Volts at Coil

I know there are lots of threads on this, but please bear with me.

I have a 68 Roadrunner that will not fire up for 8 months (but cranks good). In that time I replaced the carburetor, coil, battery, cap, rotor, ignition switch, plugs, ballast resistor and ECU module. Now I think it's a voltage problem so have done lots of voltage and resistance checks and checked reluctor gap. With battery at 12.7 V, I measure 11.5 V at +coil with key in start without cranking and 10.2 V at +coil when cranking. Also, get 11 V at starter relay battery connection when cranking -- in case I want to try to run a wire from there to the coil (but haven't tried that yet). Shouldn't 10.2 V at +coil be enough to fire it up? I get sparks from the coil secondary wire (disconnected from distributor) to ground when cranking, but seems weak, but this is with ignition switch in run with 8 V at +coil (using remote starter), not in start where voltage is greater at 10.2 V (using key to crank). Isn't it normal to get a couple volts less than battery at coil (12.7 - 10.2) when cranking or is that too much drop? Does battery itself (if I ran a wire from it to coil) stay at full voltage when the starter is operated?

Please help.

you may want to check the ground at the ecu or even the ecu wiring harness. the engine will run at 6v to the coil. also check the fuel supply to make sure nothing amiss there.

If you have been trying to start this thing for months I strongly suggest you pull the plugs and give a couple of shots of oil down the cylinders before going further.

It may be just lack of compression after starting/flooding many times. If nothing else, this can't hurt. And, don't worry about the oil fouling the plugs, it will be fine.

Does it pop at all? If it does, the above may be enough to get you going.

Good luck to you.

Yes 10.2 during crank should fire the coil, but I think that’s on the low side.
All batteries will show a voltage drop under load, that’s what a load tester measures. Healthy batteries drop less under load (cranking) than weak.

What's the history? New installation or did it run OK and then just quit starting?

10.2v is fine on cranking. I was working on a no-start situation on a '66 Barracuda recently. Due to a bad underhood harness I was dropping to 8v while cranking with a cheap Chinese distributor that won't fire at less than 10v. As stated above, ground on the ignition box is critical and one of the most overlooked problems.

It ran OK until about a year ago, then progressively got worse. At first I could start it but had to crank it a lot and pump the pedal 2-3 times or more then it slowly fired up. Then I realized I was pumping the gas too much when I saw black carbon marks at the tail pipes on garage floor. I had recently reinstalled the original 383 manifold with a new holley carb (it did run some after that for a while). Now when I try to start it, I'm afraid to pump it more than once to avoid wet plugs. After I realized it was flooding, I replaced the wet plugs with new ones (but it has not fired at all even before that). I have been checking each time to see fuel is being delivered from the accelerator pump, but only pump once or 1-1/2 times. I try to start it every once in a while after replacing the items I mentioned. I have had the car since 1969. THANKS!

It ran OK until about a year ago, then progressively got worse. At first I could start it but had to crank it a lot and pump the pedal 2-3 times or more then it slowly fired up. Then I realized I was pumping the gas too much when I saw black carbon marks at the tail pipes on garage floor. I had recently reinstalled the original 383 manifold with a new holley carb (it did run some after that for a while). Now when I try to start it, I'm afraid to pump it more than once to avoid wet plugs. After I realized it was flooding, I replaced the wet plugs with new ones (but it has not fired at all even before that). I have been checking each time to see fuel is being delivered from the accelerator pump, but only pump once or 1-1/2 times. I try to start it every once in a while after replacing the items I mentioned. I have had the car since 1969. THANKS!

(I don't know if I'm replying with the quote right -- this may be redundant, I will have to see how it looks, sorry)

I did check the ECU ground previously and it was good. I think it will run OK (I get 8 V with key in run), if I could get it started.

It does not pop at all. I see what you're saying about the compression.
Wouldn't you think once on the pedal would be enough to get the mixture to the plugs and fire it? I can see gas in the carb from the accel pump. I don't want to flood it each time.
THANKS!

Give it a quick shot of starting fluid, if it fires immediately the problem is likely fuel related.

If you have 13.5 v to 14.5 v at the battery while running, with the proper 0.5 Ohm ballast resistor you should be getting about 9v to 10v at the coils positive. But that voltage may have a volt or two drop when going thru the firewall bulkhead connector twice if the connectors are dirty or the wires under the dash are damaged.

What ignition system and what brand and part number spark plugs are you using?
Where do you live and how much moisture is in the air where you live?
How old is the gas in the tank your trying to use?
I've had issues like yours with starting with old dead, no octane and won't evaporate properly, gasoline.
I buy 100 octane low lead aviation fuel at my local airport to use in the cars I have that don't get driven much, that fuel has a 18 month shelf life so it doesn't go flat, bad, as quickly as todays pump gas
On the Holley carbs I will fill the fuel bowls up with fresh fuel with a syringe so the motor will start and run for a short while on decent fuel and put some heat into the motor to help it run on the old fuel in the gas filter and fuel line and tank.
You may need to drain the gas tank and fill it with new gas if your gas in the tank is more than 6 months old and you live in a area that gets above 80F during the spring, summer and fall
I have a 1969 Dart GTS 383 4 speed car that I haven't driven in the last two years so I'm facing the same deal this year as your dealing with now
I did fill it with 100 LL but it has sat for over two years now, I have started it and moved it a little but no trips of any distance, less than a mile in our neighbor hood and driveways
BTW, it is illegal to use leaded fuels in this country on normal street vehicles

Cab, you hit the nail on the head! I started thinking about the age of the fuel in my tank and looked back to see it was 19 months old! I don't put many miles on it so it's not unusual to go a year without burning a full tank and have never had fuel go bad before. I looked back to see that the last time I was able to start it was last July and it had been hard to start for about several months before that. It was ethanol-free gas, don't know if that makes a difference on how long it should last before going flat. With a couple second spray of starting fluid, it started right up at fast idle, then the rpm came down in a few seconds and I was afraid it was going to stall and I would have to spray again, but that one spray was enough to heat it up enough to let it continue to run on the bad gas and take a drive (9 miles, my regular route). Hopefully I can drive it a few times this way to burn the rest of the tank (8-9 gallons?) then get new gas -- probably just a part of a tank at a time to keep it fresher. THANKS!

You made me start thinking (see my reply today to Cab). It did fire immediately with staring fluid and the problem was bad fuel. Thanks for the suggestion.

I'd add 1-2 gallons of fresh to the stale gas and then do a 70 mile loop run and top it off. Keeping a part filled tank promotes condensation and RUST.