No spark! Bad coil?

#939095 Sat Apr 27 2013 09:33 PM

I think I have a bad coil. First off, I'm trying to start the 250ci in my 66 C10 parts truck. The engine in the 66 is a 250 from a 1971 or 72 C10. I don't have a wiring diagram for a 71-72 truck, so I am making a few assumptions here.

From the firewall, the wiring harness appears to be identical to my 1965 except for 1 wire which is black. Please see the photo for clarification.

I have tried several combinations of yellow and black going to the positive side of the coil but I have no spark. I even disconnected the distributor side coil wire and connected a jumper wire to the negative side of the coil and still don't get any spark to the engine block. I confirmed that I have 12V coming to the positive side of the coil.

I measured 2 ohms across the coil terminals with it out of the truck and nothing connected.

So do I have a bad coil? Any other tests I should run? Is the yellow wire a "resistor wire"?

engine bay starter and coil wiring

Re: No spark! Bad coil?

Joined: Dec 2012

Posts: 97

Wrench Fetcher

Can you measure any voltage between neg side of coil and ground with dist disconnected? Ignition should be on for this test.

Last edited by hd4unm; Sat Apr 27 2013 09:40 PM.

Re: No spark! Bad coil?

Joined: Dec 2012

Posts: 97

Wrench Fetcher

I'll give it a try tomorrow. What range should I set my multimeter at?

You should be getting right around 12volts plus on neg side too if your battery is fully charged. No volts or much less more than a few .xx could be trouble.

You mentioned 2ohms resistance before, seems like a lot for a coil unless it has built in resistor. Could be the reason for the reading if you don't have a resistor on the firewall.

Last edited by hd4unm; Sun Apr 28 2013 11:46 AM.

Re: No spark! Bad coil?

Joined: Nov 2002

Posts: 1,008

Shop Shark

Another molested truck. frown

The yellow wire looks correct, but I'd check for a splice inside the harness. The yellow wire should provide 12v to coil on start only. more below

The blue wire, and black are suspect.

The blue wire should be sender to gauge, no clue (unless) why it would go to starter.

The black wire has been added(?). Check for voltage on the black wire with ignition on/off. The shop manual shows a W/O purple wire to the same side of the starter solenoid as the yellow wire. I believe the W/O ppl wire is a resistance wire to drop the voltage to the coil when running to 9v. IIRC I've also seen the the resistance wire spliced into the yellow wire in the harness. Hope that helps.

Larry


I don't own a vehicle that isn't old enough to drink.

Re: No spark! Bad coil?

Joined: Sep 2011

Posts: 2,361

'Bolter

The blue wire should be sender to gauge, no clue (unless) why it would go to starter.

The black wire has been added(?). Check for voltage on the black wire with ignition on/off. The shop manual shows a W/O purple wire to the same side of the starter solenoid as the yellow wire. I believe the W/O ppl wire is a resistance wire to drop the voltage to the coil when running to 9v. IIRC I've also seen the the resistance wire spliced into the yellow wire in the harness. Hope that helps.

Larry

The photo may not be clear, but I assure you that the blue wire does NOT go to the starter. It goes from the oil pressure switch to the dash gauges (somehow). grin

Also, I cannot find a ballast resistor anywhere. My FSM doesn't show a ballast resistor in the wiring diagram for the C10 trucks.

I will pull the wire harness and carefully inspect the wires for a splice. I am unsure if this engine harness came with the 250ci from the donor '71 truck OR if it is the original harness from the '66 truck (which had a factory 250ci).

I did check the coil with the starter cranking and got better than 10,000 volts at the coil's negative post connected to ground.

I get 12.74V at the coil's negative post with the key turned to IGN but not cranking.

I'm thinking maybe the coil isn't bad after all.

Last edited by Lugnutz; Sun Apr 28 2013 02:05 PM.

Re: No spark! Bad coil?

Joined: May 2005

Posts: 8,467

Sir Searchalot

Yel to pos coil is correct. Purple to switch is correct. take off black at starter. Not sure what it is yet. Neg side of coil to distributor. All these wires should be checked carefully for breaks/shorts and to be sure they are connected as described. Assuming you are cranking engine? and not doing some kind of static test? Be sure you have not blown a resistor or a fuse or a fusible link somewhere. Make sure coil/distributor are actually grounded. Slap a spare coil in there.


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Re: No spark! Bad coil?

Joined: Sep 2011

Posts: 2,361

'Bolter

Yel to pos coil is correct. Purple to switch is correct. take off black at starter. Not sure what it is yet. Neg side of coil to distributor. All these wires should be checked carefully for breaks/shorts and to be sure they are connected as described. Assuming you are cranking engine? and not doing some kind of static test? Be sure you have not blown a resistor or a fuse or a fusible link somewhere. Make sure coil/distributor are actually grounded. Slap a spare coil in there.

I've tried a few of the mentioned combinations, but with no luck. I'm off to my FLAPS to get a cylinder compression gauge and see what readings I get. I realize that it has nothing to do with the ignition system, but I need to know what I'm looking at before I spend any money on this engine.

Re: No spark! Bad coil?

Joined: Nov 2002

Posts: 1,008

Shop Shark

Sounds like the resistance wire has been removed or bypassed. The resistance wire is there to drop the voltage to the coil with the ignition on. The drop in voltage to the coil is to improve the point life. The 12.74 voltage at the coil would be great for an HEI distributor, but not points. The voltage at the coil neg. fluctuates indicating points open/close. The only reason I could see for the coil not fire would be not enough dwell on the points.

Checked the distributor to see that the rotor is pointing to #1 at TDC?

OK on the blue I see it now. my oops

Larry


I don't own a vehicle that isn't old enough to drink.

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