Lose spark after cranking - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**Detour** · 04-04-2025, 11:47 PM

The weekend project has turned into 5 months and I can't remember what things looked like for reference. I replaced the starter, alternator, points, cap, rotor, condenser, wires, plugs and found most of the wiring under the hood melted. So then I began cleaning it up and replacing with proper colors according to a wiring diagram I found on this forum.

Now the problem is I have spark during crank but as soon as you let go of the key you lose the spark. I have 9.8v from the starter to + coil during crank, but again 0v when you let go of the key. What I assume is the factory resistor wire is still in place going to the coil (cloth style insulation wire that goes to the correct pin on firewall block).

Battery is good 12.5v or better depending on how recently it came of the battery tender.

Any guess on what I am missing here.

**dmjlambert** · 04-05-2025, 12:16 AM

If, when you go to 0V at the coil in the run position of the key, you can verify you have 12V at the IGN UNFUSED terminals of the fuse panel, then I would say you have a failed resistor wire or corroded connections of the resistor wire. Replace the resistor wire with a new good copper wire, delete the yellow wire going to the starter, and replace the coil with a 12V coil as discussed here; that's probably what I would do. https://www.67-72chevytrucks.com/vbo...83#post9285783

**Sheepdip** · 04-05-2025, 10:39 AM

I had that problem rear its head and it was the ignition switch that was bad. Bad "On" contact to be specific. Just another possibility.

**Detour** · 04-05-2025, 10:50 AM

dmj- Thanks for the info and link. My local Napa has the Echlind part in stock and there is a 20% off promo going on, so I placed the order and already got a Ready for Pickup reply so heading up there next.

Sheep- Thanks, I'll keep that in mind in case the above part and a new wire doesn't fix the problem.

**dmjlambert** · 04-05-2025, 10:55 AM

Before doing work, check for the 12V at the IGN UNFUSED terminals, that should tell you if the problem is the ignition switch or resistor wire.

**Detour** · 04-05-2025, 11:02 AM

Curious is the wiring block on the firewall a connector that comes apart or do the wires pass through it? If I can easily disconnect it, I would be able to check continuity through the resistor wire, correct?

**dmjlambert** · 04-05-2025, 11:49 AM

Yes it comes right off by taking it loose with a 3/8 nutdriver. Checking continuity is probably not as good as just checking for voltage, because checking for voltage shows continuity or lack of continuity under load. The IGN UNFUSED terminals on the wiring panel connect to the resistor wire, which connects to the the coil yellow wire and starter yellow wire. That is why I suggest to check for voltage at the IGN UNFUSED terminals when you see no voltage at the coil. If you don't see voltage at the IGN UNFUSED terminals with key in on position, you don't need to be worried about the resistor wire or wires to coil until you fix that lack of voltage at the IGN UNFUSED terminals, perhaps the problem is the ignition switch as Sheepdip points out.

The resistor wire is the white cloth covered wire and inside the engine harness non-adhesive vinyl wire wrap is where it is spliced to the yellow wires. Here is a thread showing pictures of what some of the connections look like. https://67-72chevytrucks.com/vboard/...d.php?t=708975

And here is a thread with info about the fuse panel. https://67-72chevytrucks.com/vboard/...98#post8004798

BROWN 70 · 04-05-2025, 04:02 PM

hi great info dmjlambert!

**PbFut** · 04-05-2025, 07:03 PM

Understanding the jump of coil wire from fuse box to coil and solenoid to coil may also help. The jump from fuse box to coil includes the resistor wire. It is there to protect the points and the coil will see from 9 to 10 volts normally. The jump from solinode to coil is there to provide approximately the same voltage during cracking. The voltage at the solinode will drop to that 9 or 10 volts due to the heavy draw from starter. The voltage at end of resistor wire will be even less than 9 or 10 volts during cranking. The splice for the end of resistor wire is actually on to the jump from solinode to coil inside the engine harness. So you will get 9 or 10v during cranking and the when you let off on the key in start position you go to run position. Solinode deenergizes and only the jump from fuse box to coil is energized via resistor wire. The fail you are having is in the jump of the resistor wire leg. Testing the ign pin on the fuse box in on position will tell you if the break is from the fuse box to splice in harness or fuse box back to, and including starter switch.

