How much voltage? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**Palf70Step** · 08-28-2004, 07:22 PM

How much voltage should be on the input (+) side of a coil? I had no voltage coming out of the coil on my 70's 250 6 popper. Replaced coil and still nothing. I didn't think it was 12V because of ther old style resistor wire thing, but seems like it is around 5V on the coil input. Is that too low?

Any hints? All fuses look good and it ran fine till it sat for a month or so. Normally I don't have fuel becuase of it gumming up a little, not it started the no sparkoe thing. Points are working and gapped properly.

Thanks.

Sweet72 · 08-28-2004, 08:07 PM

I believe 5 volts is a little low, as I recall it should be around 8 volts after the engine is started and a full 12 with the starter solenoid engaged. I don't believe an ordinary multimeter is capable of reading the 25,000+ volt output on the coil. Check for a poor ground, as it sounds like corrosion has caused a bad connection somewhere. Doesn't the neg terminal go to ground on the coil?

**Palf70Step** · 08-28-2004, 08:27 PM

Nah...neg goes to the distributor.

Tx Firefighter · 08-29-2004, 06:37 AM

Put a test light onto the negative terminal, which goes to the distributor.

Now, crank the engine.

The test light should flash quickly as you crank the engine over.

If it does not flash, check your points.

68C15 · 08-29-2004, 07:23 AM

the reason you have no voltage at the - side is because that is close to ground & all the current is being used up before that. do what TX said. what voltage is at the + side? should be above 10.5 V while cranking & about 7 volts with key on not cranking. if the light does not flash but you have the needed voltages check the coil resistance & the points. if that is all ok think about what may have happened while sitting (hungry mouse, moisture in cap, etc...)
if no voltage while cranking check the wire coming from the "S" post on starter to the + side of coil (check the "S" post for ~12V while cranking[may be bad sol. not supplying voltage])

08-28-2004, 07:22 PM	#1
Palf70Step State of Confusion! Join Date: Jul 2002 Location: Gulfport, MS USA Posts: 46,761	How much voltage? How much voltage should be on the input (+) side of a coil? I had no voltage coming out of the coil on my 70's 250 6 popper. Replaced coil and still nothing. I didn't think it was 12V because of ther old style resistor wire thing, but seems like it is around 5V on the coil input. Is that too low? Any hints? All fuses look good and it ran fine till it sat for a month or so. Normally I don't have fuel becuase of it gumming up a little, not it started the no sparkoe thing. Points are working and gapped properly. Thanks. __________________ 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!

08-28-2004, 08:07 PM	#2
Sweet72 Registered User Join Date: Nov 2003 Location: Alexandria, VA Posts: 1,534	I believe 5 volts is a little low, as I recall it should be around 8 volts after the engine is started and a full 12 with the starter solenoid engaged. I don't believe an ordinary multimeter is capable of reading the 25,000+ volt output on the coil. Check for a poor ground, as it sounds like corrosion has caused a bad connection somewhere. Doesn't the neg terminal go to ground on the coil? __________________ '72 GMC SWB C1500 Custom, frame-off in progress. 383 SBC, 9:1CR, Comp Cams XE262H, Scat internal balanced crank, Eagle SIR 6" rods, Keith Black dish pistons, Dart Iron Eagle 72/180 heads, Weiand Stealth intake, Stewart stage I water pump, Holley 4bbl vac sec, TH350 with B&M Shift Improver Kit. 12 bolt positraction.

08-28-2004, 08:27 PM	#3
Palf70Step State of Confusion! Join Date: Jul 2002 Location: Gulfport, MS USA Posts: 46,761	Nah...neg goes to the distributor. __________________ 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!

08-29-2004, 06:37 AM	#4
Tx Firefighter Watch out for your cornhole ! Join Date: Jun 2003 Location: Azle, Texas Posts: 14,162	Put a test light onto the negative terminal, which goes to the distributor. Now, crank the engine. The test light should flash quickly as you crank the engine over. If it does not flash, check your points. __________________ I'm on the Instagram- @Gearhead_Kevin

08-29-2004, 07:23 AM	#5
68C15 blood type; Retumbo Join Date: Jan 2004 Location: next to my reloading bench Posts: 10,269	the reason you have no voltage at the - side is because that is close to ground & all the current is being used up before that. do what TX said. what voltage is at the + side? should be above 10.5 V while cranking & about 7 volts with key on not cranking. if the light does not flash but you have the needed voltages check the coil resistance & the points. if that is all ok think about what may have happened while sitting (hungry mouse, moisture in cap, etc...) if no voltage while cranking check the wire coming from the "S" post on starter to the + side of coil (check the "S" post for ~12V while cranking[may be bad sol. not supplying voltage]) __________________ Man rule #77...if you own a 67 stepside with a caddy 472 you will never be in danger of loosing you man card



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