72 Sierra Grande wiring - still at it

[Bigblock50]

I'm still working on cleaning up the snarled wiring on my 72 GMC Sierra Grande. I discovered yesterday (on the glovebox label) that the truck has the "camper wiring" option which accounts for a lot of the spaghetti under the hood. The installation of the extra wiring is so poor I have to believe it was a dealer who tried to get it done. Some of the wiring matches the shop manual's "mobile home" diagrams, though the truck only had a slide-in camper.

Anyway, I reinstalled the main junction block wiring according to the circuit diagram and help from this forum, then gingerly hooked the circuit up. No disaster. With the key turned "Off", I got lights and horn as expected. With the key turned "ON" I got the blower motor and radio. Going to "Ignition" turned off the radio (OK) and unfortunately did not activate the solenoid/starter.

I have three leftover unconnected wires from the crazy quilt harness. First, a black wire, maybe 18 gauge, that APPEARS to be attached to the solenoid (very hard to see that) and was attached at the other end to a junction block mounted on the passenger side front fender. Then there are a pair of wires, blue and yellow, running together in wrapping tape that emerge from the cowl bundle near the heater blower and connected into the snarl of wires near the junction block.

Big question is why the ignition switch is not sending power to the solenoid and probably not to the HEI ignition module. Looking at that today, but testing ideas welcome.

Bill in Virginia

[HO455]

I would look at the neutral safety switch. It is between the ignition switch and the solenoid. If it is not aligned correctly the contracts will be open.

[cj847]

Check for 12V+ with key on at the power wire for the HEI.

The black wire goes from the junction block on fender to the connector block on the firewall. Mine has been removed, so not sure how important it is.

Regarding NSS mentioned by previous poster. You can disconnect it and check both sides for 12V+ when key is in start position.
Alternately you can make a jumper harness for it. Just be sure to fix it before you start daily driving it.

[Bigblock50]

Thanks to all for recent advice. I’ll follow up this weekend.

Bill

[Boog]

Maybe post up some pics. We may see something to help.

[LockDoc]

Originally Posted by Bigblock50

I have three leftover unconnected wires from the crazy quilt harness. First, a black wire, maybe 18 gauge, that APPEARS to be attached to the solenoid (very hard to see that) and was attached at the other end to a junction block mounted on the passenger side front fender. Then there are a pair of wires, blue and yellow, running together in wrapping tape that emerge from the cowl bundle near the heater blower and connected into the snarl of wires near the junction block.

Big question is why the ignition switch is not sending power to the solenoid and probably not to the HEI ignition module. Looking at that today, but testing ideas welcome.

Bill in Virginia

I'm betting the wire you are calling Blue (the one wrapped with the Yellow) is actually the 12 gauge Purple wire that is supposed to connect to the "S" terminal on the starter solenoid. Many times that wire will change color from Purple to Blue from the heat. Unwrap it a little bit and I'll bet it is Purple. Connect that wire to the "S" terminal on the solenoid and the starter will work. If you have an HEI you don't need the yellow wire.

LockDoc

[Bigblock50]

LockDoc - I will look at that today. One problem with the big block engine layout is that the solenoid connections are invisible and unreachable from any angle. You have to pull the starter to reach it. But I have a small snoopscope that might do the trick.

[LockDoc]

Originally Posted by Bigblock50

LockDoc - I will look at that today. One problem with the big block engine layout is that the solenoid connections are invisible and unreachable from any angle. You have to pull the starter to reach it. But I have a small snoopscope that might do the trick.

That would let you know if there is a wire connected to the "S" terminal on the solenoid or not. Let us know what you come up with.

LockDoc

[Bigblock50]

Extremely hard to see the solenoid even with the snoopscope, but I could tell that there were three things connected to it: the big battery cable, a wire connected to a short post, and a third wire connected to a regular screw terminal. Could not tell any colors. The unidentified blue wire I’ve mentioned is pure blue all the way. No sign of a purple wire. But I did find one oddity, see the attachment to this note. The picture is of wiring on the engine side of the firewall to the right of the distributor. There you see a cut off 12 gauge red wire. What would that be for?

I checked all the fuses in all ignition switch positions, using a neat reference on the forum. Everything checks out as it should, power is feeding the right things except for the ignition circuit.

Bill

[LockDoc]

Originally Posted by Bigblock50

Extremely hard to see the solenoid even with the snoopscope, but I could tell that there were three things connected to it: the big battery cable, a wire connected to a short post, and a third wire connected to a regular screw terminal. Could not tell any colors. The unidentified blue wire I’ve mentioned is pure blue all the way. No sign of a purple wire. But I did find one oddity, see the attachment to this note. The picture is of wiring on the engine side of the firewall to the right of the distributor. There you see a cut off 12 gauge red wire. What would that be for?

I checked all the fuses in all ignition switch positions, using a neat reference on the forum. Everything checks out as it should, power is feeding the right things except for the ignition circuit.

Bill

Not sure on the Blue wire then. I downloaded your picture and added some arrows. The single and double arrows indicate what I think is the adapter for the camper tail light wiring. If that is what it is you can actually remove it and plug the original tail light wire connector into the firewall plug. That may get rid of some of the extra wires in that area.

LockDoc

[Bigblock50]

Nothing done today, I was making hay while the sun shone. Rain the rest of the week - in Virginia, can't even mow. I'll run the jumper test tomorrow and check the HEI.

[LockDoc]

Originally Posted by Bigblock50

Nothing done today, I was making hay while the sun shone. Rain the rest of the week - in Virginia, can't even mow. I'll run the jumper test tomorrow and check the HEI.

Sounds good.

LockDoc

[RustyPile]

Originally Posted by Bigblock50

Nothing done today, I was making hay while the sun shone. Rain the rest of the week - in Virginia, can't even mow. I'll run the jumper test tomorrow and check the HEI.

Good deal..

[Bigblock50]

First test - neutral safety switch jumper. Hooked up a 12 gauge jumper exactly as shown above. First turn of the key to “Start” yielded a faint, and I mean faint, click from under the hood. Successive tries yielded nothing.

HEI power test next.

I guess suspicion focuses on the solenoid itself. When the fusible link blew, possibly the solenoid shorted out at the same time? I’d have to drop the starter to even see the solenoid let alone test/replace it.

[Bigblock50]

Second test - HEI has power.

[Boog]

Did you start with cleaning and inspecting your battery cables, especially the ground? Both ends.

[VetteVet]

Here is a write-up and some visuals that may help.

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