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Old 09-15-2019, 08:12 AM   #1
mb_camper
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Default Weak Spark

Hello, I am brand new to this forum as well as flatheads so please bare with me.

I have a 1946 Super Deluxe with (as near as I can tell) somewhere between a 49 and 53 flathead 8 in it. The car runs and drives but seems to me to have weak spark and lately has been hit or miss when I try to start it. I measure around 3-4 volts at the coil with the points open and in my research have come to understand that is about correct for the 46 flatty due to a condenser under the dash but only for a couple of model years. I also have a condenser that is mounted on the firewall next tot eh voltage regulator. My question is this, if my motor was originally intended for later model vehicle that would not be equipped with the under dash condenser, does the coil want to see the full 6 volts? Additionally what purpose does the firewall mounted condenser serve? Any help would be appreciated, like I say I am new to this vintage vehicle so please be gentle!

I suppose I should add that is a stock 6 volt pos ground set up.
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Old 09-15-2019, 08:52 AM   #2
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Default Re: Weak Spark

I kinda think the condensor next to the regulator is NOT the ignition condensor....Condensor is attached to the distributor housing , these hardly ever fail ...
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Old 09-15-2019, 09:25 AM   #3
mb_camper
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Default Re: Weak Spark

Do you know what its function might be?
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Old 09-15-2019, 09:31 AM   #4
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Default Re: Weak Spark

could be for radio static ,I have seen them mounted on the generator
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Old 09-15-2019, 09:31 AM   #5
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Default Re: Weak Spark

Most likely noise suppression for a radio. What distributor does your engine have? Front of the engine pancake style or the later distributor that looks more like a modern one? And, welcome to the Barn!
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Old 09-15-2019, 11:33 AM   #6
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Default Re: Weak Spark

My (original when I got it) '51 Ford had radio suppression condensers on the voltage regulator, generator, and even the oil pressure sending unit.
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Old 09-15-2019, 03:32 PM   #7
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Default Re: Weak Spark

Radio interference reception, that makes sense considering where it is. it is not the crab style disto it is the modern stile just in front of no 1 plug. thanks for the warm welcome!
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Old 09-15-2019, 05:06 PM   #8
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Default Re: Weak Spark

If you want a reliable condenser (although it is an externally mounted unit), contact me.
Attached Images
File Type: jpg cap1.jpg (28.4 KB, 28 views)
File Type: jpg IMG_1236.jpg (62.7 KB, 30 views)
File Type: jpg IMG_1326.jpg (84.5 KB, 35 views)
File Type: jpg Dual Coil.jpg (82.0 KB, 38 views)
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Old 09-18-2019, 07:38 AM   #9
mb_camper
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Default Re: Weak Spark

Thanks for all of the advice so far, but I am still hoping to hear someone weigh in on what the proper voltage for a circa 1950 flatty should be at the coil. I am still concerned that the reduced voltage for the 46 chassis is causing a problem for the more recent motor.
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Old 09-18-2019, 07:43 AM   #10
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Default Re: Weak Spark

It depends on the coil you are using and the only way to know is to measure the coil resistance. But, in most cases a "can" coil will not require an external resistor if it is still a 6v system.

You ideally are wanting somewhere in the 3.5 amp range in the ignition circuit. Current is calculated as Current (A) = Voltage (V) divided by the Resistance (R). As an example, if your coil measures 1.5 ohms (a common value for "can" coils in a 6v system), then A = V/R would be A = 6V/1.5 ohms or 4A. The amperage is a little high, but a common configuration.

In a real circuit there is additional resistance in the circuit in addition to the coil and the voltage can be higher than 6v, but that is the general idea. Your voltage at the coil should be around 6v.

If the car is not using a "can" coil, but one of the original Ford style coils, then the resistance of the coil will determine what ballast resistor is required. Just as an example, if a coils resistance is say 0.75 ohms then you would want around a 0.75 ohm ballast resistor.

Also keep in mind that if the car is still using positive ground and the coil is not wired with the correct polarity, it will reduce the coil output. On a positive ground system the (-) terminal connects to the ignition switch and the (+) terminal connects to the distributor.

