Voltage at cranking - less than 12V

[stallion112]

I've been running an MSD 6A box for over 20 years with a Ford magnetic pick up distributor. The distributor was worn out so I decided on a Ford distributor with points (to keep the small cap due to limited room between the motor and expansion tank, and to keep the cost down) but use the Pertronix Ignitor to replace the points.

The motor will only run for a few seconds (rough) and then stall out. I've played with the timing but it sounds close enough to at least start and continue running.

Long story short, I've verified the wiring is correct, the Pertronix pick up is working, and the MSD box is working.

The only discrepancy I can find is that I do get 12.84 volts with key on, but it drops down to 9.3 volts when cranking. Pertronix and MSD both require a full 12 volts while cranking. The "ignition on" wire is wired directly from the battery so no ballast resistor is involved.

I'm assuming the voltage drop is due to the Tilton high torque starter. The car doesn't have any accessories, no heater, defroster, etc.

Is there a trick to wiring to maintain a full 12 volts while cranking?

[twobjshelbys]

Why don't you just drop a spare battery in place of the existing one??? Swap it from another vehicle.

[cycleguy55]

Originally Posted by stallion112

I've been running an MSD 6A box for over 20 years with a Ford magnetic pick up distributor. The distributor was worn out so I decided on a Ford distributor with points (to keep the small cap due to limited room between the motor and expansion tank, and to keep the cost down) but use the Pertronix Ignitor to replace the points.

The motor will only run for a few seconds (rough) and then stall out. I've played with the timing but it sounds close enough to at least start and continue running.

Long story short, I've verified the wiring is correct, the Pertronix pick up is working, and the MSD box is working.

The only discrepancy I can find is that I do get 12.84 volts with key on, but it drops down to 9.3 volts when cranking. Pertronix and MSD both require a full 12 volts while cranking. The "ignition on" wire is wired directly from the battery so no ballast resistor is involved.

I'm assuming the voltage drop is due to the Tilton high torque starter. The car doesn't have any accessories, no heater, defroster, etc.

Is there a trick to wiring to maintain a full 12 volts while cranking?

Suggestions:

1. If the battery hasn't been load tested recently, have it checked out.
2. If the battery tests okay, then check and clean all ground and battery to starter connections.

[stallion112]

Yes, I didn't mention but the battery was load tested and checked out good. I'll see about swapping it out for another one to see if it makes a difference.

[Tommy]

No expert here, but a Google search "expert" says it is normal for voltage to drop into the 9s during start. Perhaps a second battery jumped in parallel with the first would provide enough current to start without dropping the voltage so low. If so, that would tell you whether it is the battery or something else.

[Blas]

Run a temporary power wire directly from the battery post to power the ignition. If that dies the trick, you are on the road again. Also check your grounds.
Blas

[Luce]

If it's running (although slow and rough) I'm assuming you've let go of the starter and the voltage has returned to 12v+ yet it still stumbles and dies.

Unless you want to carry around double batteries like a heavy duty diesel pickup, voltage will sag into the 9's while cranking.

If you've only eyeballed and guessed at the timing rather than having a light on it, my bet is you're a little retarded (not you personally, your timing) You'll do no harm pushing it up 5 degrees sitting at an idle and seeing if it'll run there.


edit- add

The spark happens when the points open, not when they close.

[stallion112]

O.K., some more background. When I hit the key for the first time after installing, it fired right up and ran for about 5 seconds. After that, it would run rough. I rotated the distributor several times about +/- 30 degrees and the results ranged from didn't start to still runs rough.

I didn't have a helper available at the time so couldn't check it with a light.

Put a light on it tonight and had someone turn it over. It was right about at TDC. Advanced it to 12 degrees and voila!! Nothing but the sweet smell of unburnt hydrocarbons. Kinda' brought tears to my eyes.

Thanks for everyone's input.

[incoming]

Especially at the prices we're paying for our petro.