**Detour** · 04-06-2025, 09:17 AM

Thanks for the help everyone. My OCD has made this turn into a multi day project.

I went to test the voltage at the fuse box and all the odd/old wiring from past accessories was driving me nuts so I started removing obviously damaged or cut wiring. There was a lamp cord with a rotary switch on it hanging down since I bought the truck and could never figure out what it was for.... Apparently it was the power to the radio. Go figure. Well chasing out a dozen abandoned wires led to me removing the dead radio, heater controls, and instrument cluster so I could better trace wires and clean things up. Found an old mouse nest and the equivalent of a 40# bag of sand up there. (I have a different cluster with all gauges that will be going in anyways)

At this point I have battery voltage at the ignition switch and at the fuse panel, but I swear I did not the other day. So I am thinking a bad wire. I tried starting it and it fired on first crank, but again as soon as I let go of the key it died.

Prior to all this I replaced most of the wiring under the hood and rewrapped it. I retained the resistor wire for the coil since I could tell it was different. I am going to pull apart and do the same for the wiring under the dash. I have 3 wires that go into the loom that were cut and I can see one wire that is melted. So will see what is hiding in the loom. Otherwise I think the resistor wire is probably bad when it comes to the loss of spark.

I'll let you guys know what the outcome is. Probably be later next week by the time I get back to the truck.

**Palf70Step** · 04-06-2025, 01:08 PM

Be aware that the wire pin-out is different between idiot lite dashes and gauge cluster dashes. You'll fry your gas gauge for one if you plug the idiot into the gauge cluster.

**PbFut** · 04-06-2025, 01:10 PM

Sounds familiar. You get in an area that puts your brain into a state that you can't keep anything straight. Even if the area is out of sight, you rip it all out and start over. Even if it cost a hundred or more bucks. The piece of mind once complete is well worth the investment in parts. Therapy is expensive. So are auto parts.

**dmjlambert** · 04-06-2025, 03:30 PM

Detour, I gotta say I love your forum screen name. It is great for a person who would say "My OCD has made this turn into a multi day project" and you've gone off and started pulling wires and fixing things that are not a part of the problem at hand. It sounds like a bunch of useful work, although you're on a detour you're not just sightseeing. Good job.

**Detour** · 04-09-2025, 09:40 AM

Quote:

Originally Posted by dmjlambert

Detour, I gotta say I love your forum screen name. It is great for a person who would say "My OCD has made this turn into a multi day project" ...

Thanks! My older screen names were ChaoticDetour, but I have shortened it up over the years for the forums.

**Detour** · 04-15-2025, 08:27 AM

Well things continued to go sideways. I did get the wiring under the dash cleaned up, rewrapped and tucked up so it wasn't rubbing anything like it was before. I then got distracted on the heater ductwork and remembering the fan only worked on high (which seemed like low) and made a ticking sound. Well... Mouse House!, heater box and upper duct was packed, dampers couldn't move, cables bent and the squirrel cage broken.

So then yesterday morning I finally plugged the harness in on the engine side so I could hear this thing run before going to work. Fired right up and ran for 5 seconds then died. YES, ignition is fixed! Turned the key again and it fired up for 5-seconds and then..... I let the smoke out. In that short time frame it melted the insulation and tape wrap off the wires. I now have a bundle of bare wires under the hood.

**PbFut** · 04-15-2025, 10:38 AM

Well that's not the outcome you wanted. Clear one of the wires was not correct. The engine harness is very basic. Should not be difficult to rebuild or simply swap out. Decide on distributor first. Then match harness to the dissy. Also, harness under dash may have taken a hit. Check that out closely.
As for heater and ac if you have it. Its going to be difficult, but one problem/project at a time. Even if you have to tear things apart twice. If you don't, before you know it, the entire front clip will be disassembled.

**Detour** · 04-15-2025, 11:35 AM

Quote:

Originally Posted by PbFut

... one problem/project at a time. Even if you have to tear things apart twice. If you don't, before you know it, the entire front clip will be disassembled.

