Lost all electrical power? 73 C30

[TheCougarine]

Hey guys, I picked up a nice '73 C30 a while back and have been driving it off and on with no problems. Today I went to start it and it turned over maybe twice and then the whole electrical system went dead. I've got no interior or exterior lights, radio doesn't come on, nothing at all works. Jumping the battery seemed to have no affect. Any idea about whats happened?

[hatzie]

Battery terminals loose.
Ground connection to alternator bracket.
BAT connection on Starter.
Fusible link at the starter.

[old Rusty C10]

i have a 20 series and at one time it had a liftgate on it.. there was some kind of power cable to the rear of the truck that used to power the electrical pump off the starter some how that shorted and did the same thing you are talking about... all I had to do was drop starter and take off extra power cable to this thing

[TheCougarine]

Thanks guys the battery cables are tight but the ground is ran to the alternator bracket, could this in its self cause the power loss? Whats the best way to get to all the connections on the starter? I looked at it from underneath but honestly I'm not sure what the various wires are that are hooked up to it. The truck did have a slide in camper in it most of its life so its possible that some random electrical stuff could still be hooked up somewhere

[hatzie]

The 73 starter has three studs. They are different size studs so mixing them up is not a problem.

The Fusible link is the small gauge wire on the 3/8" stud with the big fat battery cable.

[SCOTI]

Loss of all power sounds like a Fusible link if it was within a short time period.

[TheCougarine]

Thank you guys, I'll check that out tomorrow. I'm assuming if its bad it'll be burnt up?

[SCOTI]

Originally Posted by TheCougarine

Thank you guys, I'll check that out tomorrow. I'm assuming if its bad it'll be burnt up?

Not necessarily. The link is designed to prevent burn through so the sheathing can be intact. They usually will be very pliable vs a typical wire (think normal coated wire & the same thing but w/no wire inside).

[TheCougarine]

Okay cool, so is there anyway to test and see if that wire is in fact the problem if its not clearly messed up? Could it be temporaily unhooked to see if it restores power to the truck?

[Dead Parrot]

If the fusible link is blown, unhooking it is the same as pulling a blown fuse, won't restore power.

Easy things first: Disconnect, clean and reconnect battery cables at the battery. Even tight, they can corrode and lose electrical connection. If that fails, disconnect and reconnect the ground on the alternator bracket for the same reason. Make sure your engine block to cab/frame ground cable is good.

The ground to the alternator bracket is factory standard for many years.

While cleaning the battery cables, check the cables themselves for corrosion. The wire inside can rot out over the years. The cable looks great but won't carry much current.

If the easy stuff fails, you will have to crawl under and check/test the link.

You can test around the link by connecting a known good length of #8 wire to the proper starter terminal and then to the junction block. Make sure the battery is disconnected while you do this. If the link blew, it blew for a reason. You don't want your face inches away from a wire if it goes incandescent. Reconnect power using the alt ground connection.

[hatzie]

The special short section of wire itself is a fuse. Usually Fusible links are sized two gauges under the wire being protected but not always. The GM wiring diagrams show the wire gauges used. PDF files of the GM wiring diagrams are in my signature.

In 1973 there are TWO fusible links in the system. They changed up where the fusible links were and how many they used on almost every year so this only applies to 1973.

#1 is a 14ga fusible link at the starter 3/8" BAT lug. It protects the 10ga RED wire that powers the Junction Stud block on the 1/4" stud. If you have the optional ammeter one of that gauges' wires will be run to a second fusible link on the same ring terminal.

#2 is a 16ga fusible link at the #10 Junction Stud. This protects TWO 12ga RED wires. One runs to the firewall disconnect plug and the other to the Alternator BAT terminal. The 12ga WHITE Alternator sense wire is on this link too.

This is the junction stud pair you'll find on the firewall. You can see there are two different size studs. In 1973 ALL of the power inside the cab comes through this junction.
In 1973 there can be a short chunk of 10ga wire with ring terminals between the 1/4" & #10 terminal or a special jumper terminal depending on when the truck was made.

[TheCougarine]

Okay surely with all this information I can figure this out. Thanks so much.

[GM72K10]

Check the main connection thru the firewall. Could have come loose. Been there, done that. You will lose all power if that happened.

[TheCougarine]

Okay I'll check that as well.
I did take some picture today and want to run them by y'all. Its apparent that at least some of the wiring on this truck has been messed with and honestly I'm not sure what is what. I'm assuming one of these on the starter is a fusible link or was. Is there any giant red flags thats jumping out to anyone that I'm just missing.

[TheCougarine]

Okay, I looked this thing over again today and have another question. I've one wire that splits into two connected with the main battery cable to the starter. One runs back back towards to front go the engine, I'm guessing the alternator possibly? The second is a short piece that loops back to the bolt to the lower left of the main battery bolt on the starter. Its the small black wire in my first picture. Is this where the fusible link should be?

