1979 just shutoff. Everything dead but the lights

[my240]

Don't want this to be too long but need to explain. I have a 1979 k10, 4x4,auto,400 sbc, shortbed ,all original with 51,000 on it. Truck has been flawless for 40 years. Still have the original uniroyal landtrac spare. A few weeks ago I was coming into driveway and it shutdown. No crank, everything dead but the headlights. I pushed it into garage and started changing things. New battery, alternator, starter (checked for frayed wires), new key cylinder( old key had no teeth left on it), new lower ignition switch, checked fuses in fuse box. It fired up and ran like a new one. Has done good for two weeks or so. Today I ran to town and when I came back to the house it shutdown again in the driveway exactly as before. Everything dead when you turn the key no fan, dash lights, nothing. I've changed all the big items but I must have missed something that must be shorting out and accidentally fixed it before. Give me you thought on where to start this time around. I really hate the thoughts of a completely new wire harness. Appreciate any ideas and sorry for length of this thread.

[johnwaynejeep]

I don't have anything for you right now, but I'll start digging around. It sounds like an electrical issue, maybe around the fuse panel. Look for any frayed/broken wires and check your ground connection under the dash. Make sure it is tight, clean and not frayed or broken.

Is there anything else you could think of that was common when the engine died those 2 times, besides your driveway? Were these short trips or had the engine been fully warmed up for a while?

I'll start digging through the schematics, good luck

[my240]

Ran into town about a 20 minute trip. Went shopping, came out it started and ran fine for 20 minute trip home. All gauges were looking good and it just quit in driveway. Dead.
Thanks for thinking. I have other vehicles to drive. This is just irritating.

[andyh1956]

Are you telling us that you changed all these parts before you diagnosed the problem & found out what component was bad?

[my240]

I guess that's right. I was taking a lot of suggestions of what it could be and----- it's not like the truck doesn't deserve the new parts. But I hear what you're saying. Problem is hard to trace.

[Desert_Eagle02]

Check the ground straps. Maybe loose or getting old/bad shape.

[Llay R. Rac]

my truck sometimes would die going over a bump, turned out to be a loose connector in the engine harness plug connector at the firewall. pushed the spade down into the plug housing with needle nose pliers, heard it click, replugged the housing , problem solved.

[my240]

I'll be checking around the wiring and grounds tomorrow.
Appreciate the replies.

[johnwaynejeep]

I took a look at all the wiring diagrams for the 79. The lighting system should have a separate power source going to the head light switch, which then feeds the head lights, marker lights etc. It does not go through the fuse panel because this is a high current circuit and it could damage the fuse panel. The starter solenoid circuit however is a low current circuit that does appear to go through the fuse panel. Your instrument panel lights are fed power by the light switch and then ground through the instrument panel.

Since your truck is not starting but the lights still work, I would suspect the problem is in the fuse panel. Probably on the ground side although I would not count out the power side as an issue either.

The ground bus bar is a metal bracket that is shaped like a taco and has 6 tabs on it for the various ground connections. Check all these. You should be able to see it mounted under the drivers side dash if you get on the floor and look up with a flashlight.

Start the truck (when it decides to cooperate) and start wiggling fuse panel wires one by one until it dies, that will be your problem. It will most likely be the main power source feeding the fuse panel or the main ground for the fuse panel.

I also suspected the electrical ignition switch which is mounted half way down the steering column. This electrical switch is controlled by the key switch which moves a metal rod up and down as you turn the key switch. I might be wrong, but it looks like the ignition switch is fed by the fuse panel. If the ignition switch had it's own separate power source then a problem here would not affect the other circuits, that's why I think the problem is in the fuse panel.

Bottom line, it looks like a problem with the fuse panel either on the cab side or the engine bay side, start with the engine bay side since it's easier to get to and is exposed to the environment more.

I'm done for the night, I'm tired

[4x4k30]

check the hot wire small 10 gauge red wire at the starter for a bad connection

[donw1986]

I would say its a loose connection probably near one of the parts you changed the first time retrace everything you changed

[my240]

Everything said here has been great information. I believe it will be a bad or loose connection somewhere around the fusebox. All the big items including the ignition switch itself have been change so I'm down to the tedious items. Again I appreciate all your time and information. I will keep you updated on what I find.