Uhhhh, yeah... my screen name was mentioned above and applies. I'm almost there. This projected started last fall as a 4 bbl carb swap from a 2-jet. At this point the radiator is at the shop being re cored, water pump, fuel pump, starter and ignition components (kept factory dizzy) have been replaced, converted to clutch fan from fixed, grill and lights are out of it (damaged from hitting an Elk), gauges, radio, heater controls and now the duct work, heater core and blower are out of it. No plans for the front clip....

But hey as soon as I figure out how to work on these things I'll have it back together in a couple of hours.

**Detour** · 04-15-2025, 11:41 AM

On the serious side. I will have be inspecting the wiring and obviously replacing some that I cooked. Sticking with the factory points set up. I have the NAPA Echlin (no external resistor) coil in it.. if I didn't cook it, and replaced the resistor wire with a 14ga white to match for wiring diagram.

Thinking events over in my head... I thought I heard the starter, slightly spin when I clicked the key over, before turning to start and it engaging the flywheel. Wondering if my jumper wire from the starter to the coil is wrong and that's what burned up, taking the neighboring wires with it.

Thoughts?

**PbFut** · 04-15-2025, 12:39 PM

If you are not running a points coil setup requiring resistor wire, you do not need the jump from the starter solenoid. It is a lead that is in an area of potential shorts. Best to eliminate if you can. The connection to the solenoid is only there to get around the resistor wire during drops in voltage while cranking the starter. If you have eliminated the resistor wire, then the jump to the solenoid is moot.

**Detour** · 04-27-2025, 07:41 PM

Got rid of the yellow wire jumper wire from the starter, replaced all the melted wires under the hood (luckily it didn't make it through the fire wall) and it fired right up. But now a couple of new problems. One is the newly rebuilt carb leaks like a sieve and Two there is no power to the dash cluster. (This truck may be cursed)

The gauges and dash lights haven't worked on this truck since I bought it, and I pulled about the loom under the dash again after the ignition meltdown and didn't see any damage. So I don't think they are related. The wiring diagram I got from a post on this forum doesn't match my cluster harness at all. Does anyone have another reference for a 67' K20? Pictures attached show my wiring colors.

**dmjlambert** · 04-27-2025, 08:14 PM

Quote:

Originally Posted by Detour

... wiring diagram I got from a post on this forum doesn't match my cluster harness at all. Does anyone have another reference for a 67' K20? Pictures attached show my wiring colors.

Yes it matches. I recommend start with this diagram:
https://67-72chevytrucks.com/vboard/...27#post9186727
Pay particular attention to this feature of the wiring diagram. The main diagram is for a warning gauge cluster and this inset shows the wire colors if you have a guage type cluster.

**dmjlambert** · 04-27-2025, 08:19 PM

Here's another couple of threads that may help

guage instrument cluster diagram
https://67-72chevytrucks.com/vboard/...36#post7630636

fuse panel diagram
https://67-72chevytrucks.com/vboard/...98#post8004798

**dmjlambert** · 04-27-2025, 08:24 PM

Keep at it one issue at a time and you'll get all that stuff figured out.

**Detour** · 04-27-2025, 10:07 PM

Thanks guys. Yeah I wasn’t looking at the inset.

**PbFut** · 04-28-2025, 03:10 PM

Given things have not worked for a while, I suspect it's the headlight switch. From what I gather from info on this forum, there are 2 types. One has an additional pin so the switch is year related. Given it sounds like your harness has been messed with, I guess you have a 50 50 shot at which one you need. Seems to me the one with more pins is backwards compatible but I am not sure on that.

04-04-2025, 11:47 PM	#1
Detour Registered User Join Date: May 2024 Location: Appleton, WI Posts: 37	Lose spark after cranking The weekend project has turned into 5 months and I can't remember what things looked like for reference. I replaced the starter, alternator, points, cap, rotor, condenser, wires, plugs and found most of the wiring under the hood melted. So then I began cleaning it up and replacing with proper colors according to a wiring diagram I found on this forum. Now the problem is I have spark during crank but as soon as you let go of the key you lose the spark. I have 9.8v from the starter to + coil during crank, but again 0v when you let go of the key. What I assume is the factory resistor wire is still in place going to the coil (cloth style insulation wire that goes to the correct pin on firewall block). Battery is good 12.5v or better depending on how recently it came of the battery tender. Any guess on what I am missing here.

