was as handy with the multimeter as you sound.

 

For this page's reference I'm going to explain how I got my car to run today, thankfully.

 

1)  I gave myself a few days off from thinking about the car, and came back this morning more clear headed.  Unplugged my fuel pump jumper at the dash (see: http://www.classiczcars.com/topic/50486-fuel-pump-wiring-harness-s/)

 

2)  I pulled the MSA voltage regulator plug and double checked the harness-side wiring to make sure the diode and jumpers were installed correctly (which they were).  [see Blue's earlier post for the relevant reference thread].

 

To my surprise I found the WHITE wire on the car's harness side loose in the molex connector and therefor probably making terrible contact.  I continued to pull the entire molex connector off and brushed, cleaned and rewired each connector into the MSA adapter plug, covering the connections with electric tape (for the meantime - rubber boot to come later).

 

I also reconnected the voltage regulator harness-side condenser to the black-wired female connector to the inner fender where the regulator used to sit.

 

3) Ran an additional ground cable from the rear passenger seatbelt mount where battery grounds, up to engine bay and bolted to the starter mounting bolt.

 

4) Cut out the bad ground wiring around my ammeter/fuel gauge and ran new 18 gauge ground to the back, reinstalled the + and - leads.

 

5) Switched the key into the ON position.  Double checked connections. Turned the motor over, double checking connections.  Plugged fuel pump jumped at dash back in, turned motor over.

 

Everything seemed good, so I went ahead and fired up the new motor.  Ran her for 20+ minutes at around 2500 rpm.  Now on to sorting out the triple 40 Mikunis and a possibly vacuum leak at the intake/manifold.  

 

Thanks again guys!

[240260280z]

And they drove happily ever after.

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