Low voltage at coil
Thread Starter
Burning Brakes
Joined: Mar 2004
Posts: 923
Likes: 4
From: Austin Tx
Low voltage at coil
My 71 BB wouldn't start yesterday, plenty of fuel and cranking, but no spark. Test light on + on the coil lights up but very dim, but when I touch + on back of alt light is bright. I ran a jumper from + on back of alt to coil and it fired right up. Any ideas?
Le Mans Master
Joined: Jan 2006
Posts: 9,192
Likes: 27
From: Boca Raton Florida
Do the same test and hit the key with the light on the + side of the coil. It should light up bright. You get full batt voltage during cranking from the starter and lower voltage while running due to the resister or hi resistance wire feeding the coil
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
when you say the test light is dim, what do you mean? if it's when the ignition is on and engine is running, a dim light would mean that you're getting power to the coil thru the resistive wire. if you're not getting power in the start position, the problem could be in the switch or that part of the wiring. test the voltage on the coil while cranking.
jeff
jeff
Tech Contributor
Joined: Jun 2004
Posts: 20,910
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
A couple quick things come to mind.
1) Excessive resistance in one of the + line feeds. There is an intentional resistance in the harness (approximately 1-2 ohms) to limit the current going through the coil and points in run mode. Sounds like you may need to start backtracking this wire to see if there is additional resistance in this line. Also check the shunt wire from the starter solenoid to the + terminal. This wire does the same function as your jumper wire to the alternator, allowing normal current despite voltage droops during cranking. This shunt wire overides the ballast wire during cranking, but is disconnected during running mode.
2) Were the points closed when you made your first measurement? With the points closed you will see a (normal) voltage drop at the + terminal. If you needed to apply direct feed (from the alternator) to the + terminal to get sufficient coil primary current while cranking, there may be a problem with the starter shunt wire mentioned above.
Good luck!
Thread Starter
Burning Brakes
Joined: Mar 2004
Posts: 923
Likes: 4
From: Austin Tx
the starter that is on the car is a non stock hi torque variety that does not have the provision for the wire that feeds the coil. This is way I bought it and it has been running fine like this for over a year. The car also has a petronix ign module and an accel coil. I don't see a ballast resistor on the firewall, but the green wire feeding 12v to the coil is wrapped with thick black tape for about 5 in before it goes into the harness with all the other wires. I'm thinking if this is where the resistance is I can clip and just put a regular wire in there. No need to step down volts if I'm running elect ign right?
Tech Contributor
Joined: Jun 2004
Posts: 20,910
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
the starter that is on the car is a non stock hi torque variety that does not have the provision for the wire that feeds the coil. This is way I bought it and it has been running fine like this for over a year. The car also has a petronix ign module and an accel coil. I don't see a ballast resistor on the firewall, but the green wire feeding 12v to the coil is wrapped with thick black tape for about 5 in before it goes into the harness with all the other wires. I'm thinking if this is where the resistance is I can clip and just put a regular wire in there. No need to step down volts if I'm running elect ign right?
You've got a hodgepodge of parts there. In a high quality electronic system there is no need for a ballast resistor. However, a lot of the aftermarket systems are just glorified electronic points. I would measure the resistance of that coil, and then contact Petronix to see what level of current their module can safely conduct.
