Grounding through throttle cable, Return spring burned the crap outta me! - SOLVED Options

[IronHillRestorations]

Was the transmission or engine case painted? Just to test , clamp one side of a jumper cable on the battery -, and the other to the engine case and see if you still get voltage at the spring

[Spoke]

That's really funky. Is the alternator charging the battery when running? What is the voltage at the battery when running?

Can you post a pic of the throttle cable and spring? Just want to see where things are connected.

[clapeza]

I got a ground cable exactly like the one in Spoke's previous post and connected one end to the alternator mounting bolt, the one shown in the alternator replacement tutorial (http://www.914world.com/bbs2/uploads/post-419-1197334371.jpg).
The other end is connected to the engine case through a bolt near the taco plate. My previous extra ground cable from the engine case has been disconnected.
As previously said, a new ground strap has been run from the stud under the trunk to the transmission. I couldn't get a wrench on the original ground strap bolt, but I did attach it to a bolt on the transmission case nearby.

With the car on, but not running, I read 0.2v from the spring to the chassis. Car running, it read 8.4v, and got hot again. I did also see that my alternator has been replaced with a remanufactured one at some point in the past, so that opens up a whole can of worms of possibilities.

The engine and the transmission have not been painted.

I'll try to get pics of the spring and throttle body, and to check the voltage at the battery when running. I did that sometime before I got the new throttle cable hooked up, and was seeing about 13.6v while running.

I need to check my alternator gauge bulb. I know the battery shouldn't charge if it's blown, but it seemed to be charging before.

[clapeza]

I did some voltage testing, all at the battery:
Car off: 12.26v
On, not running: 12.1v
Running, idle at 1000 rpm: 12.6v
Running at 2500 rpm: 13.2v
Running at 2500, with throttle cable and return spring disconnected: 13.6v

One thing I forgot to test was voltage between the throttle body and chassis with the cable and spring disconnected while running, just for completeness.

Here's the picture of my throttle body, with the throttle cable and return spring in place. I did remove the air filter assembly, for clarity. (The fuel line is not as close as it appears to the return spring.)

[Spoke]

This is really weird. The entire engine case, FI components, spring, and cable should be at ground potential. The FI plenum should be connected to the engine case. The engine case is connected to the trans which has the ground strap to the chassis.

I'd never think of testing this but what is:

1) engine case voltage to chassis ground?
2) FI plenum voltage to chassis ground?
3) fan shroud voltage to chassis ground?
4) Pull alternator adjustment cover and measure alternator case voltage to chassis ground.

There should never be a voltage from the return spring to chassis. That whole lump should be at ground potential.

[76-914]

Have you tried pulling one fuse at a time until the voltage drops to 0?

[Spoke]

Have you tried pulling one fuse at a time until the voltage drops to 0?

Interesting thought. Along those lines, try disconnecting the voltage regulator (With the engine off!!) to disable the alternator and see if the spring still heats up. This will prove that the alternator is not grounded.

[clapeza]

I did some quick testing this afternoon, and came up with a theory afterwards.

First the data:
Engine running, throttle return spring disconnected:

1) Engine case to chassis ground: 0v

2) Fan shroud to chassis ground: 0v

3) Alternator case to chassis: 0v

4) FI plenum/Throttle body to chassis: 12v !!

My theory: Obviously, something is introducing 12v into the plenum/TB. What if it is the AAR? If the wire is wonky and it's not letting its current into its innards, but rather passing straight into the case, through the plenum, into the throttle body, and then through the throttle return spring...which is acting like the heating coil that normally resides inside the AAR?

I haven't had time to test it yet. All I have to do disconnect the AAR wire and see if I get a voltage through it and heats up the spring. I'll try it tomorrow and report back.

What'cha think?

Possible?

[Spoke]

It is possible that AAR wire is shorting to its case or shorting to the plenum. Absolutely disconnect the AAR wire and make sure the wire is not touching anything. The wire is always 12V.

[ChrisFoley]

Your plenum must be insulated from the crankcase or there would be enough current flow from a shorted AAR to pop the fuel pump fuse.

[clapeza]

GOT IT!!

I disconnected the wire to the AAR and started it up. I checked the voltage to the plenum and the throttle body with the return spring disconnected. With the wire connected, I got 12v. With the AAR wire disconnected, it read 0v.

I connected the spring, and it stayed cool. I wiggled the spring a bit, and where previously I'd see a little sparking, now there's none.

The fresh coat of paint on the plenum must be enough to electrically isolate it from the engine case. When I take it out to replace the AAR, I'll clean off where it touches the engine case.

Thanks, everyone, for all your help on this problem. I couldn't have solved it without your guidance and input!