Author Topic: no power to distributor  (Read 624 times)

Offline TheBigHat

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no power to distributor
« on: October 28, 2021, 09:43:24 PM »
I have a 78' K20 that is not getting 12v through the distributor wire. I haven't been able to figure out where that wire's source is. I did find a fuse link on the firewall right above the power booster in a bundled wire which had a blown fuse in it. I thought I may have found the problem. That turned into another problem because after I replaced it with what I thought was supposed to be a 32v fuse the truck still wouldn't start. I tried to check the fuse again to see if it may have instantly blown from a short somewhere, possibly the reason the fuse blew to begin with but the fuse wasn't blown. However the crusty fuse link did at that point break on the end. Now I don't know what that is for and still have no power to my distributor. I have headers and was able to probe both sides of the fusible link and got light on both sides. I put a jumper on the broken fuse link to see if that would work also, no luck. Now I have a broken fuse link I don't know what is to and no power to my distributor.

I've ruled out the ignition switch, that didn't fix it.

Offline bd

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Re: no power to distributor
« Reply #1 on: October 29, 2021, 01:27:42 AM »
The factory wiring manual is your friend:  1978 Wiring Manual

Power routing to the distributor is fairly simple.  It runs from the starter solenoid 3/8" battery cable stud through a 6" long 14-Gauge fusible link and 10-Gauge red wire to the double-stud firewall junction block.  The two studs of the firewall junction block are jumpered together via a flat bus.  From the firewall junction block, power passes through a 16-gauge fusible link and 12-gauge red wire to the firewall bulkhead connector where it passes into the cab and a 6-way splice taped up inside the dash wiring harness.  From the taped splice, two of the six 12-gauge red wires route directly to the ignition switch, supplying power to the switch.  From the ignition switch, a 12-gauge pink wire runs back out through the firewall bulkhead connector to the distributor B+ terminal of the HEI distributor cap to power the ignition system.

Using an incandescent test light clipped to good clean grounds, do you find B+ power on both studs of the firewall junction block?  Do you find B+ on the STOP, TAIL and ACC fuses in the fusebox?  With the ignition switched ON, do you find power on the "CLSTR FEED" and "B/U LPS   DIR SIG" fuses?

Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline TheBigHat

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Re: no power to distributor
« Reply #2 on: October 29, 2021, 08:40:13 AM »
Thanks for your reply, I'll be looking into all of that today. I took it to an "electric shop" yesterday and for some reason he felt compelled to completely remove my junction block and took a hand grinder to it. I think I will start at the junkyard getting another one of those, get it back to normal there and start over. I have ran it by hotwire and everything did seem to work though.
« Last Edit: October 29, 2021, 09:01:12 AM by TheBigHat »

Offline TheBigHat

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Re: no power to distributor
« Reply #3 on: October 29, 2021, 12:08:19 PM »
Everything on my fuse panel is hot except for pnl. I also probed every wire that looked like it could be pink there were about four or five and even an orange one they are all hot going into the back of the fuse panel. The spot that has no power at pnnl has a fuse that appears to not be blown. I'm considering just running a spatula connector to the distributor from the ignition side of the fuse block is there anything wrong with doing that?

Offline TheBigHat

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Re: no power to distributor
« Reply #4 on: October 29, 2021, 02:34:58 PM »
I finally threw in the towel. I ran 14 gauge from the IGN tab on the fuse block to the distributor cap, then scotch spliced a section of 16 guage to the electric choke. It's not factory, but it's working. I see a harness replacement being added to my long term to do list. Now I have to figure out how to get power to my instrument panel lights. Were they on the same circuit as the ignition coil wire? Suspecting there's things on this truck that have been messed with in it's 43 years.

Offline bd

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Re: no power to distributor
« Reply #5 on: October 29, 2021, 04:02:28 PM »
Speaking as a retired professional, I think you are applying bad judgment.  Did you look at the wiring diagram I linked?  It includes a schematic diagram of the bulkhead connector that shows which wire cavity the 12-gauge pink wire (Ckt 3) to the distributor uses to feed through the firewall. 

Your decision to run a 14-gauge wire to feed the distributor from the 15-amp protected fusebox tap is inviting trouble that includes randomly blowing the fuse, overheating the distributor feed wire, and possibly limiting the ignition system to less-than-optimum performance.  Regarding that distributor feed wire gauge, 12-gauge is adequate and factory for your 1978 K20, although 10-gauge is optimum and the gauge used in later years of production.  It is alright to power the electric choke from that IGN tap, although I recommend that the choke wire passes through a two-prong oil pressure switch for the best drivability.  But the original OEM distributor feed wire is protected strictly by a fusible link, not a readily replaceable fuse.  The reason is that the fusible link is equivalent to an ultra-slow-blow fuse rated at perhaps 35-40 amps.  How does the 15-amp fast-blow fuse in the fusebox compare?

