72 Sierra Grande wiring - still at it
 

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

Re: 72 Sierra Grande wiring - still at it
 

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.

Re: 72 Sierra Grande wiring - still at it
 

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.

Attachment 2050749

Re: 72 Sierra Grande wiring - still at it
 

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

Bill

Re: 72 Sierra Grande wiring - still at it
 

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

Re: 72 Sierra Grande wiring - still at it
 

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

Re: 72 Sierra Grande wiring - still at it
 

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.

Re: 72 Sierra Grande wiring - still at it
 

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

Re: 72 Sierra Grande wiring - still at it
 

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

Re: 72 Sierra Grande wiring - still at it
 

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

Re: 72 Sierra Grande wiring - still at it
 

Thanks for that info. I'll do that once I figure out why the ignition switch does not activate the solenoid and HEI!

Even with the HEI installation (looks like a vintage GM setup), I should still find the purple wire from the ignition switch on the solenoid, yes? See the attached graphic from the internet. To actually see the wires I'd need to drop the starter, always fun.

Re: 72 Sierra Grande wiring - still at it
 

Yes, there should be a Purple wire connected to the "S" terminal on the solenoid but lets back up for a minute here.

1. Is your truck an automatic or manual?

2. If it is an auto it will have a neutral safety switch at the bottom of the steering column inside the truck. This switch will have two purple wires connected to it. Unplug the connector with the two Purple wires as shown in "cj847's" post #3 and make a jumper as he shows and see if the starter engages when the key is in the start position. Make sure the transmission is in the Park or Neutral position.

3. As far as the HEI, in your first post it sounds like you are not sure if the HEI is getting power or not. You need to make sure by testing the wire to the HEI with the switch in the "ON" position. It should show 12 volts on that wire.

Report back with your findings on these two things if you would please.

LockDoc

Re: 72 Sierra Grande wiring - still at it
 

Truck is automatic. I gave the neutral safety switch a careful check. Ignition switch “on” feeds power to the dark green wire for the backups. Ignition switch to “start” feeds power to one of the two purple wires.

I didn’t have time Today to test the HEI except to note that it has several wires attached. I’ll check them all tomorrow.

I did note while in the cab that when a load is put on, say headlights and heater blower, the ammeter does not respond. Not sure if it should or not. I checked the two shunt fuses, they’re fine.

Re: 72 Sierra Grande wiring - still at it
 

First --- you didn't conduct the test as instructed by Loc Doc.. Be that as it may, if you had the shift lever in either the Park or Neutral position, voltage should appear on booth those purple wires. Your results indicate you have a bad or misadjusted Neutral Safety switch.. If you didn't have the lever in either of those positions, the NSS is working as it should..

Now --- Go back and conduct the test with a jumper installed as Loc Doc instructed.. Report back with your findings....

Re: 72 Sierra Grande wiring - still at it
 

There should only be one wire feeding the "BAT" terminal on the HEI and it should show 12 volts on the multi-meter when the ignition switch is in the "ON" position.

Don't even worry about the ammeter at this time.

LockDoc

Re: 72 Sierra Grande wiring - still at it
 

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.

Re: 72 Sierra Grande wiring - still at it
 

Sounds good.

LockDoc

Re: 72 Sierra Grande wiring - still at it
 

Good deal.. :)

Re: 72 Sierra Grande wiring - still at it
 

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.

Re: 72 Sierra Grande wiring - still at it
 

Second test - HEI has power.

Re: 72 Sierra Grande wiring - still at it
 

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

Re: 72 Sierra Grande wiring - still at it
 

Boog - thanks for the reply! The cables are essentially new and very clean, likewise the ground connection. Before the fusible link incident that started all this aggravation, the truck started great, the starter spun at very high rpm. My one clear picture of the top of the solenoid shows everything in order. But it’s not a great picture ...

Re: 72 Sierra Grande wiring - still at it
 

Any thoughts on the outcome of the tests I did? Jumper did not activate the starter. HEI has power. Next step to drop the starter/solenoid and examine?

Re: 72 Sierra Grande wiring - still at it
 

One other thing you might try is to connect the jumper from the purple wire in that connector, (which ever one goes to the starter), to the "IGN UNFUSED" terminal on the fuse block. You won't need to turn the key to the "Start" position to see if the starter kicks in, just turn it to the "ON" position. If the starter kicks in turn the key back to the "OFF" position right away and disconnect the jumper. Make sure the transmission is in Park or Neutral.

Disclaimer: Since I am not there to see what is going on, you do any tests I suggest at your own risk..... I'm sure I could have the problem diagnosed in 15-30 minutes if I was there, but it is hard to convey some procedures by exchanging messages.

LockDoc

Re: 72 Sierra Grande wiring - still at it
 

Hearing that single faint "click" indicates the purple wire is connected.. It could be the solenoid is bad, or it's possible that wire terminal is not tight or voltage is low (weak battery??) for some reason. Diagnose the problem and find what's actually wrong, don't just start slinging parts at it...

Under any event, I'm curious as to why there is a second red wire attached between the distributor "batt" terminal and the solenoid, the one with the fuse.. It's not needed with a Delco HEI -- remove it while you're under there...