The Stovebolt Site Forums The Shop Area Engine Shop No spark - multimeter troubleshooting

1962 235 that I had running beautifully a year ago. Got a new battery yesterday and went to start and quickly figured out I have no spark at plugs now. This is a 12v conversion with a ballast resistor on firewall.

I suck at all things electric. But here’s what I’ve got so far:

Engine cranks very well
With ignition on and points closed:
12v at one ballast terminal, 5 volts at the other
5 volts at positive terminal at coil, about 0.25v at the negative and at the distributor.

Those reading make some sense (not sure what the ballast resistor should drop the voltage to though). The closer you get to ground (closed points in this case), the lower the voltage will be. The start circuit bypasses the ballast resistor, so maybe that has an open circuit, meaning no voltage to the coil when cranking.
You should see close to 12V at the coil input terminal WHEN CRANKING.

.25 V downstream of the coil with the points closed is a bit too high, but not terrible. Bump the starter over until the points open, and the voltage at all the connections you tested should jump up to battery voltage since no current is flowing. 5V at the coil input id s bit low - - - -if the resistance of the coil primary is correct, you should see 8-9 volts downstream of the resistor or at the coil + terminal. It's possible you might have an "internal resistor" coil in series with your ballast resistor. That would result in a weak spark, but not a "no spark" situation. Try cleaning up the points by scrubbing them with a pencil eraser or a clean business card. Don't file or sand them!
Jerry

What do I use my points file for if I can't use them to file points? (be nice - moderator is always watching)

Your nails? LOL!

Fingernails, maybe? Once a set of points gets scratched up by a file, they don't last very long at all, particularly since both contacts end up flat, instead of one of them having a little radius.
Jerry

Too late on the sandpaper. But it was just 600 grit and just a couple swipes.

What’s a bit puzzling is the fact it ran like a champ last time it ran. I checked spark at a different cylinder and did have some spark but pretty weak. Good spark at high tension lead.

If I’m low at the + side of the coil, and the downstream side of ballast resistor...is that a sign of bad ballast resistor?

Try jumping out the resistor and if it starts, you’ll know that is the problem or at least part of the problem. It won’t harm anything! If it doesn’t start you’ll know that it’s down to distributor, coil or the wiring.

I don’t know if you know this or not but a 12v conversion that has ballast resister is also supposed to have a starting bypass circuit. Usually a wire from the starter switch that bypasses the ballast resistor on starting to supply a full 12v to the coil. It almost sounds to me that you are missing this bypass circuit. Many people run their trucks without this circuit but it’s not recommended especially if you live in colder climates.

Good luck!

Most coils that need a resistor function best with around 9 to 9.5 volts applied to them, so if your voltage at the battery posts is 12.5V or so, you should be able to put both voltmeter leads across the resistor and see about a 3V drop with the points closed. If the points have some resistance instead of providing a "zero volts" drop when they close, the voltage the resistor drops will be less. The sum of all the voltage losses in the circuit will always equal battery voltage, so let's say the resistor drops 3V, and the coil primary drops 8.9V., then the voltage upstream of the points will be 0.6V with a battery terminal voltage of 12.5. That indicates the points are a little dirty, as the drop across the points should be virtually zero. With clean points, the resistor will consume a bit more voltage, assuming the current flow through the coil windings stays the same.

Try pulling the coil wire out of the distributor cap and holding it about 1/2" from a ground such as the cylinder head as someone spins the starter. You should get a spark every time the points close, then open. The "old mechanics' tale" about a "hot blue spark" went out of style about 50 years ago when resistance plug wires came along- - - -a normal spark will be sort of a yellow-orange color unless you happen to be running solid wire conductors. As long as the spark will jump 3/8" or more, you've got plenty of voltage to fire a plug efficiently. If you're getting a good spark at the coil wire, but little or no spark at the plugs, check the distributor cap and/or rotor for problems.
Jerry

Well, I just decided I was gonna try to start the darn thing and away she went. Settled down once the choke was pushed in and acted like it would idle for a year. Vac gauge hooked to manifold read 22 and steady.

Before that, I did test voltage everywhere with the points open and it was 12.8 at every spot.

Not sure what the deal was yesterday.

Yes, this has that bypass circuit.

Flooded, maybe? If it happens again, try holding the throrrle wide open and crank it for several seconds. Since the advent of "instant start" fuel injection, flood clearing has become a lost art!
Jerry

Yeah, I definitely didn’t have spark yesterday. Probably some gremlin that will haunt me later. Flood procedure is handy with small engines too.

You think the low voltage downstream of resistor is coil and not the resistor itself?

Coil failures are few and far between, but it's more likely that you might have a coil with the wrong resistance in the primary winding. Try measuring the resistance of the primary with both wires disconnected. A coil that will flow approximately 5 amps with 9.5 volts applied to it will have somewhere near 1.9-2 Ohms of resistance. 5 amps is the maximum a set of points will handle reliably without burning.
Jerry

FYI, the symptom for a failed resistor, is when the engine starts but stops as soon as the key/foot starter is released.

Back to this saga. Engine has been run successfully several times since this thread began, but the weird voltage has been bugging me so I tried a new balance resistor last night and checked voltages just for experimentation purposes.

Old ballast resistor:
12.5 volts at battery terminals
11.7 volts at resistor input (I swear I used to have full battery voltage here, but I guess I didn’t document it)
5-ish volts at resistor output

New ballast resistor:
12.5 at battery
11.7 at resistor resistor input
3.5-ish volts at resistor output

So...if 9 volts to the coil input is considered about right...the new resistor took me the wrong direction.