04-05-2025, 12:16 AM	#2
dmjlambert Senior Member Join Date: Apr 2016 Location: Cypress, TX Posts: 4,051	Re: Lose spark after cranking If, when you go to 0V at the coil in the run position of the key, you can verify you have 12V at the IGN UNFUSED terminals of the fuse panel, then I would say you have a failed resistor wire or corroded connections of the resistor wire. Replace the resistor wire with a new good copper wire, delete the yellow wire going to the starter, and replace the coil with a 12V coil as discussed here; that's probably what I would do. https://www.67-72chevytrucks.com/vbo...83#post9285783 Last edited by dmjlambert; 04-05-2025 at 12:17 AM. Reason: punctuation in my long crappy sentence and clarity

04-05-2025, 10:39 AM	#3
Sheepdip Senior Member Join Date: Nov 2018 Location: Waterford California Posts: 2,893	Re: Lose spark after cranking I had that problem rear its head and it was the ignition switch that was bad. Bad "On" contact to be specific. Just another possibility.

04-05-2025, 10:50 AM	#4
Detour Registered User Join Date: May 2024 Location: Appleton, WI Posts: 37	Re: Lose spark after cranking dmj- Thanks for the info and link. My local Napa has the Echlind part in stock and there is a 20% off promo going on, so I placed the order and already got a Ready for Pickup reply so heading up there next. Sheep- Thanks, I'll keep that in mind in case the above part and a new wire doesn't fix the problem.

04-05-2025, 10:55 AM	#5
dmjlambert Senior Member Join Date: Apr 2016 Location: Cypress, TX Posts: 4,051	Re: Lose spark after cranking Before doing work, check for the 12V at the IGN UNFUSED terminals, that should tell you if the problem is the ignition switch or resistor wire.

04-05-2025, 11:02 AM	#6
Detour Registered User Join Date: May 2024 Location: Appleton, WI Posts: 37	Re: Lose spark after cranking Curious is the wiring block on the firewall a connector that comes apart or do the wires pass through it? If I can easily disconnect it, I would be able to check continuity through the resistor wire, correct?

04-05-2025, 11:49 AM	#7
dmjlambert Senior Member Join Date: Apr 2016 Location: Cypress, TX Posts: 4,051	Re: Lose spark after cranking Yes it comes right off by taking it loose with a 3/8 nutdriver. Checking continuity is probably not as good as just checking for voltage, because checking for voltage shows continuity or lack of continuity under load. The IGN UNFUSED terminals on the wiring panel connect to the resistor wire, which connects to the the coil yellow wire and starter yellow wire. That is why I suggest to check for voltage at the IGN UNFUSED terminals when you see no voltage at the coil. If you don't see voltage at the IGN UNFUSED terminals with key in on position, you don't need to be worried about the resistor wire or wires to coil until you fix that lack of voltage at the IGN UNFUSED terminals, perhaps the problem is the ignition switch as Sheepdip points out. The resistor wire is the white cloth covered wire and inside the engine harness non-adhesive vinyl wire wrap is where it is spliced to the yellow wires. Here is a thread showing pictures of what some of the connections look like. https://67-72chevytrucks.com/vboard/...d.php?t=708975 And here is a thread with info about the fuse panel. https://67-72chevytrucks.com/vboard/...98#post8004798 Last edited by dmjlambert; 04-05-2025 at 11:56 AM. Reason: more info

04-05-2025, 04:02 PM	#8
BROWN 70 Registered User Join Date: Aug 2010 Location: central oregon Posts: 176	Re: Lose spark after cranking hi great info dmjlambert!

