1979 F-150 w/ AC and gauges

Trying to get fire to my recently-dropped-in 400. It is running an MSD 8350 ready-to-run distributor and Blaster SS coil. Coil-to-dizzy wiring is taken care of by provided MSD wiring harness. Too easy. Now I just have to get a keyed-12V source to the coil. To keep it simple, I first thought of using the old coil hot wire. In my wiring diagram, I think it's red/green. I broke out the multimeter to verify it was a keyed 12V source, cause the MSD system needs 12 volts, nothing less.

The battery is putting out 12.83 volts.

I installed the battery, cab to engine ground, neg battery cable to block. Don't know if it matters, but the starter is still uninstalled (trying to resolve some header issues in parallel...grrr).

Anyway, with the key out/ignition switch OFF, red meter lead on the red/green coil wire, black meter lead grounded, voltage is 0. As expected.

Turn the key to ON, repeat the voltage measurement at the red/green wire and come up with 12.4-12.5 volts. Curious...I reckoned it would have been less than that what with the pink-ish resistance wire/ballast resistor thingy.

Relayed these numbers to MSD's tech folks, and they said I'm good to go. Just plug that wire to my coil, connect the wiring harness, and FIRE.

QUESTIONS:

1. Is the hot coil wire indeed the red/green wire?
2. If so, shouldn't the voltage have been less than 12 volts because of the pink resistance wire thing aft of the ignition switch ON?
3. Does any of this matter, since my multimeter seems to have verified that this red/green wire is a keyed 12V source?
4. I've also got to find a home for my Edel 1806 electric choke hot wire. Would y'all connect it to this red/green wire in addition to the new coil?

Thanks!
Justin

 

Well I can only help with ?# 4 right off Edelbrock web site:

Q: How do I hook up my electric choke (voltage)?

A: The Electric choke on our Performer Series and AVS carburetors needs to be hooked to a “keyed” (while the key is in the on position) 12-volt power source, with a good ground. Verify there is no voltage when the key is in the off position. Do not connect the positive wire to the Ignition Coil, ballast resistor or Alternator.


Hope this helps.

 

The voltage drop across the resistor wire ONLY happens when there is current flow through the circuit. So it would seem to me that the MSD ignition module does not conduct current without the dissy going round and round. My GM HEI 4 pin module also does not have current flow without the engine turning. Check the coil positive again at idle to see if there is a voltage drop.
Jim

 

Didn't I complete the circuit with my multimeter, which acts as a wicked-large resistor, when I took the measurement? When I checked the resistance from the battery to that coil positive wire terminal, I was getting around 1.3 ohms. That means it was flowing 12.83/1.3 = 9.87 amps without the engine turning, right?

So you're saying that when the engine is running, that coil positive wire won't have the 12.5 V that I'm seeing now? Hmm...

Right now, I'm seeing (engine off, key ON) a ~ 0.3 V drop from the battery to the coil positive. That seems a bit large to be just the resistance from the wires. Ballpark, it'd take around 275' of 16 gauge wire to equal the 1.3 ohms I saw.

What causes the voltage to be different at the coil positive when the engine is turning over? What should I expect it to be?

I don't want to ruin this ignition system because the coil "input" voltage would be lower when the engine turns. And I can't test your idea unless the engine is turning...

 

A Digital Volt Meter (DVM) is designed to sample the voltage source without changing it… so it won’t load the circuit.

I am not sure if the 1.3 Ohm measurement you got is correct, but it is close to what the resistor wire would be.

The .3 volt drop might just be the main battery dropping when you turn on the key and thus put some load on the battery.

The coil voltage will be lower at the coil positive at idle, only if there is a resistor wire in the circuit before the coil.
To test this… with the KEY ON and the DVM connected to the coil positive note the voltage…. Then momentarily ground the coil negative to see if the voltage reading drops.
Grounding the coil NEG will create the current flow needed to see the voltage drop across the resistor wire.

You can still use the original coil positive wire to power a relay to get full voltage to your new ignition system (coil) if needed.

Low voltage wont ruin the new system… but it wont work as it should… meaning a lower coil output as the coil is just a step up transformer… higher input means higher output to the plugs.
Jim

 

Thanks for the help! Think the proverbial "lightbulb" is coming on... Sorry...that was awful. Anyway...