[hatzie]

Originally Posted by TheCougarine

Okay, I looked this thing over again today and have another question. I've one wire that splits into two connected with the main battery cable to the starter. One runs back back towards to front go the engine, I'm guessing the alternator possibly? The second is a short piece that loops back to the bolt to the lower left of the main battery bolt on the starter. Its the small black wire in my first picture. Is this where the fusible link should be?

One of the two should be half of the ammeter feed. The other is the Power feed to the Junction Block. I thought that both of these wires ran up through the tube along the bellhousing and block.
Your Previous owner did a bunch of re-wiring. You'll need to pick through it and find out what he modified.

The alternator sense and charge wires should run to a fusible link at the Junction Block.

This is a Snippet with just the engine bay from the 1973 GMC CK Light Truck Factory wiring diagrams. If you download the 1973 wiring diagrams PDF you'll be able to expand the images to take a much closer look anywhere you wish. I can generally run them up to 200% magnification.



▲▲▲This▲▲▲ is a JPG snippet from the 1973 wiring diagram PDF.

Looking at the starter. The diagram shows three connections on three different size studs.

The 3/8" ring terminal with two connections is your fusible link connection point at the starter.

The wire marked 16B-105B attaches to a Fusible Link marked 20OR-105C.
Reading those numbers is not difficult. So let's pick some apart.
16B-105B means 16ga BLACK - Circuit 105 Segment B.
20OR-105C means 20ga ORANGE - Circuit 105 Segment C.

You'll see descriptive notes as well. Between 20OR-105C and 14D/BRN-2B you see ▼FUSIBLE LINK▲ telling that those two segments are fusible links.

A lot of optional wiring is shown in this diagram so you will not have all the wires shown.

There are two types of Junction Studs shown. Yours' has the Special double ring terminal so look at the top J-Stud drawing for what should be connected.

RPO C60 AC is a sub-harness that's not shown on the main diagram. It's a stand alone pane section on one of the next few pages.
The J-Studs have an inline glass fused connection to the RPO C60 AC sub-harness in 1973.
Here's the engine bay side of that sub-harness showing the 15AMP inline blower motor fuse on the J-Studs...

[TheCougarine]

I appreciate the help and direction as well as the diagram. I was worried that it was looking like someone had decided to do some rewiring so hopefully its nothing to crazy, the truck was running great and everything electrical, minus the blower motor worked fine so I'm hoping its just something simple. Its just a matter of finding it.

[clockdoc]

I had a similar problem with my '78 C10. The symptoms were exactly as you describe. Battery voltage looked good, battery cable connections were tight, fusible links checked out, cleaned up the ground connection at the alternator bracket and actually got into the fuse block looking for the problem. much to my embarrassment, It turned to a bad connection at the positive battery post.

Even though it was tight, it was so corroded at the clamp/post interface that the resistance was too high to run anything. Tip-off was when I noticed that, when measuring battery voltage to ground, I got a lower reading from the clamp to ground than I did from the post to ground.

Cleaned the post and clamp thoroughly with wire brushes, remade the connection and the truck came back to life.

In the end, I sure felt foolish, but I sure was happy.

Hope yours is as simple to fix.

Clockdoc

[TheCougarine]

I'll have to check that out as I've yet to find a fusible link thats burnt up. I did have an odd thing happen today though. While looking at the truck today I suddenly noticed the dome light had come back on so the cab had power, unfortunately turning the switch on killed the power again. Turning the headlights on would also kill the power. Since then I've not been able to get the dome light to come on again. Am I looking at some kind of short somewhere possibly?

[clockdoc]

Could be a short somewhere but clean your terminals really well before you go any further. The intermittent light reinforces the high-resistance battery connection possibility.

Clockdoc

[TheCougarine]

Okay I'll do that tomorrow. It'd be great if that was all thats wrong. I've also found about a half dozen wires ran to the back of the truck that are just hanging by the bumper not connected that I'll have to also see about.

[Wgesnerjr]

Just as a side note:

I had an Optima battery fail just like this in my caprice wagon. Turns out it was an internal short in the battery.

Drove to some friend's house no problem. Then went to leave and it gave two turns of the engine and everything (and I mean everything like battery was disconnected) died. NO power anywhere. Dead as a junk yard car. By the time the tow truck got there and started loading, the lights would come on but then go off. Still would die if I turned the key.

Next day. traced it down to the battery. Changed it out and no more problems.

Might want to swap batteries if you can, just to check.

[TheCougarine]

When it dries up here I will see about that. The battery is about 6 years old judging by the sticker on it so thats definitely worth a look.

[SCOTI]

Originally Posted by TheCougarine

I'll have to check that out as I've yet to find a fusible link thats burnt up. I did have an odd thing happen today though. While looking at the truck today I suddenly noticed the dome light had come back on so the cab had power, unfortunately turning the switch on killed the power again. Turning the headlights on would also kill the power. Since then I've not been able to get the dome light to come on again. Am I looking at some kind of short somewhere possibly?

Try using a different batt to eliminate the current one as the source of a short. As far as I've ever experienced, once a Fusible link goes, it's done & you wouldn't have an intermittent issue.

[TheCougarine]

Yeah thats kinda what I assumed, I'll see if I can get the battery checked out in the next day or so.