4 April 2017

How to make your classic car’s ignition system dependable

Last week, I introduced the idea that, when a vintage car dies while under power, most of the time the cause isn’t some random lightning bolt from the blue. It is instead one of The Big Six (ignition, fuel delivery, cooling, charging, belts, ball joints). In the next six installments, we’ll drill down into each one. Let’s begin with the ignition.

On just about every car from the 1920s through the mid-’70s, the ignition system consists of the distributor (the housing and shaft that rotates inside it, along with the stuff that’s attached to it – the cap, rotor, points, and condenser), plus the ignition coil, ballast resistor if there is one, spark plugs, plug wires, and the voltage supply to the whole thing.

Distributor and coil on a 1973 BMW 2002tii.
Distributor and coil on a 1973 BMW 2002tii.

Here’s how it works. The points have two spring-loaded faces that are pulled closed against each other, but are forced open by a little nylon block that rides on the lobes of the distributor shaft. Current flows through a set of primary windings in the ignition coil, through the closed points, to the condenser, and then to ground. This sets up a magnetic field inside the coil. As the engine turns, it spins the shaft, whose lobes push against the nylon block, forcing the points open. When that happens, the current is abruptly shut off. The field collapses, creating a voltage in the coil’s secondary windings that’s high enough to jump across the gap in the spark plugs. This current then flows out the center of the coil, to the center of the distributor cap, where the spinning rotor distributes the current to each of the spark plugs. As the shaft spins faster, springs and weights cause the spark to advance (to occur sooner).

While part of the reason we love vintage cars is their simplicity, this quaint, almost Victorian, reliance on mechanical triggering—the opening and closing of points—is in fact a major source of unreliability. The faces of the points get burned and pitted, and the nylon block that rides on the shaft lobes wears down, both of which cause the point gap to shrink. One day, the gap becomes so small that the points no longer open, the coil no longer fires, the car dies, and you sing the little John Hartford song I mentioned last week.

Those pesky points
Those pesky points.

Another thing that can happen is that the little nylon block can crack, or the glue holding it to the points can fail. In either case, suddenly there’s nothing for the shaft lobes to push against, and… dead car.

Close-up of points with rotor removed
Close-up of points with rotor removed. Note how the nylon block is on the high point of one of the shaft lobes, and how this forces the points open. Now, imagine what happens when that nylon block wears or breaks off.

Coils can sometimes fail, but make no mistake about it – suspect the points first.

Before a road trip, you should do three things. First, visually inspect the points, then check the dwell. Dwell is the number of degrees out of 360 that the points are closed (that they “dwell” together). You measure dwell with a dwell meter. You should look up the spec for your car, it’s about 60, 40, and 30 degrees for four, six, and eight cylinder engines, respectively. Remember that dwell goes opposite from point gap. If the dwell is too big, you need to make the point gap wider. If the dwell too small, you need to make the point gap narrower. Sure, if the points are obviously burned and pitted, replace them, but it’s really the dwell that’s important. If you measure the dwell and it’s is really big, it means the points are barely opening, and it’s likely your car will soon die. Adjust the point gap to properly set the dwell before it does.

Because dealing with points can be a pain, many folks who road-trip their vintage cars replace the points and condenser with a small electronic triggering module. This eliminates the need to check dwell, set point gap, and worry about that nylon block flying off on while you’re crossing the Tappan Zee Bridge.

Next, check the connections to the distributor, coil, and ballast resistor. These are usually push-on spade connectors, and heat, oil, and vibration take their toll on them. If the spade connectors slide loosely onto their tabs, squeeze them gently with pliers to get them to bite. If the wire going into the connector is badly frayed, cut the connector off and crimp on a new one. And be sure the condenser is screwed tightly to the distributor. That’s its ground connection. If it’s loose, the car will misfire like mad.

Lastly, inspect the plugs, plug wires, and distributor cap. If the plugs are corroded or fouled, replace them. If the plug wires are cracked or missing their rubber insulating boots, the spark can arc to ground instead of going through the plug. Make sure the distributor cap isn’t cracked or has carbon tracks along the inside.

Do these things, and you’ll spark happily on down the road.

(Next week: The fuel delivery system.)

Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel Magazine for 30 years. He is the author of Memoirs of a Hack Mechanic and The Hack MechanicGuide to European Automotive Electrical Systems. Both are available from Bentley Publishers and Amazon. Or you can order personally inscribed copies through Rob’s website: www.robsiegel.com.

24 Reader Comments

  • 1
    Dave A. Arizona April 5, 2017 at 16:50
    Too bad you didn't mention converting to an electronic ignition system. Little maintenance and much more reliable.
  • 2
    Bill Fraley Ballston Spa, NY April 5, 2017 at 17:02
    I recently installed an electronic distributor unit in my '1968 E-Type Jaguar. Worked fine for a while until very suddenly it didn't No idea why at that point. Installed another of the same brand (Pertronix) and that failed, too. Seems the manufacturer has determined that because of EMI issues, solid core (read: copper) spark plug wires eventually cause the unit failure and so recommend soft core plug wires. This is now included in the text instructions for the Pertonix unit. Don't know if any other electronic mfg has the same problem. It's another thing to check if your engine suddenly stops running.
  • 3
    Bruce Madden Atlanta, GA. April 5, 2017 at 17:17
    We install Petronix which finally gets rid of the troublesome point gap problems, the main cause of poor performance. They are reasonably inexpensive(especially if one figures in labor costs) and very easy to install; would highly recommend it for any of our neanderthal cars.
  • 4
    woodboatchick wisconsin April 5, 2017 at 18:18
    My 69 XK-E had a small hole in the rotor, enough to mess it all up. Finally decided to install the "electronic triggering module" and its accompanying coil. Will get him out soon, so will know if the money was well spent.
  • 5
    Mel North Carolina April 5, 2017 at 19:00
    There is an old saying among 'old school' mechanics: "About ninety percent of what appear to be fuel problems, are actually ignition problems." From my experience, I believe this. Thanks for the article.
  • 6
    bill yankey Vegas April 6, 2017 at 13:24
    started working on cars at a toyota dealership in 73. the condenser is not a part that goes bad very often ..a poor condenser will usually cause the points to pit and burn quick. My suggestion is if you have a condenser that doesn't fry your points keep it in the system.
  • 7
    Tim Sullivan Farmington Mi April 6, 2017 at 15:23
    Dwell affects timing, one of my favorite, now obscure automotive bits of knowledge, great to share when anyone is looking under the hood of a vehicle.
  • 8
    Jerry Coale Holly, Mi. April 6, 2017 at 15:33
    In the 70's I was a parts mgr. for a BLM dealership & have had a number of MG's & Jag's. My current 74 B-GT still has points and condenser - I carry capt/rotor/points/rotor in the cubby. Can quickly be back on the road - - who carries a spare elec. ing. and get going again ?? AND by God it came with the system and I will not upset the Queen by altering !!
  • 9
    SK SoCal April 6, 2017 at 16:03
    I have Pertronix on one of my cars, but I keep the old points, rotor, and condenser in the glove box. If the electric ignition fails, I have a reliable back-up. I've noticed a deterioration of quality in new points/condensers over the years. I replaced the wires, cap, rotor, points, and condenser on a Dodge 360 before going on a trip. 100 miles into it, started sputtering, missing on the freeway. I was able to limp off to a gas station, and through trial & error figured that the new condenser went bad. I had the old one in the glove box, put it on: problem solved. I kept that condenser on until I sold the Dodge.
  • 10
    Gary Thornton 44629 April 6, 2017 at 05:52
    Great article on ignition systems. There is one little thing that has left people sitting along the road to that fails, that little thing is the condenser. I had two of them fail over the years, one failed right after I had tuned my car up at about 200 miles.
  • 11
    Robert Arkansas April 6, 2017 at 18:19
    Most condensers today are not American made. They often fail. Keep your original condenser!
  • 12
    jac vestal ny April 6, 2017 at 07:50
    don't foeget you dielectric grease on connectors....
  • 13
    David Bagley Runville Pa April 6, 2017 at 09:56
    79 mg Midget intermittent spark, found wiring harness under dash was not fully seated. I pushed it together and secured with a cable tie. Being intermittent made it tough to find. Engine would turn over just no spark.
  • 14
    Denny Elimon Mahomet ll April 6, 2017 at 12:00
    Discussion--points versus electronic ignition
  • 15
    GR Stackhouse Wayne, PA April 7, 2017 at 05:37
