[mrdreex]

Ok, I am not sure if all GM wiring is similar, but what I have is a '70 Nova with electrical issues. I am currently trying to figure out what goes to what under the hood with what I have. What is absolutely confusing the heck out of me is this: I am following a Haynes wiring diagram for my '70, and the right side of the fuse block (when looked at from the front of the car towards the back) cross-references and matches just fine. Now, the left side is a different story. No matter which way I turn it or rotate it, I cannot get any of the wires/color combinations to match up. I know that this was an externally regulated car at one point, but that regulator is long gone, along with a whole bunch of wires that have been cut.

My question is this. Is there only one place where a 12V+ source is supplied to the fuse block? How/where do you hook up the starter to the purple wire, to the fuse block?! What all should be connected to be able to have power to the lights?

Thanks!

[wildman1932]

Your going to need to know some basic wiring fundamentals. The purple wire is the start wire that energizes the starter Bendix ("S"). But it does not hook to the fuse box, it goes to the ignition switch. To the "S" terminal. Or start wire .The power to the head lights on older cars do not go to the fuse box ether, they get power from the light switch. And the light switch gets power from the battery side of the ignition switch. The turn lamps are fused through the flasher fuse and goes through the turn signal switch.
I did not get real technical, but in general terms.
Dave
Daves Hot rod Shop KC MO.

[mrdreex]

Originally Posted by wildman1932

Your going to need to know some basic wiring fundamentals. The purple wire is the start wire that energizes the starter Bendix ("S"). But it does not hook to the fuse box, it goes to the ignition switch. To the "S" terminal. Or start wire .The power to the head lights on older cars do not go to the fuse box ether, they get power from the light switch. And the light switch gets power from the battery side of the ignition switch. The turn lamps are fused through the flasher fuse and goes through the turn signal switch.
I did not get real technical, but in general terms.
Dave
Daves Hot rod Shop KC MO.

Hey Dave,

So the purple wire doesn't go to the fuse block at all? I am not sure if it is even fused, just know that it 'plugs' into the block itself. I can find where it 'plugs' into the fuse block (engine side) by looking at my Haynes diagram. From there it goes to the neutral safety switch (which is another problem, my car is manual, not sure if it's even in there), correct? Is looking at it from this way is coming at it 'backwards' (as in, this wire doesn't receive the 12V+ directly, but rather is switched from the ignition)? As far as getting power to that wire, I *think* it's supposed to come from a 12V+ source from another wire (10g red) coming from the horn relay. I will try to get some pictures later tonight. Thanks for any help.

[wildman1932]

IT COULD BE PASSING THROUGH THE FUSE BOX ( TO GET TO THE OUT SIDE OF THE CAR,) BUT NOT FUSED. FOR MANUAL TRANS JUST HOOK THE TWO PURPLE WIRES TOGETHER (BY PASS THE N/s SWITCH) 12v POSITIVE COMES FROM THE IGN SW. FOR THE STARTER. (PURPLE)

[mrdreex]

Originally Posted by wildman1932

IT COULD BE PASSING THROUGH THE FUSE BOX ( TO GET TO THE OUT SIDE OF THE CAR,) BUT NOT FUSED. FOR MANUAL TRANS JUST HOOK THE TWO PURPLE WIRES TOGETHER (BY PASS THE N/s SWITCH) 12v POSITIVE COMES FROM THE IGN SW. FOR THE STARTER. (PURPLE)

Ok, but as far as where the ignition switch gets the 12V from, where does that come from? My wiring diagram shows a 10g wire coming from the horn relay to the fuse block. I am not sure, but is that where the ignition switch gets the 12 volts from? The power from the horn relay comes from the battery AND the alternator (??) Is that correct? What is weird is I applied a 12V+ hot wire directly from the battery to the relay, and the connection grounded out (voltmeter read 0) after applying it the tip of the relay. Is that expected? And, yes, this is my first time having to debug wiring, so please bare with me, I am a noob.

This is what I'm going off of. Thanks Dave.

[Joe G]

Looks like you could use a better wiring diagram. The answer to what wire powers the ignition switch is on the next page - where all the wires on the right side of the diagram you posted are continued. Looks like it's the red wire - 12th up from the lower right corner.

Here is a link to a better and more complete wiring diagram.

(click here)

Hope this helps.

Joe G.

[mrdreex]

Originally Posted by Joe G

Looks like you could use a better wiring diagram. The answer to what wire powers the ignition switch is on the next page - where all the wires on the right side of the diagram you posted are continued. Looks like it's the red wire - 12th up from the lower right corner.

Here is a link to a better and more complete wiring diagram.