[my240]

Went down to garage this AM and got in the truck. Everything came on; fan,seat belt lights, buzzer, etc. Truck started right up. I shutoff three or four times and it started back every time. Started and brought up to temp, and started pushing and pulling on wires under hood, firewall, under dash, around fuse box, etc. Can' get it to shut down. It's pretty cold now and i'll go down again later. My problem is trusting the truck to get me back home if I take it out. Do I need that blower motor resistor connected to run truck. The new one I put in starts the fan on high instead of low when key is turned on. Is there a neutral safety switch that could be loose or grounding out? I'll stay after it and keep you updated.
Thanks
Bob

[Desert_Eagle02]

My guess is still a loose or bad connection or ground. That would explain the intermittency. Trying to find wiring issues can sometimes be a real pain.

Yes there is a neutral safety switch.

Also I reread your original post. So this is only happening after the truck has been driven and is up to normal operating temp?

[my240]

That's been the case. But most of the time even if you leave it for a few hours it's still dead. Today kind of surprised me that it started. I'm going to pull off and clean all the grounds I can find today. When it quits there isn't any accessories going like radio, heater, wipers, etc so I don't think it's a power draw or something with them. All gauges, oil, temp, ammeter, seem to working fine,? Will head back to garage shortly and see if it starts again.

[johnwaynejeep]

you can also check the electronic ignition for loose/damaged wires. This is the electronic control unit that I mentioned earlier. It's mounted on the top side of the steering column about half way down the column. It is connected to the key switch via a rod that runs down the column.

[hatzie]

You changed the starter. There are TWO Fusible Link power connections on the 3/8" BAT terminal of the starter. The 1.0mm²/16AWG fusible link feeds the headlamps regardless of the condition of the main power wire coming into the cab.

The larger 2.0mm²/14AWG fusible link feeds the fuse panel via the J-Stud on the firewall. This fusible link at the starter is probably your Gremlin.

The fuse panel has a second fusible link connection to the J-Stud on the firewall but your repairs didn't disturb that connection. This fusible link also protects the big fat 12AWG RED wire on the alternator BAT stud along with the wire to the bulkhead plug and on to the fuse panel.

Break out the test lamp and meter. Check for volts at the J-Studs. I bet you disturbed a dodgy connection in the fusible link feeding the J-Studs when you changed the starter out... just enough to make it work for a while.

NAPA either stocks or can get Belden 784696 fusible link wires with 3/8" ring terminals already installed. You'll still get the joy of soldering it in place and covering the connection with adhesive lined heat shrink tube tho. If it looks like the pictures you should slide 2" of adhesive shrink tube over the wire end of the terminal as a strain relief.

https://www.napaonline.com/en/search...h_form-allprod

[my240]

This is really interesting and something I know very little about. OK, the truck has started fine for the past two days and I have poked at every wire I could to try and shut it down. Explain why everything is fine and running for an hour or so then die. Did this fusible link get hot and shut down the fuse block? Everything dies. Then after a few hours cooling down it starts again? This really makes sense, even if I don't completely understand it.

[andyh1956]

[quote=hatzie;8149375]You changed the starter. There are TWO Fusible Link power connections on the 3/8" BAT terminal of the starter. The 1.0mm²/16AWG fusible link feeds the headlamps regardless of the condition of the main power wire coming into the cab.

The larger 2.0mm²/14AWG fusible link feeds the fuse panel via the J-Stud on the firewall. This fusible link at the starter is probably your Gremlin.




These Fusible Link wires should be inside the L shaped steel conduit tube at the RR of the engine above the starter, correct?

[Getter-Done]

One of the fusible link is down by the starter.
There is another on the firewall above the distributor either to the left or right of it I can not remember. These have gave me trouble over the years also.

[hatzie]

Originally Posted by Getter-Done

One of the fusible link is down by the starter.
There is another on the firewall above the distributor either to the left or right of it I can not remember. These have gave me trouble over the years also.

The number of fusible links at the starter and J-Studs varies depending on the model year the harness was manufactured to fit.