Jerry mentioned it’s possible I have a coil with improper resistance. I did measure the primary last night and it was 1.3 ohms. What else would I need to do to test the coil? Or just should I just try new one and test the same spots for voltage?

Edit: I did forget to disconnect wires before checking the coil primary voltage.

My understanding is that the old ballast resistor readings were made before you cleaned the points. Reducing the resistance across the points by cleaning them will increase the voltage drop across the resistor. (Kirchhoff’s Voltage Law)

If you want to compare your old and new resistors, set your multimeter to ohms and check them that way.

Good comment....I should clarify.

1) I don’t recall at what point I cleaned the points. Probably before taking any of the readings in the original thread....some weeks ago.

2) The voltage readings of the two different resistors took place just last night (on the firewall of the truck). All other variables should be the same. The old was taken before removing from the truck. The new was taken after installing on the firewall. I will measure the resistance of each disconnected from the system as well.

Had a small breakthrough this evening. Long story, but the coil that’s on the truck is apparently a 6v coil. I had a guy do the brakes for me before even bringing it home after buying it. He installed the coil and all the verbiage was facing the engine and I never had it off until tonight.

From what little reading I’ve done tonight, sounds like a 6v coil will still function ok in a 12v system (it’s about ohms and not volts) but at least I can understand now why I was getting 3.5 or 5 volts on the resistor output.

It seems like you are dealing with lots of variables and inconsistencies. Troubleshooting these situations can drive you crazy.

Whenever I get into a mess like this I step back and organize myself and my test plan. You have at least 3 variables you need to reconcile. Which resistor produces what results, which coil should you be using, and is the wiring and ignition switch working correctly?

As a starting point I would connect a jumper wire directly from the starter terminal/battery to the coil after disconnecting the wire from the resistor. Determine if the engine starts and runs as you expect. Change only one variable at time as you troubleshoot.

Remember that the major reason for the resistor is to reduce the voltage across the points when the engine is running. The lower voltage reduces the heating and arcing of the points so they last longer.

The idea behind a "resistor bypass circuit" for starting is to simply give a slightly hotter spark to get things going. In my experience if everything is in good condition that circuit is not essential.

I will not even start the discussion about whether a coil is a 6 volt or 12 volt and if one will operate properly on the opposite voltage.

Agree that isolating variables is the best path forward.

For now, I have the old resistor back on the truck.....and I am going to get a proper 12v coil designed for use with an external resistor.

From there, I will see if I have voltage readings at all the specified points that are considered more textbook. And if they are, I will go back and re-visit plug gap, dwell, timing, etc.

This whole mess started with a a sudden no-start problem and verified no-spark symptom a few weeks ago. In spite of all the weirdness with my coil and whatever else, the truck has run pretty well every time except that one. I may well have been chasing some ghost all along....but at least I finally got around to looking at the coil the other dude installed.

Update with new coil installed (12v for use with external resistor).

This may be a purely academic exercise at this point, but if the consensus is that I should be getting 8-9 volts on the coil side of the ballast....I’m determined to figure out why I don’t. Spark seems to be adequate but I just want to learn something.

With points closed and old delco remy resistor installed:
@ ballast input - 11.9v
@ ballast output - 5.5v
@ coil negative - 0.18v

I think I will measure the resistance of both the old and new ballast resistors this evening.

Edit: After carefully re-reading one of HRL’s posts, perhaps I will take another crack at cleaning the points as a next step.

JW51 You know you can bag-up all that ignition trouble with an HEI,our hi-tek 7to1 compression engines recognize modern ignition and run great on it. Why beat yourself=up with that sorry antique ,wore-out ignition ??? Make your own,keep a spare under the seat !!!

Try replacing the condenser.

Fair point, I’m just too stubborn to give up.

If I can’t figure out points ignition....I don’t “deserve” the upgrade. Just for clarity, from a functional standpoint it seems to work just fine even as it is. I just want it to be working as designed.

It's amazing that untold millions of vehicles that used point-type ignition systems traveled billions of trouble-free miles and did things like winning WW II- - - - -all without a single HEI ignition in sight! I'm reminded of the meme showing a Spartan soldier holding an M-16- - - - -the caption reads "Weapons change- - - -Warriors don't!"
Jerry

This most likely is not the problem but I have seen where a fine scratch inside the distributor cap near the rotor contact going down to the bottom was grounding out the voltage.
The engine was cold and the outside humidity high. Collected moisture in a scratch or a crack will act as a conductor to high voltage.
Didn't have another cap so I wiped the inside with a clean rag. It started up and ran.

Just to close the loop on this. After extensive testing, isolating this and that, resistance checking this and that....that I can now conclude.....

It doesn’t matter.

This quest started with me thinking I had too low voltage from the ballast to the coil. I had in my head that less than the textbook 8-9 volts might be causing weak or unreliable spark.

4.5 volts from ballast to coil = bright white spark from distributor wire while cranking that will easily jump 1/2” or more. This setup was the new ballast that is 2.2 ohms and the wire from the starter to coil disconnected.

5.6 volts from ballast to coil = Same results and this is how I chose to leave it. The old ballast registers about 0.6 ohms. Starter to coil circuit disconnected.

If nothing else, all this silliness made me much more familiar with the entire ignition circuit. It also gave me occasion to clean up some sketchy looking terminal connections and throw some shrink wrap over ancient crusty wires with cracked insulation from the 50s.

Thanks to all for your input.