(click here)

Hope this helps.

Joe G.

Thanks Joe, I actually have that diagram in my Haynes book, but that is alot better quality/resolution. I was out there tonight, and managed to make sense of somemore stuff, although this came up. There is one point where a 12V+ hot source enters the cab, and that is through a 10G wire from the horn relay (which is connected directly from the battery). Once that enters the cab through the firewall, it is then split into three more wires: one to the light switch, one to the ignition connector, and one to the fuse box to power some electronics that are fused. When I stuck my voltmeter onto ANY of the split red wires (12V+ hot wire from battery), it would read 0. I disconnected everything, including the light harness from the switch and the harness that goes to the ignition switch. Still nothing on my voltmeter (yes I double and triple checked my ground). So, I started back from the hot wire at the firewall/fuse block entrance. I disconnected the part from the outside of the fuse block (where the hot wire pin is) and stuck my voltmeter in there. It read 12.5 volts. Once I connected it back to the fuse block, it would change back to 0 volts.

What does this mean? Does this mean that there is a ground missing somewhere? How can I go about checking/getting that hot wire past the fuse block? Thanks!!

[bullheimer]

not sure what youre doin when you put your voltmeter on but it measures a difference in potential. if you put it on a hot and measure to another hot, there is 12V to GROUND on both, but your meter will read zero. always put one of your probes on a known ground to do your measurements, (for the most part)

[mrdreex]

Originally Posted by bullheimer

not sure what youre doin when you put your voltmeter on but it measures a difference in potential. if you put it on a hot and measure to another hot, there is 12V to GROUND on both, but your meter will read zero. always put one of your probes on a known ground to do your measurements, (for the most part)

yes, I was using the chassis as the ground. I have a bolt drilled to the frame, studded through the trunk (where my battery is at) and the negative terminal from the battery bolts there. Could it be a bad pin on the inside (on the fuse block, where the hot wire comes in)? I didn't even think to check for that..

[mrdreex]

So I managed after a lonnnnng time under the dash to get the 12V+ source out to the lights. What confuses me is this, and I hope somebody can tell me exactly what this means. At my headlight harness (the plastic connector), I unhooked all the wires inside of the connector (12V+ high beam, 12V+ low beam, and the ground). I stuck the positive side of my voltmeter onto the 12V+ source (wire) coming to the left headlight, then touched the voltmeter ground onto the frame. It read 12.44 steady. Cool, I have power past the firewall now! I stuck that 12V+ wire onto the headlight, then voltmeter tested it again. Still 12.44 volts at the wire. Now, as SOON as I stuck the ground (from the headlight) onto the prong, my voltmeter jumped to 0.

W T F DOES THAT MEAN!

Is there a loose ground/ground not connected somewhere? This kept happening every step of the way from the junction at the firewall ALL the way to that 12V+ source out of the firewall. What IS good news is that I managed to get a 12V+ to the starter wire when the ignition key is all the way out (like when you're trying to start). I have a painless box that I will be hooking most of my aftermarket stuff up to (water pump, fuel pump, fan, etc), and I would REALLY like this to be street legal. I need to get all the lights to work. Thanks for any help.....

[75gmck25]

I think your headlight troubleshooting is working okay, but needs a little explanation. When you plugged in the ground connection on the headlight (and I assume you left your positive probe on the 12 volt terminal of the headlight), you provided a ground to the positive side of the meter. You then measured the difference in potential between that ground point (through the headlight) and body ground.
The ground potential shouldn't quite go to zero, but it it will be close, since you are only measuring the loss of voltage across the headlight. Its kind of like putting the volt meter leads on two positive power sources - it reads zero because they are both at 12 volts (no difference in potential).

Another couple of points about the wiring:
- Chevy uses the starter lug as a power distribution point. Its not a very convenient place to reach for troubleshooting, but it has a very direct connection to the battery. All the smaller power wires (10 gauge or 12 gauge) connected to that lug should have fusible links. Its fairly hard to see if the link has burned through because its under the car in the dark.
-- Make sure you find and check the instrument panel ground wire. There is usually one wire (10 or 12 gauge) that grounds all the instruments. If that wire has been disconnected, it causes a lot of problems with the dash. Some lights/instruments manage to find ground through other paths, so you might have lights or instruments that work only when the brake pedal is pressed or when the turn signals are on.

Bruce

[Joe G]

OK. Now you said that the power wire for the ignition and the headlight switch comes off the horn relay, right? I believe you've got that wire for the headlight switch, etc. hooked to the wrong terminal on the horn relay. I believe it is on the terminal that is supposed to go the the horn switch instead of the terminal that goes to the battery and the alternator. This would explain how the voltage drops to near zero when you complete the circuit - the power to the headlight switch is passing through the horn relay coil, which can't carry enough current to light a headlamp.

