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?

[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