Electrical question

[chevelless502]

I recently acquired a 1972 C20. It has sat for about 10 years. We decided to try to see if we could get it started. We hooked up a battery in it and turned the key nothing at all. It has no interior lights or head or tail lights even. Battery checks out fine. We have power at the back of the alternator and there is power on the engine side of the firewall at the bulkead connector. At the fuse block inside there is power to it. But here is the issue. As soon as you turn the key to the start position you loose all power at the fuse block. Any ideas what might be causing this? Thanks in advance.

[RustyPile]

What do you do to make the power reappear at the fuse block??

[chevelless502]

Originally Posted by RustyPile

What do you do to make the power reappear at the fuse block??

Turn the key back to the off position. Its like its all backwards.

[RustyPile]

Did you clean the battery cables before you connected the battery?? It sounds like you have a bad connection that won't carry high amperage.. A clue might be as to why the truck was parked 10 years ago...

[67C10Step]

Sounds weird. Have you gotten yourself a wiring diagram yet? If not, I encourage you to do so as it will save you a bit of frustration.

[rpmerf]

Schematics:
http://outintheshop.com/faq/wiring/w...s/image002.jpg
http://outintheshop.com/faq/wiring/w...s/image003.jpg

Agree, sounds like a bad connection somewhere.

So to verify.
Key @ off, 12V @ the alternator and 12V @ the fuse block.
Key @ start, 12V @ the alternator and 0V @ the fuse block.

Keep an eye on battery voltage while you turn the key. If the voltage stays the same, it's a bad connection. If the voltage drops, it's a high load / short.

I would deal with lights first. I have a feeling once lights are figured out, the starting issue will be resolved. Also, it's easy to just turn on the light switch.

[chevelless502]

I guess I should clarify a bit. I loose all power as soon as the key is turned to the "on" position. Doesnt have to be all the way at the "start" position. Sorry about that.

I have a wiring diagram i just found on here last night so going to start to try to chase things down on it. Maybe can find something there as well.

Anyone have a diagram for the key switch that tells the electrical flow through it? Im leaning a bit towards it being shorted out maybe?


Thanks for all of the ideas and help so far.

[franken]

As said above, likely a bad connection. This could be on the ground side as well.
Having voltage doesn't mean any current can be drawn. When the key is on no current can be drawn due to the bad connection so the voltage reading goes to nothing.
Follow the red wire from the battery to the junction block (unplug them and plug back in) on the firewall connecting to the fuse panel and check/fix all connections.
Trace the ground path from the battery to the cab and check/fix all connections.

[toolboxchev]

If you have a stock wiring system under the dash do a check on the following wires.

12 gauge Purple, powers the starter solenoid.

12 gauge Red, powers the ignition switch.

If you loose power at the Red wire when you turn the key I would suspect a bad switch or short to ground.

I have had to replace 3 ignition switches that burned up due to heater issues.

Also look for burned connectors at the heater switch. You can download the wiring diagram from this website if you cannot get it from the attachment.

[dmjlambert]

Do you continue to have power at the screw terminal on back of the alternator when you turn the key to on, or do you lose power there? If you have power there but no on any terminal in the fuse box, you may have corrosion in the bulkhead connector. You can see which position the 12 gauge red wire goes into on the back of the bulkhead connector, and remove the bulkhead connector. I think you use a 3/8 nut driver to remove it if I recall correctly. Check out those terminals in there.

[dmjlambert]

Also, I have found detecting power at the fuse box to be tricky if you have rusty fuse holder terminals. You really have to scrape down to bare metal with your tester probe. And finding a spot in the cab to connect to non-rusty and non-painted bare metal for a ground is also hard to do.

[Wgesnerjr]

IIRC, there might be a difference in the ignition switch tilt vs. Non tilt. I think they worked different in that one "pulled" on the switch to start and one "pushed". So a non tilt switch in a tilt column might work backwards. Or vice versa.
Just a thought.

[RustyPile]

Originally Posted by Wgesnerjr

IIRC, there might be a difference in the ignition switch tilt vs. Non tilt. I think they worked different in that one "pulled" on the switch to start and one "pushed". So a non tilt switch in a tilt column might work backwards. Or vice versa.
Just a thought.

That does not apply in this case.. 1972 C/20s had the ignition switch mounted in the dash..

[Wgesnerjr]

Crap! Forgot about that.

[Killer Bee]

OP, I see it's been a few days since your last reply..

please let us know if you're still having trouble and I'll try to walk you through getting it started and a diag strategy to find your gremlin..

good luck!