I gotta run now, but I'll be back later to explain further if need be.

Joe G

[mrdreex]

Originally Posted by 75gmck25

I think your headlight troubleshooting is working okay, but needs a little explanation. When you plugged in the ground connection on the headlight (and I assume you left your positive probe on the 12 volt terminal of the headlight), you provided a ground to the positive side of the meter. You then measured the difference in potential between that ground point (through the headlight) and body ground.
The ground potential shouldn't quite go to zero, but it it will be close, since you are only measuring the loss of voltage across the headlight. Its kind of like putting the volt meter leads on two positive power sources - it reads zero because they are both at 12 volts (no difference in potential).

Another couple of points about the wiring:
- Chevy uses the starter lug as a power distribution point. Its not a very convenient place to reach for troubleshooting, but it has a very direct connection to the battery. All the smaller power wires (10 gauge or 12 gauge) connected to that lug should have fusible links. Its fairly hard to see if the link has burned through because its under the car in the dark.
-- Make sure you find and check the instrument panel ground wire. There is usually one wire (10 or 12 gauge) that grounds all the instruments. If that wire has been disconnected, it causes a lot of problems with the dash. Some lights/instruments manage to find ground through other paths, so you might have lights or instruments that work only when the brake pedal is pressed or when the turn signals are on.

Bruce

So, does that mean everything is working perfectly? As in, there are 12 volts on BOTH sides of the wires (12 volts at the 12V terminal, and 12V at the ground terminal)? Shouldn't there be light showing up if so?? (assuming that all the ground from that point on in the circuit are properly connected) Should I be checking for current draw at that point instead of voltage? I even applied 12 volts directly from the battery to the positive terminal ON the lights, but, nothing happened. No light show. Nothing. This leads me to believe there is a (bad) ground somewhere on the end of the circuit somewhere that is bad/not connected. Is that the right thinking?

It would do the exact same thing when I would push the brake pedal down (as if trying to stop and the switch completing the circuit, plunger coming up). The orange 12V+ wire coming in would show 12.5 volts, and as soon as I pressed the pedal down, all voltage went away..even at the junction block where the 12V+ hot wire is coming in.

As for the main power source, are you sure that it's supposed to come off the starter lug? In the diagram I attached earlier, it shows the 10g wire (bold) coming to the horn relay, and other wires stemming off of that. I don't see anything coming from the starter, except the purple wire which is the ignition wire. I plan on using the starter lug for the power to all my accessories that I will be custom wiring myself (electric water pump, fan, fuel pump).

It's actually not imperative that I get the ignition to work, it's just important that I get all lights/turn signals/brake lights working. This will be a street driven car, and would like it to be legal.

What should I be checking/doing next? Thanks for all the help!

[Joe G]

Originally Posted by mrdreex

......... I even applied 12 volts directly from the battery to the positive terminal ON the lights, but, nothing happened. No light show. Nothing. This leads me to believe there is a (bad) ground somewhere on the end of the circuit somewhere that is bad/not connected. Is that the right thinking?
......
What should I be checking/doing next? Thanks for all the help!

Based on that experiment, I agree that the grounds are suspect.

Have you tried it with a different battery? Just a thought.

I would check good for grounds in the following places:
battery to the engine,
engine to the frame,
firewall to the engine,
radiator support to the frame,
and the headlights to the radiator support.

And I'm sure you've already cleaned the battery terminals.

I rewired a '66 Caprice a few months ago, and ran into a bunch or weird problems. It turned out to be that some the bulkhead connector terminals had pushed back into the fuse box area when the bulkhead connectors were plugged in.

Sounds like you're on the right track. Good luck.

[mrdreex]

Originally Posted by Joe G

Based on that experiment, I agree that the grounds are suspect.

Have you tried it with a different battery? Just a thought.

I would check good for grounds in the following places:
battery to the engine,
engine to the frame,
firewall to the engine,
radiator support to the frame,
and the headlights to the radiator support.

And I'm sure you've already cleaned the battery terminals.

I rewired a '66 Caprice a few months ago, and ran into a bunch or weird problems. It turned out to be some the terminals in the bulkhead connectors had pushed back into the fuse box area when the bulkhead connectors were plugged in.

Sounds like you're on the right track. Good luck.

I guess it would be a good time to mention that the car is a roller right now. No motor/tranny in it yet. The battery is brand new (as of last week), but no, I have not tried another battery. I may try that tomorrow.