Resistor ignition switch wire

I am looking for a confirmation that my suspicions are correct. I havent made any measurements with a meter yet but that will be my next job when I get some time to devote to this issue. I am working on other areas of the car and I am not driving it at the moment. When the car has sat for a few weeks and the battery voltage is a little lower than full charge, the engine will crank over fine but it wont start. Not even a pop or spit. If I connect a jumper wire from the battery (+) to the ignition switch side of the ballast resistor (white ceramic type), the engine will fire right up. I can let the engine run and charge the battery a bit then pull off the jumper and the engine will continue to run. If I dont wait for the battery to charge and pull the jumper too soon, the engine will die and wont re-start.
I suspect that I may have an ignition feed wire with the resistor built in and I am using the ceramic ballast resistor as well in the ignition feed circuit to the coil. This is the way the wiring was when I purchased the car so I assumed it was correct. It would seem that if I have 2 resistors in the ignition feed line to the coil that I am not getting enough voltage to the coil to get it to fire. The distributor has been upgraded with a Mallory pointless ignition adaptor, and I have a new coil that was supposed to be the OEM 6v type not 12v type. Any thoughts.

A stock Mustang has a built in resistance wire (behind the dash) feeding the coil, the coil then connected to the distributor. The resistance wire lowers the voltage to the coil and points. This is done to prolong the life of the points. There is another wire at the starter solenoid that bypasses the resistor wire for starting and is only powered while cranking the starter. That 12 volt bypass generates a hotter spark during cranking for easier starts. From your description of the problem, you may have an issue with that bypass wire. There should be no need for another resister in series. Pointless distributors often require the full 12 volts. Mustangs with the factory tach need the resistance wire in series with the coil to function properly. With some pointless distributors, getting the factory tach to work can be tricky.

Thanks for the reply Doug. I feel a little stupid as I found out that I asked a similar question just before Christmas last year (Ghost in the machine). The car has been sitting and I tried to fire the motor the other day. I had to use the jumper so I thought I would pose the question to the forum. Between the answers last year and your comments, I know I just need to get into it and get all the voltages right. I am rebuilding the hurst shifter at the moment. When I get that completed then I will get into this ignition thing. I'm likely to replace the distributor and coil with 12 volt electronic units and be done with it all. Thanks again for the advice.

The attached PDF is a snippet of the Shop Manual for 1973 Fords. It shows a lot of information about the primary ignition system wthat may be of use to you.

Thankyou for this post as it answered alot of my questions. Ive burned thru 2 brandnew points systems and have had no tach since her debute last July. currently 25 F so will have to wait.

You can verify the voltage using a volt meter at the ballast and after the ballast. Do this with the ignition on and the car running.

I am a bit confused about your last sentence " and I have a new coil that was supposed to be the OEM 6v type not 12v type."
Can you expand on this? Are you saying the coil also has an internal resistor?

Great post MRGMHALE. Your PDF document gives me some specific information to test around. The diagrams in the file are just what I needed (I hate reading). As for Will e's question. When I bought the new coil, the instructions said that I needed to have a ballast resistor or a resistor in the power feed wire from the ignition switch to the coil.
As mentioned, I am more inclined to switch everything over to a true 12 volt system (electronic distributor and coil) and not have to mess with all this resistance stuff and terminal "I" on the start relay. Simplicity is better for me. Thank you all for your comments.

Mark Jones said:

Great post MRGMHALE. Your PDF document gives me some specific information to test around. The diagrams in the file are just what I needed (I hate reading). As for Will e's question. When I bought the new coil, the instructions said that I needed to have a ballast resistor or a resistor in the power feed wire from the ignition switch to the coil.
As mentioned, I am more inclined to switch everything over to a true 12 volt system (electronic distributor and coil) and not have to mess with all this resistance stuff and terminal "I" on the start relay. Simplicity is better for me. Thank you all for your comments.

Click to expand...