The fact that all of the fuses and power taps in the fusebox are 'hot' with the ignition switched ON is a telltale that power delivery through the ignition switch is complete at least to the fusebox and probably to the bulkhead connector.  The only real question that you need to sort out is whether power is flowing through the bulkhead connector into the engine compartment.  If the appropriate bulkhead connector cavity is 'hot' in the engine compartment, the next questions are whether a 12-gauge wire occupies that cavity, and then where does it go from there?  Not a difficult process, if a little tedious.

Make sure that the battery cable connection to the starter solenoid is suitably tight and that the attached fusible link does not stretch if tugged.  Check the fusible link at the firewall junction block with a similar tug.  Then use your test light to probe the appropriate cavity of the firewall bulkhead connector, using the linked wiring diagram as your guide.  Bear in mind that original wire colors will degrade and can change significantly over time.  So don't be too surprised if the original pink wire appears white or purple or orange or brown or red where it has been exposed to underhood heat, chemicals, and petroleum lubricants.

Don't shortchange yourself.  Sort it out properly.

Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline TheBigHat

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Re: no power to distributor
« Reply #6 on: November 01, 2021, 12:08:23 PM »
Thank you, I have been waiting for your reply. I was able to probe the inline fuse that resides on the solenoid and get light. One wire has a hard case fuse and the other does not. They both have power. If I knew all the answers there'd be no post asking someone for help - that you're offering, thank you. I think I'll.make the time today to get to the junkyard to find a replacement for the two studded distribution block the so-called auto-electrician destroyed for me. You are talking about the block located above the power booster? I found two fusible links there, one still good, the other contained a 23v glass fuse that was blown. I replaced the blown fuse and nothing changed. So I checked the fuse to see if it instantly blew. It did not, however the wire crumbled off at the crimp. So, I alligator clipped the ends together to see if anything would change. Nothing seemed to change. I would.much rather find the problem and repair it correctly. I need to get where I'm going.in the meantime. So, your.help is very appreciated.

Offline bd

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Re: no power to distributor
« Reply #7 on: November 01, 2021, 01:59:07 PM »
You are welcome to any assistance I can offer.

The two-stud firewall junction block is imaged at the very bottom of this post.


A fusible link is not the same as a cartridge fuse... 





A fusible link is a 6" length of highly specialized wire located at the "hot" end (the end of the wire closest to the power source) of a high-current feed wire.    When a fusible link fails, it will sever (melt) like a normal fuse, but the encasing insulation may visually appear intact, distorted, or split.  Because the wire is severed, if tugged, a failed fusible link will stretch like a rubber band or pull completely apart.  Hence the instruction to "test" links by giving them a firm tug.  You can find more information on fusible links in, Fuses and Circuit Protection.


The inline glass fuse that you found melted should be rated at 20 amps; it feeds the HVAC high blower through the blower relay.  It has no bearing on anything other than the high blower function when the fan speed is switched to high.  FYI - At some future point in your project, I would replace that inline glass fuse holder and fuse with a Mini-ANL 20.  The OE plastic inline fuse holders used by the factory have a propensity to melt and distort resulting in poor high blower performance.


Rest assured that help is available for you to sort this out as long as you are attentive, persistent, willing to follow directions, and exhibit patience.  Reread through what has been posted to date to wrap your arms around how the system is wired from the factory.  Refer to the factory wiring diagram often and grow accustomed to using it.  Ask questions about anything you don't understand clearly.  And, don't cut corners.  Diligence will get you where you want to be.

« Last Edit: November 01, 2021, 02:06:18 PM by bd »
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline TheBigHat

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Re: no power to distributor
« Reply #8 on: November 01, 2021, 02:32:54 PM »
Wow, thank you! I just got back from the junkyard, I picked up a new junction block and took the harness from the bulkhead back to the starter. I was having trouble following the diagram honestly. But with a whole junk harness to probe and follow along with the diagram I hope to hone my skills. I'll get the band-aid off her soon hopefully! Thank you very much, glad to be a part of this forum!

Offline JohnnyPopper

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Re: no power to distributor
« Reply #9 on: November 01, 2021, 07:41:27 PM »
bd your green truck needs a bath...jus sayin  ;D
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline bd

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Re: no power to distributor
« Reply #10 on: November 01, 2021, 07:44:40 PM »
JP, my "green truck" is actually burgundy.   ;)
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline JohnnyPopper

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Re: no power to distributor
« Reply #11 on: November 02, 2021, 12:33:07 PM »
Ahh, so someone else owns the green filthy firewal::)

Stop S.B. Abuse!!

1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline bd

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Re: no power to distributor
« Reply #12 on: November 02, 2021, 01:13:03 PM »
Unfortunately, I can't claim that mine is much cleaner.   :'(
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)