    Don't forget to grease (dielectric) the dizzy rotor shaft where the nylon block rides. They used to supply a little capsule of grease with new points sets for this but I haven't seen one included for years.
  • 16
    Buddy howle Greenwood sc April 7, 2017 at 19:50
    I got a 1966 Ford Mustang 6 cylinder.It has electronic ignition, cuts off when the engine gets hot. it will start right back up.
  • 17
    Thomas Madere LaPlace,La April 7, 2017 at 21:05
    At 73 years old I have had a lot of experience with conventional point ignition systems and from my experiences point were usually the last of my problems. Condensers, coils and ballast resistors were the most common failures, can't ever remember a set of points causing a problem.
  • 18
    Michael Fremont, Ca April 7, 2017 at 10:36
    I tried the Petronix electronic ignition twice on a 73 Toyota Land Cruiser. I found out if you leave the key in the ignition on without the motor running that will burn out the pick up head up. I actual seen smoke coming out of my distributor cap and the car would no longer start. When I replace my points I replace the condensor at the same time, cheap insurance.
  • 19
    Ben L. Coral Springs Fl April 7, 2017 at 23:20
    I have a Smallblock 66 corvette I installed a Petronix electronic ignition in it 20 years ago and have no problems with it I also noticed better fuel economy and power. You can't see it under the cap so it looks stock.
  • 20
    Steve Bauer Austin, TX April 7, 2017 at 00:52
    Great article. Two things came to mind. The condenser is electrically parallel to the points with a separate ground, acting as a sort of shock absorber for the points, reducing arcing as the points open. Current doesn't flow to ground from the points to the condenser. When the points are closed, the positive side of the condenser is grounded and it discharges through the points. As the points start to open, the coil current flows through the condenser instead until it's "full" instead of arcing across the point contacts when they are slightly open. Second, an easy way to check for arcing plug wires is to run is to start the car when it's dark and open the hood. Rev the engine a few times, and any arcing plug wires should be readily apparent.
  • 21
    Bill Rosen Redmond WA April 9, 2017 at 11:02
    I remember my dad telling me to keep a match book handy, because if the points got burned I could always use the strike pad as sandpaper to clean them up until I could replace them. Also, always inspect the distributor cap electrodes (inside the cap) for corrosion.
  • 22
    Robert White House, Tennessee April 12, 2017 at 23:17
    I have a 1971Camaro with a 350 engine. In 1988 l replaced the points and condenser with electronic conversion from Allison. It's ​the type with the Hall effect LED sensor. I haven't had the distributor cap off since. That's over 28 years! Maybe l should just to see how things look.
  • 23
    Don Ellsworth Girard Pennsylvania April 15, 2017 at 00:53
    I was tooling around in my 1971 Alfa Romeo Spider Veloce when it sputtered, then ran, then sputtered again and died. It seemed like fuel but it turned out to be points sticking open. WD-40 to the rescue.
  • 24
    Billy Sacratomato April 21, 2017 at 15:27
    Best concise explanation of the ignition system I have ever read. A skillful writer can be very helpful by explaining complex things in few words! As for the ignition systems, I have no idea why it took so many years for a jewel like the Pertronix to come out, as simple as it is in design. Maybe it was a Russian conspiracy with a former US president that was responsible. I put the P1 in my '66 Shelby many years ago, and it has been flawless!. The dual point setup was a total PITA in prior years. But, after initial installation I noticed a lack in accelerator "snap" and was initially disappointed with the change. So I then installed their powerful coil and was amazed at the increased overall HP I got ... around 20-30! Now, this could have been related to a possibly dying old coil, which often happens with classics that aren't kept on top of regularly, or maybe the old coil was part of the issue. Whatever, my change-over was probably the best update to my HiPo 289 that I ever made over the years, besides a rebuild. To make sure I covered everything, I put on the Pertronix plug wires also, but really didn't notice any change from them. Anyhow, having zero connection to Pertronix, I highly recommend their ignition systems, and using all of their units together. In fact, I think they should sell the ignitor with coil as a package, to get the full potential out of it, and possibly eliminate some of the issues some people have had with their systems.

Join the Discussion