Originally Posted by andyh1956

These Fusible Link wires should be inside the L shaped steel conduit tube at the RR of the engine above the starter, correct?

The fusible links don't run through that conduit just the RED harness wires attached to them.

The original 1979 GM fusible links are only around 6-8" long attached to the 3/8" Starter BAT terminal on a single ring terminal.
They end before the conduit with plastic cylinders moulded over the splices to the RED harness wires they protect.
I usually clip the RED wire right next to the plastic cylinder, strip the wire and the fusible link back about 1/8" or a little more, tin the newly exposed ends, slide a chunk of heat-shrink over each fusible link as far away from the tinned end as possible, solder the new fusible links to the tinned ends of the harness wires, and slide the shrink tube(s) up to shrink it in place over the new splice(s).

The Fusible links at the starter are in a pretty inhospitable location. They are exposed to road debris, regularly get covered in oil, and get heated up by proximity to the exhaust. 40 years or so is a pretty decent service life under those conditions. It's not shocking that the insulation and wire would eventually break down.

[Getter-Done]

Hatzie
I should have went into a little more detail.
That way it would help the next guy.
The Square-Body's I am most familiar with are 77 & 78. I have worked with 73&74 a little. I have parted out a 6.2 liter 80 something model which had a lot of wiring harness. I also have an 84 model.
I will try to take Pic when it warms up to help out more.

[hatzie]

Originally Posted by Getter-Done

Hatzie
I should have went into a little more detail.
That way it would help the next guy.
The Square-Body's I am most familiar with are 77 & 78. I have worked with 73&74 a little. I have parted out a 6.2 liter 80 something model which had a lot of wiring harness. I also have an 84 model.
I will try to take Pic when it warms up to help out more.

I've owned, serviced, and modified 1973-1981, 1984, & 1985 squares including an M1008 CUCV with NATO compliant military hybrid 24v/12v electrical.
I design, service, and install AVL equipment in public transit vehicles for a living so reading all kinds of wiring diagrams from darn poor to excellent quality is not a foreign concept.
I have original hard copies of the factory parts books, wiring diagrams, and manuals that I scanned for all of the square body trucks from 1973-1991. I have an intimate knowledge of where to find the information in question.

This is a picture of stock unmolested 1979 K20 starter wiring that matches the 1979 wiring schematics with two fusible links...
The wire on the tiny #8 "S" stud is the CRANK wire.
The big hairy cable attached to the 3/8" BAT stud is from the battery.
The smaller wires crimped to the ring terminal at the 3/8" BAT stud are the fusible links. If they are original they will end in two white or black cylinders covering the splice to the headlamp and main J-Stud feed wires.



This is a similar harness end from an 82 El-Camino showing two separate gauge fusible links on one ring terminal. The lighter gauge link is the headlamp wiring feed. This clearly shows the factory plastic cylinders covering the factory splice points from the fusible links to the harness wires. These are the three wires that come out of the metal conduit. You can see the shiny area on all three jackets where the wires rubbed on the bell shaped end of the conduit. The PURPLE CRANK wire has faded to pink after nearly 40 years.


Just an example of the changes made over the years.
This next picture does not show a 1979 setup...
This is a different year GM where GM used ONE fusible link spliced to TWO harness wires. This is NOT correct for 1979...

[hatzie]

This is a fine example of a fatigued fusible link that completely failed and broke off in service.
The OPs vehicle hasn't progressed this far... YET.

[my240]

This is an excellent reply and clarifies a lot of my questions. I'm going down now and get the measurements of these fusible links and will pick them up tomorrow and change them. I feel good that this may solve the problem. It ran good today when I was moving it around in the garage but it only had to run a few minutes. I do have a 2-post lift which makes life so much easier. Let me ask one other stupid question. Trying to think what this shut off episodes have in common and all I come up with is when I pound on the brake in driveway at the gate, it quit. Today the brake light warning light came on on the dash. Could anything there be shorting out the ignition or power.
Thanks much for all info and will keep you updated.
bob

Looked at this wiring at starter and mine is the 2 going into 1. looks like your next to last pic that says not 1979. Sure looks original and it's going to be real close to that steel wire holder tube when i cut them off. About 3/4 inch to work with. Does that tube come off?