04-05-2025, 07:03 PM	#9
PbFut Senior Member Join Date: Feb 2022 Location: Ca Posts: 1,026	Re: Lose spark after cranking Understanding the jump of coil wire from fuse box to coil and solenoid to coil may also help. The jump from fuse box to coil includes the resistor wire. It is there to protect the points and the coil will see from 9 to 10 volts normally. The jump from solinode to coil is there to provide approximately the same voltage during cracking. The voltage at the solinode will drop to that 9 or 10 volts due to the heavy draw from starter. The voltage at end of resistor wire will be even less than 9 or 10 volts during cranking. The splice for the end of resistor wire is actually on to the jump from solinode to coil inside the engine harness. So you will get 9 or 10v during cranking and the when you let off on the key in start position you go to run position. Solinode deenergizes and only the jump from fuse box to coil is energized via resistor wire. The fail you are having is in the jump of the resistor wire leg. Testing the ign pin on the fuse box in on position will tell you if the break is from the fuse box to splice in harness or fuse box back to, and including starter switch.

04-06-2025, 09:17 AM	#10
Detour Registered User Join Date: May 2024 Location: Appleton, WI Posts: 37	Re: Lose spark after cranking Thanks for the help everyone. My OCD has made this turn into a multi day project. I went to test the voltage at the fuse box and all the odd/old wiring from past accessories was driving me nuts so I started removing obviously damaged or cut wiring. There was a lamp cord with a rotary switch on it hanging down since I bought the truck and could never figure out what it was for.... Apparently it was the power to the radio. Go figure. Well chasing out a dozen abandoned wires led to me removing the dead radio, heater controls, and instrument cluster so I could better trace wires and clean things up. Found an old mouse nest and the equivalent of a 40# bag of sand up there. (I have a different cluster with all gauges that will be going in anyways) At this point I have battery voltage at the ignition switch and at the fuse panel, but I swear I did not the other day. So I am thinking a bad wire. I tried starting it and it fired on first crank, but again as soon as I let go of the key it died. Prior to all this I replaced most of the wiring under the hood and rewrapped it. I retained the resistor wire for the coil since I could tell it was different. I am going to pull apart and do the same for the wiring under the dash. I have 3 wires that go into the loom that were cut and I can see one wire that is melted. So will see what is hiding in the loom. Otherwise I think the resistor wire is probably bad when it comes to the loss of spark. I'll let you guys know what the outcome is. Probably be later next week by the time I get back to the truck.

04-06-2025, 01:08 PM	#11
Palf70Step State of Confusion! Join Date: Jul 2002 Location: Gulfport, MS USA Posts: 48,486	Re: Lose spark after cranking Be aware that the wire pin-out is different between idiot lite dashes and gauge cluster dashes. You'll fry your gas gauge for one if you plug the idiot into the gauge cluster. __________________ Bill 1970 Chevy Custom/10 LWB Fleetside 2010 Toyota Tacoma PreRunner SR5 Double Cab - DD Member of Louisiana Classic Truck Club (LCTC) Bill's Gallery Life isn't tied with a bow, but it's still a gift. Live simply. Love generously. Care deeply. Speak kindly. Leave the rest to God!

04-06-2025, 01:10 PM	#12
PbFut Senior Member Join Date: Feb 2022 Location: Ca Posts: 1,026	Re: Lose spark after cranking Sounds familiar. You get in an area that puts your brain into a state that you can't keep anything straight. Even if the area is out of sight, you rip it all out and start over. Even if it cost a hundred or more bucks. The piece of mind once complete is well worth the investment in parts. Therapy is expensive. So are auto parts.

04-06-2025, 03:30 PM	#13
dmjlambert Senior Member Join Date: Apr 2016 Location: Cypress, TX Posts: 4,051	Re: Lose spark after cranking Detour, I gotta say I love your forum screen name. It is great for a person who would say "My OCD has made this turn into a multi day project" and you've gone off and started pulling wires and fixing things that are not a part of the problem at hand. It sounds like a bunch of useful work, although you're on a detour you're not just sightseeing. Good job.

04-15-2025, 08:27 AM	#15
Detour Registered User Join Date: May 2024 Location: Appleton, WI Posts: 37	Re: Lose spark after cranking Well things continued to go sideways. I did get the wiring under the dash cleaned up, rewrapped and tucked up so it wasn't rubbing anything like it was before. I then got distracted on the heater ductwork and remembering the fan only worked on high (which seemed like low) and made a ticking sound. Well... Mouse House!, heater box and upper duct was packed, dampers couldn't move, cables bent and the squirrel cage broken. So then yesterday morning I finally plugged the harness in on the engine side so I could hear this thing run before going to work. Fired right up and ran for 5 seconds then died. YES, ignition is fixed! Turned the key again and it fired up for 5-seconds and then..... I let the smoke out. In that short time frame it melted the insulation and tape wrap off the wires. I now have a bundle of bare wires under the hood.

