1978 C10 Electrically Dead

[clockdoc]

Since my first post about a leaking windshield, I've been driving my "new" truck (1978 C10 shortbed/305 V8) and making minor improvements as convenient. Today, I pulled the truck into the garage to install a new radio, cut it off and, a little later tried to restart it. It cranked but didn't start. Tried again and it turned a few revolutions and went dead. Investigation indicates that the fuse panel has no incoming power. I have no lights, horn, radio, etc. Nothing!

I have a Chilton's and a Haynes. Neither has an overall wiring diagram. I'm suspecting a fusible link. Do any of you have any other ideas?

I'll really appreciate any suggestions.

[buffydores]

bad ground

[ray_mcavoy]

I'd recommend checking the battery cables to make sure they have good clean & tight connections. Check both ends of both cables.

Thanks to hatzie, you can download a full factory wiring diagram (and factory service manuals) here --> http://67-72chevytrucks.com/vboard/s...d.php?t=545416

[clockdoc]

Thanks very much for the references. They should prove to be very helpful. I measure about 13.5 volts from the positive battery post to every chassis or engine ground I can reach. Fuse panel is dead

[ray_mcavoy]

Originally Posted by clockdoc

I measure about 13.5 volts from the positive battery post to every chassis or engine ground I can reach. Fuse panel is dead

Okay, that indicates that your negative battery cable / ground is probably okay.

But with the fuse panel being dead, I'd recommend checking voltage between ground and various points in the system. Specifically, check for voltage at the starter solenoid BAT stud and at the 2-stud junction block on the firewall. If you don't have power at the starter solenoid BAT stud, the problem is most likely the positive battery cable or it's connections.

If you do have power at the BAT stud, but not at the 2-stud junction then it could possibly be a fusible link. Similarly, having power at the 2-stud junction but not at the fuse panel could also be a fusible link. However, those generally don't fail unless there is a dead short to ground or a major overload of the main feed wire. Bad (loose or corroded) connections on the other hand seem to be much more common.


Oh, one other thing to check ... take a look at the purple wire going to the starter solenoid "S" terminal. If that wire has chafed or melted and is shorting to ground, cranking the engine could cause it to blow a fusible link in the main feed wire.

[clockdoc]

Thanks Ray. I should have included that I had power at the starter solenoid. I plan to check out the firewall junction block today. One new piece of info__I had inadvertantly left the starter switch in the run position and, out of the blue, the heater blower started running. Ran abou10-15 secondsand quit. Seems like a probable high resistance connection. I appreciate your input.

Alex

[clockdoc]

A little more info. I have voltage at the alternator, both lugs of the junction block and at the large red wire entering the firewall side of the fuse block. No voltage at fuses.

Is the firewall side of the fuse block basically a large plug with stab-in connectors to the cab side?

[RodsGreen10]

Last time that this happened to me I melted the fusible link "the little white crimp cover" melted on the header. Truck was dead wouldn't do anything like you described. Popped the hood and coming off the starter was that connector laid on the header melted. Freaked me out when it wouldn't start and nothing would work. And to top it off I was hauling azz up the road. But pucker moment lol. Good luck.

[ray_mcavoy]

Originally Posted by clockdoc

A little more info. I have voltage at the alternator, both lugs of the junction block and at the large red wire entering the firewall side of the fuse block. No voltage at fuses.

With power on the red wire entering the firewall bulkhead connector, but not on the fuse box inside, it sounds like you have narrowed the problem down to a bad connection in that bulkhead connector.

Originally Posted by clockdoc

Is the firewall side of the fuse block basically a large plug with stab-in connectors to the cab side?

Close. The fuse box is basically stacked on top of the inside half of the bulkhead connector. The terminals inside the connector are Packard / Delphi 56 series 1/4" wide blade terminals (there might be one or two 59 series 5/16" terminals in there as well). Both are still available if you find one that needs to be replaced.

Working from under the hood, you can separate the connector by unscrewing the retainer bolt in the center. I think it has a 3/8" hex head but some years might be metric. It is normal to find some dark gooey dielectric grease inside the connector that was used to seal out moisture.

[clockdoc]

Thanks for the information. I tried to separate them this afternoon__got the bolt loose but couldn't get it all the way out. Just kept spinning. Tugged and pried with some slight loosening but was afraid to pry too hard lest there be a hidden fastener somewhere. I'll try again tomorrow with a little more determination, having read what you have said.

This is frustrating. I've never had to attack a fuse box before. Had to trace a lot of disconnected circuits but always had a functioning fuse box.

Thanks again,
Alex howell

[ray_mcavoy]

You're welcome. The retainer bolt has a snap ring on the inside that keeps it from coming all the way out. So that's normal for it to back out so far and then just spin.

However, the connector still being stuck together is not a good sign. That could be an indication of some terminals that are stuck together ... possibly due to a weak connection getting hot and melting the surrounding plastic a bit.

[clockdoc]

I finally got the back cover off. Apparently was stuck around the perimeter. Aside from nasty looking grease, everything looked OK. Only one of the connecting spades inside had power. Is this normal? Doesn't seem right.

Alex

[hatzie]

Originally Posted by clockdoc

I finally got the back cover off. Apparently was stuck around the perimeter. Aside from nasty looking grease, everything looked OK. Only one of the connecting spades inside had power. Is this normal? Doesn't seem right.

Alex

That's normal. There's only one power passthrough on the 78 bulkhead plug. It's wired directly to the two stud J-Block on the firewall.

[clockdoc]

Thanks. I was tied up with taxes yesterday but will try to check out the switch today.

Alex Howell

[clockdoc]

Well, The truck's running again! Kind of hate to admit what the problem turned out to be, but I've got to man-up and fess up.

The positive post on the battery had a side-mount-to-post adapter installed. When the ignition was off and there was no load on the battery, there was full battery voltage at the solenoid post and at other appropriate places. However, the resistance at the adapter's interfaces was so high that,when the ignition was turned on, the voltage dropped to 2-3 volts. Key must have been in the acc. position when I read a dead fuse box. Caught this when I noticed a small difference in voltage when probing the battery post as opposed to the adapter.

Since I had removed under-dash stuff I decided to go ahead and replace the radio which had a dead right speaker circuit.

Thanks to all respondents. Due to the good help, I learned a lot about the truck!

Alex Howell