The "resistaance stuff" is actually qquite eimple once it gets broken down. But, there are some misconceptions that have been oft repeated, even taught, for year after year that I hope to clear up. A lot of folks have heard, and been taught, that when the ignition key is in the Start or crank position the ballast resistor or resistance wire circuit is bypassed to give the ignition coil a full 12 volts to help the engine sart. That is not quite the case. The reality is the ballast resistor or resistor wire are bypassed so the ignition points and ignition coil can get "full battery voltage," which when cranking the engine is only about 10 volts or so. With our First Generation Mustangs in ignition switch the Run position the resistor wire reduces the battery voltage (assume 12 volts, although it is usually a little higher) down to 5 - 10 volts or so (manual says 4.5 - 6.9 volts, but I often see a little higher than that). So, when cranking the engine over the voltage to the ignition points and ignition coil can't be higher than the bazttery's effective cranking voltage - but that is still a little higher than the running voltage to the coil.

If you are burning through ignition points and/or ignition coils, you may have a faulty ignition point condensor, or the resistor wire has been bypassed and you are sending full battery voltage to the points and ignition coil. Or both. If you decided to move to a Pertronix o other electronic ignition system you likely will need to use full switched battery voltage. There are a few places with "apparent" switched voltage under the hood, but the easy ones are not full battery voltage. The Circuit #4 White wire with Black Stripe from the alternator stator terminal is only 1/2 battery voltage, and is also AC, not DC, power. So using that is not going to be much help - at least not using it directly.

If you have a 72 or 73 Mustang there is a Red wire with Yellow Hash markers that feeds a carburetor Throttle Position Solenoid (which or may not be present with your carburetor). That circuit has full switched battery power which can be tapped into for an aftermarket ignition system if they require full battery voltage. An alternative is to tap into the Windshiwld Wiper Motor's red wire (Circuit #63), which is also a full battery voltage switch circuit.

Another possible solution is to use a simple voltage relay, where the triggering circuit can be Circuit 4, 16, 640, or 63, where any of those circuits can cause the relay to direct voltage from the battery to the ignition system power terminal. I have a YouTube video that shows how to provide switched battery voltage with a 1973 Mustang, which can be adapted for use with an ignition system or other items (headlight relay for running lights, etc.). That YouTube video is at:

https://youtu.be/kCfZygdUUds

I have some schematics you may find useful when looking for the Circuit 640 or 63 (or 16 or 4), but it is best to provide the schematic relevant to your specific model year Mustang, as there are a few differences over the years in some areas. If you would like the schematic for your model year Mustang just let me know which year you have in a reply to this thread.

Mark Jones,
I terminal on the starter relay is as simple as it gets unless you plan to have ignition module inside the car.
Boilermaster

boilermaster said:

Mark Jones,
I terminal on the starter relay is as simple as it gets unless you plan to have ignition module inside the car.
Boilermaster

Click to expand...

The "I" terminal for the Starter Relay provides the resistor voltage for the ignition coil, except when the engine is being cranked over, at which point it sends "full battery voltage" into Circuit 16 which impact the ignition coil's positive primary terminal. But, the "full battery voltage" is not 12 volts, as during cranking the battery voltage drops to about 10 volts (should be no lower than 9.6 volts). Thus, it is not an ideal voltage source for a full 12 volt electronic ignotion system. BUT!!! Circuit 16 (Starter Relay "I" terminal) is a good source of power to trigger a voltage relay, which relay can be wired to send relay managed battery power to an electronic ignition system requiring 12 volts of switched power.

The voltage at the battery will drop while the engine is cranking, a couple of volts is normal. So, if the battery is fully charged measuring the battery across the positive and negative terminals should show 10 or 11 volts. Less than 9.6 volts indicates a battery problem or a starter load problem.

Assuming the cranking voltage of the battery is 10.5 volts, a normal coil resistance of 1.40 ohms and a resistor wire resistance of 1.35 ohms leaves only 5.4 volts to supply voltage to the ignition coil. So even the reduced voltage at the "I" terminal will still supply substantially more voltage (approximately double) to the ignition coil during starting (cranking) when using the resistor bypass circuit that runs from the "I" terminal directly to the ignition coil.

The resistance of the coil and resistor wire only comes into effect when the points are closed; with them open, there's no current draw and the system will see full battery voltage.