04-15-2025, 10:38 AM	#16
PbFut Senior Member Join Date: Feb 2022 Location: Ca Posts: 1,026	Re: Lose spark after cranking Well that's not the outcome you wanted. Clear one of the wires was not correct. The engine harness is very basic. Should not be difficult to rebuild or simply swap out. Decide on distributor first. Then match harness to the dissy. Also, harness under dash may have taken a hit. Check that out closely. As for heater and ac if you have it. Its going to be difficult, but one problem/project at a time. Even if you have to tear things apart twice. If you don't, before you know it, the entire front clip will be disassembled.



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04-15-2025, 11:41 AM	#18
Detour Registered User Join Date: May 2024 Location: Appleton, WI Posts: 37	Re: Lose spark after cranking On the serious side. I will have be inspecting the wiring and obviously replacing some that I cooked. Sticking with the factory points set up. I have the NAPA Echlin (no external resistor) coil in it.. if I didn't cook it, and replaced the resistor wire with a 14ga white to match for wiring diagram. Thinking events over in my head... I thought I heard the starter, slightly spin when I clicked the key over, before turning to start and it engaging the flywheel. Wondering if my jumper wire from the starter to the coil is wrong and that's what burned up, taking the neighboring wires with it. Thoughts?

04-15-2025, 12:39 PM	#19
PbFut Senior Member Join Date: Feb 2022 Location: Ca Posts: 1,026	Re: Lose spark after cranking If you are not running a points coil setup requiring resistor wire, you do not need the jump from the starter solenoid. It is a lead that is in an area of potential shorts. Best to eliminate if you can. The connection to the solenoid is only there to get around the resistor wire during drops in voltage while cranking the starter. If you have eliminated the resistor wire, then the jump to the solenoid is moot.

04-27-2025, 07:41 PM	#20
Detour Registered User Join Date: May 2024 Location: Appleton, WI Posts: 37	Re: Lose spark after cranking Got rid of the yellow wire jumper wire from the starter, replaced all the melted wires under the hood (luckily it didn't make it through the fire wall) and it fired right up. But now a couple of new problems. One is the newly rebuilt carb leaks like a sieve and Two there is no power to the dash cluster. (This truck may be cursed) The gauges and dash lights haven't worked on this truck since I bought it, and I pulled about the loom under the dash again after the ignition meltdown and didn't see any damage. So I don't think they are related. The wiring diagram I got from a post on this forum doesn't match my cluster harness at all. Does anyone have another reference for a 67' K20? Pictures attached show my wiring colors. Attached Images

04-27-2025, 08:19 PM	#22
dmjlambert Senior Member Join Date: Apr 2016 Location: Cypress, TX Posts: 4,051	Re: Lose spark after cranking Here's another couple of threads that may help guage instrument cluster diagram https://67-72chevytrucks.com/vboard/...36#post7630636 fuse panel diagram https://67-72chevytrucks.com/vboard/...98#post8004798

04-27-2025, 08:24 PM	#23
dmjlambert Senior Member Join Date: Apr 2016 Location: Cypress, TX Posts: 4,051	Re: Lose spark after cranking Keep at it one issue at a time and you'll get all that stuff figured out.

04-27-2025, 10:07 PM	#24
Detour Registered User Join Date: May 2024 Location: Appleton, WI Posts: 37	Re: Lose spark after cranking Thanks guys. Yeah I wasn’t looking at the inset.

04-28-2025, 03:10 PM	#25
PbFut Senior Member Join Date: Feb 2022 Location: Ca Posts: 1,026	Re: Lose spark after cranking Given things have not worked for a while, I suspect it's the headlight switch. From what I gather from info on this forum, there are 2 types. One has an additional pin so the switch is year related. Given it sounds like your harness has been messed with, I guess you have a 50 50 shot at which one you need. Seems to me the one with more pins is backwards compatible but I am not sure on that.

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