Voltage drop 3 volts cranking at coil

1964 FHC, went to a pertronix ignition, and she doesn’t want to fire when hot. When she cools down for an hour she fires up. There is a distinct difference firing while cranking or when I get off the starter button. She seems to fire as I get off the starter rather than when cranking.

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Voltage at the control box is battery voltage. At fuse no 6 it is ~8 volts. Haven’t checked the ignition light wires yet because I am reluctant to go farther into the dash, but I may need to. Coil voltage running is 6 volts, cranking is 3 volts, and on just ignition is 8 volts at the coil and fuse 6. There is a brown and yellow wire going to the ignition switch that isn’t noted in the manual. Any suggestions, tips or thoughts would be welcome, as well as how to get into the dash panel holding the speedo and Tach.

There was another similar thread recently, Pete. Your symptoms suggest that you should be looking at an external ignition ballast resistor if present, and separate wiring, if present, that exclusively supplies the coil whilst cranking. Good luck! IMHO.

Among other things that can be an indication of a worn starter. Occurs when the starter is drawing so much current you don’t get a decent spark… then, as soon as you stop cranking, you get a good spark for one last compression stroke which can be enough to start the engine.

But that 8 VDC at fuse number 6 is not so good either - worth finding out why that is.

That is an issue with 6V Porsche 356 systems. When a typical 12V Pertronix is added to replace points, the starter draws so much juice in the 6V system the Pertronix can’t work. There is a 6V Pertronix now, but the point is, it’s a similar symptom.

This actually sounds like a bad ignition switch more than anything else.

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Sounds to me you have taken quite a few voltage readings, and you have very low voltage several places. Now, simply trace back and see where those voltage drops are. As Michael suggested, possibly the ignition switch, and that certainly is a likely suspect, but it could also be at many other places, including battery connections, fuse connections, fuse holder, fuse, or maybe even a faulty battery. You obviously have a meter, now go for it! Check them under load.

for any older electrical system, I have 2 wired switches with alligator clips

  1. battery + to coil, with an on/off switch
  2. momentary push switch from + battery to solenoid (thicker wires)
  3. DMM on battery

If the car now starts, that narrows the search quite a lot

in my opinion, unless a battery reads over 12.50V (after siting overnight), it should be considered suspect, and tested

take note of the DMM reading while the starter cranks over

Battery voltage is 12.4 and I would have expected a bit more. Another symptom when I tried the pertronix coil was no spark (way less voltage than it wanted) but, when I switched the ignition off, I cot a single cylinder bump as it fired. Not sure if this implicates the ignition switch or if it just corroborates the starter grabbing all the juice. The NY wire on the switch and not on the wiring diagram is a puzzler. The clear plastic sheath over the connection blade got scorched at one time too… we’ll check the battery side voltage under cranking load.

Do i take the dash top off or just undo the tach/speedometer panel to access the connections behind.

Until I swapped my original starter with a high-torque, I rarely got it to fire up before letting go of the starter button.

My big ugly starter in poor condition drew so much juice the house lights dimmed - and at the house next door too.

NASA called once…I almost drew a few satellites into the Ohm’s Law Vacuum, (OLV) …what? It’s a thing…


My '64 uses a Crane/Allison igntion system. There are two versions…one with ballast resister and one without. Mine uses the resister and associated extra wiring (similar to the later S2 arrangement). A friend has the same arrangement in his '64, but connected the resister incorrectly, and it does exactly what you describe…fires when the key is released. Anyway, I’d check with Pertonix about the need for a resister in the system.

Why are you removing the dash top?

I didn’t - I was wondering how to get the dash out or access to the ignition light at the bottom of the speedo. I also have the waving speedometer needle issue now, so I probably will be needing to access that at some point.

I haven’t found a ballast resistor in the system - and now the coil is getting 12.4 volts when not cranking and 5+ volts running. not sure about the drop - so where does the ballast resistor hide when it is there? My manual does not show the resistor in the circuitry.

Agreed - the starter starts attracting planets when the button is pushed. The fuel pump apparently is a pretty big current draw as well…


Some ballast resistors are wires rather than more obvious traditional resistors

I have often removed the speedo by reaching up under the dash ( bottom panel removed). You’ll probably have to get under there and look the first time you do it but eventually it can be done while seated behind the wheel. Remember there are two cable connections, the usual and the odometer reset.

As a follow up, my son Stewart (a mechanic) and I kept on chasing voltages and continuity from the control box (340) and the fuse panel and on to the coil. We focused on the ignition circuit (fuse 6) and checked continuity of the wires, power feed, and voltage at the coil. with repeated removal and re-attaching of the wires to the fuse box tabs, the voltage at the coil kept climbing, and continuity improved - so I am assuming I have some light corrosion on the fuse box that comes with time. I also had some non-functional switches a few months ago and gave them the same treatment - so perhaps I should spend a day detaching and reattaching the darn connections… that said, we’re still low at the coil - but we got the pertronix coil to work - better coil wire connection (was loose on the Lucas Coil) and we reattached the vacuum advance…seems pretty darn peppy and the pertronix coil allows the restart when the engine is hot - probably the higher zap from the flamethrower…

Still curious about the brown and white wire on the ignition switch that isn’t shown on the wiring diagram…it got a bit crispy on the end at some point…which is a bit disconcerting…

Made my morning…:joy::joy::joy::joy::joy:

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I have a new ignition switch coming and will try to swap that out this weekend. The voltage drops seem to be diminishing as I check the wires (remove and re-install them on the connecting tabs - so there is definitely a bit of schmutz on the connector pins. I think the car sat a lot in Santa Barbara before I bought it, so the salt air may have been a bit of a culprit. Fuel pump is running fine and not really drawing a whole lot (once isolated and tested) - so I am coming back to connections and the ignition switch. I got the little wrench from XKs so I don’t bugger up the dash when I take the bezel (escutcheon) off (with any luck). stay tuned! I am also looking at going back to points and condenser, which seemed to work fine and started hot every time…