Dead short?

[4oldcars]

All of sudden my truck won't start. At least it's in the garage. I thought the starter was shorted because it was making a hissing/crackling noise after I charged the battery. I replace the starter but no help. I have recharged the battery. It shows about 10-12 volts and lites a tester, however when I hook up the cables it shows a small spark, a small bit of smoke and the test light won't work with the cables hooked up. I unhooked it. Any ideas? Help please.

[gmachinz]

Sounds as though you have a hot lead grounding out somewhere....may have to inspect all the loom for a melted wire-may be hidden inside the conduit itself-rhese trucks don't have much for underhood wiring-I'd pull it and inspect everything.

[starterman99]

Take the battery and get it load tested.

[4oldcars]

I left the alt unplugged and with the batt hooked up it still won't crank. I traced pwr to the ignition but no pwr coming out of ignition sw in the start position. Pulled ignition and it's black/burnt inside. I'm still concerned about the alt tho. Should I change it? There is no visable wiring damage under hood or under dash. Then too, is the batt any good now? I can see where this could get expensive.

[4oldcars]

After changing the ignition switch I'm back to square one. Still no power on the purple wire coming out of the ignition switch in start position. Purple wire goes to starter. How can this be? Any help appreciated.

[VetteVet]

Originally Posted by 4oldcars

After changing the ignition switch I'm back to square one. Still no power on the purple wire coming out of the ignition switch in start position. Purple wire goes to starter. How can this be? Any help appreciated.

Where are you checking it for power? Check it at the neutral start switch plug from the key. You should have power at the key switch when you turn the key to start. You can jump the purple wires on the NSS plug if you have power there and if the truck starts, the NSS may be out of adjustment or bad. Be careful as the truck may start in gear.

Have a look at this thread.

http://67-72chevytrucks.com/vboard/s...d.php?t=655903

[4oldcars]

If life could be so easy - - -
PO eliminated the neutral sw. My truck will start in gear (when it starts). I found one of the plugs for it with a dark and a light green wire. Cannot find any other wiring to it.
I guess I should unwrap the harness and trace the purple wire from the ignition sw?
I changed the column a while back and put in a neutral sw, but it has no wiring, yet.

[picklito]

If you've still got the factory style starter wiring, then you have an "always hot" starter solenoid. Probably GM's worst idea ever. I'd begin at the starter, and trace every wire down there back up. One of them is fried from exhaust heat.

[4oldcars]

Could you/someone explain the "always hot" soleniod.

[picklito]

Good question. Sorry I left that vague. Here's a GENERAL account of how GM did their wiring. They used this basic plan for most of their car and truck models. This is not specific to any one year or model.

When I said always hot I'm referring to the fact that GM ran the large positive battery cable all the way down the frame past the exhaust to the large lug on the solenoid. It's live 12v ALL THE TIME. This cable is famous for being fried to a crusty mess over the years. I've seen them with virtually no insulation left. If this one grounds to the frame or chassis you can have serious damage... fire or exploding battery. Or, if you're lucky, the battery is just completely dead with no fireworks. So, this particular problem is pretty easy to find (if it didn't find you first).

Then, you typically have a (usually purple) wire coming from the ignition switch. Turn the switch to START and this wire sends a 12v signal from the ignition switch to the solenoid S terminal. It's also right down there by the heat/exhaust. If this wire grounds out somewhere between the ignition switch and it's course down to the solenoid... you typically have no problems until you hit START. Then you might get smoke, smell, other lights (dome, dash) dimming each time you hit START as it rushes current to ground. This one can be tricky unless you go under there and find the melted/shorted wire. If you test for 12v at the SUPPLY side of the ignition switch all will test fine. If you test at the side that becomes hot when you turn the key to start, this gets tricky. If the shorted wire is connected to the ignition switch while you test, you will often get very weird test results. It may show a decrease in voltage drop across the switch. THIS IS ALSO A COMMON SOURCE OF THE IGNITION SWITCH SMELLING, SMOKING, OR LOOKING FRIED WHEN YOU OPEN IT UP. The ignition switch is not designed to handle the current load of a wire to ground. It is only capable of sending a 12v "signal" to the solenoid so that the large positive battery cable can do its work.

Finally, some solenoids (varies with year) have a wire from the I terminal of the solenoid that takes 12v back up to the ignition coil when you hit START. This is to bypass the ballast resistor in the coil's RUN voltage supply, which is reduced to just under 10V by a resistor or resistor wire. If you have a short to ground on this I terminal wire it can behave similarly to the S wire shorts.

So, what started as a quick answer became a book. Sorry! Hope it helps.

I guess, while I'm here, I should mention that some GM models also got their main chassis power from the large positive battery cable down there. So, on some models, there are EVEN MORE wires sitting down there in the heat, grease, and road grime of the under chassis environment... Fortunately, although our trucks wired their starters this way, they kept the chassis power up top in the engine compartment. It comes from the smaller "side wires" that come off the battery cables and go to the junction blocks on the fender. fun fun FUN!

[VetteVet]

Combine this with the thread I linked in post 6 and with the drawings and you will have a better understanding of how the starter and solenoid works. VV

http://67-72chevytrucks.com/vboard/s...d.php?t=655903

[4oldcars]

Thanks picklito and vettevet. I don't feel as dumb as I was - - but I still haven't found my problem. I'm leaning towards the neutral switch wiring that ain't there. I'm going to have to pull my gauge cluster to find what was done and hopefully I'll find my problem.

[4oldcars]

Checking further - - - I have continuity on the selinoid from the battery cable to the "R" post. Is this right?

[VetteVet]

No the R post is only hot when the solenoid is engaged. There is a set of contacts inside the solenoid that closes when power is put on the S terminal purple wire which connects the battery cable with the starter windings (via the starter brushes) and sends voltage through the R terminal wire to the ignition coil to provide a hotter spark during starting.

If you have power on this R terminal from the battery cable and the wire from there is connected to the coil, you will have a constant drain on the battery.

[starterman99]

Originally Posted by 4oldcars

Checking further - - - I have continuity on the selinoid from the battery cable to the "R" post. Is this right?

If the ignition switch is in the on position and the wire is hooked up to the R terninal, the anser would be yes. with the switch off or wire not connected to R terminal the anser would be no continuity.