*Second attached photo is of Bubba's Ignition Service checking a Ford coil.
Attached Images
File Type: jpg Coil Measurements.jpg (58.1 KB, 41 views)
File Type: jpg Bubba Checking Coil 1.jpg (25.4 KB, 35 views)

Last edited by JSeery; 09-18-2019 at 08:15 AM.
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Old 09-18-2019, 08:33 AM   #11
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Default Re: Weak Spark

A 1946 will have a resistor 'left side under the dash' to drop coil voltage. I always thought of this being a "Ford thing" of that period probably something to do with the
original type coil they used that needed a resistor. I have always questioned that?
At my age I have yet to come upon any 1949-53 Ford product with a coil resistor.
Here is the common sense when with this resistor = turn ignition on and you get maybe
1/2 of 6vts at the coil more or less, now hit the starter that sucks juice. Lucky the
coil will light a flashlight, But they did work with the 'square' coil. Now you have a I call it a beer can coil used on all 48 - 53 trucks and 49 - 53 cars. My positive proof I have a
fleet of trucks one 1950 F6 bought new one 1954 F800 bought new by my father 6volt
positive grd. 1950 F8 mighty 337 V8 no resistor on any. Do you have a volt meter and
connect to the minus side of the coil if you get less than 6 volts you have the resistor.
Connect a jumper wire from battery to coil = meter should jump to 6volts now hit the starter should start on a dime ...........oh make sure the wire going to the points and
condenser is good they do fray over time. sam J Seery said it all..

Last edited by big job; 09-18-2019 at 08:37 AM. Reason: spelling
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Old 09-18-2019, 10:17 AM   #12
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Default Re: Weak Spark

Ford eliminated the in line resistor in 1949.
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Old 09-18-2019, 06:42 PM   #13
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Default Re: Weak Spark

Great info guys, that gives me a direction to go in. I will be sure and post results so the next guy that needs this post see the results as well!
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Old 09-22-2019, 05:30 PM   #14
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Default Re: Weak Spark

Okay so time for an update. So I started the car and warmed it up, then I jumped negative side of the coil to the negative battery post (remember positive-ground 6 volt set up) and the car died and would not restart. It wasn’t completely without fire because it did stumble once or twice but did not catch. At this point I believe my coil is all but dead which is weird considering the negative terminal on the battery post runs into the car through the ignition switch and back out to the coil so jumping it should only have provide a full 6 volts at the terminal instead of the 3-4 I have there with resistor under the dash. Thoughts?
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Old 09-22-2019, 06:47 PM   #15
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Default Re: Weak Spark

I already posted how I would have done this. But for starters, what coil are you using? It is going to make a difference depending on the coil you have.
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Old 09-22-2019, 07:51 PM   #16
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Default Re: Weak Spark

MB IF YOUR COIL WAS WIRED BACK TO FRONT ,eg then you may have fried the points ,
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Old 09-22-2019, 07:54 PM   #17
mb_camper
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Default Re: Weak Spark

There are no markings on the coil other than 6 volt. That is about all the info I have. I will start taking the reading you suggested but I suspect I will have to change the coil first. I appreciate the info you provided in you first post and would like have done that first but I work 3 jobs and usually only get a few minutes here and there to tinker, often in the dark. Thanks all!
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Old 09-22-2019, 07:55 PM   #18
mb_camper
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Default Re: Weak Spark

It is a generic can style coil
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Old 09-22-2019, 08:06 PM   #19
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Default Re: Weak Spark

With three jobs you are lucky to have any time at all. I have one more than full time job and it's a struggle to find time to work on cars. That said, if you have a can style coil AND a resistor, that is probably your problem. Wiser heads than mine can set you straight.

Last edited by mhsprecher; 09-23-2019 at 11:39 AM.
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Old 09-22-2019, 10:05 PM   #20
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Default Re: Weak Spark

On my Model A. Had a fair spark but would not start when hot. Checked coil primary. Between pos and neg on coil. Meassured 5.3 ohm. Primary to secondary (output) was 9,000 ohms. Found my old coil. Primary was 1.6 ohms with 9000 secondary. Problem solved. 1in srark jump now. No start problems. Check coil
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