More data...

I hooked the Blaster SS coil into the coil positive red/green wire. I attached a ground wire to the coil negative terminal, but did not ground the coil, yet.

Again, battery voltage around 12.8. Turn key to ON, red DMM lead on coil positive terminal, black lead grounded, read 12.45 V. So it's still an open circuit, but there's 12.45 volts that could be making it to the coil inlet.

Without moving the leads, I then grounded the coil with the aforementioned negative coil terminal wire. This completed the coil primary circuit, and the voltage drop across the coil was ~ 4 volts!

So does that mean that when everything is hooked up and the engine is turning, that of the ~12 volts available in the primary coil circuit, the resistor wire is eating up about 8 of 'em?

 

With a stock setup, the resistance of the primary coil winding and the resistance of the ballast are pretty close - so the voltage divider between the two is pretty even. That puts the running voltage of the coil around 7 volts or so. This isn't exact, since the coil is constantly grounded and ungrounded by the ignition module - so the actual RMS value shifts some.

Just a quick primer: voltage only drops across resistance when current is actually passing through the resistance. If you connect the RED/GREEN wire to the positive terminal of the coil, leave the negative terminal of the coil ungrounded, and read the voltage at the positive terminal with the key on, it's deceiving to say that 12 volts is applied to the coil, because that's an open-circuit voltage. Once the coil is grounded and the circuit is completed, current flows through the ballast resistance, and the voltage available at the coil drops. And, this is what you're seeing.

You're saying that with the coil grounded, the voltage at the coil drops from 12 volts to about 4 volts (too low for the application). That means that the primary winding resistance of your coil is less than that of the stock coil, which is not recommended for an otherwise stock setup. The voltage drops based on the ratios of the two resistances. If the primary winding resistance is equal to that of the ballast resistance, then exactly half of the 12 volts from the key will drop across the coil primary winding, and half drops across the ballast. If the primary winding resistance is lowered (as is the case with many aftermarket coils), then less drops across the coil, and more drops across the ballast (which is what you are seeing).

In your case, you're upgrading the entire ignition system, so what you need to do is consult MSD's documentation for how to connect everything. If your documentation is saying to connect 12 volts to the coil, then you can NOT use the original coil wire, as it is downstream of the factory ballast. It will read 12 volts open-circuit since there is no load, but the running voltage will be less as voltage drops across the ballast. You must find an actual 12-volt source in this case. Such a source will stay 12 volts even with the coil connected as a load, because there is no resistance in the wire to create a voltage drop (Ohm's Law). The resistance of the factory ballast, however, is on the order of 1.4 ohm. That's not much in broad terms, but considering that the primary winding resistance of many coils is on the order of 1 ohm as well, the relationship starts to matter.

 

Y'all rock. Thanks for squaring me away!

Now, I've gotta figure out what do next. I don't really want to try to rewire the back of the ignition switch...sounds gnarly.

Could I use the old coil positive wire to power a relay that switches "on" a 12-ga wire straight from the battery? If so, what kind of relay would y'all recommend?

 

Here is a good voltage divider calculator web site.

Voltage Divider Calculator

I used 12.45 volts
R1 = 1.4 ohm (resistor wire)
R2 = .7 ohm (coil primary)
Output voltage = 4.15V (Voltage drop across coil)

The way I see it your only choice is to use a relay as you new IGN system will be drawing more the twice the current as the original one did and you really don’t want the extra current going through the stock wiring or IGN switch. The relay will be energized by the original coil positive wire down stream of the resistor wire. I would use the power output stud on the ALT as a HOT power source for the “BOSCH” style relay (pin 30) commonly sold at the auto parts store.

Dorman/Conduct-Tite 4 pin 30 Amps 12 Volts universal relay (84601) | Relay and Resistors | AutoZone.com

Here is a good link with a lot of info on how relays work.
Relays

Having the resistor wire in the circuit with the new relay won’t be an issue as the relay coil resistance will be around 75 ohms. If you run the numbers in the voltage divider calculator you will see what I mean.
Jim

 

Great advice; I was going to say the same thing.

 

Thanks so much...it all makes sense now.

I'll check the actual resistance tomorrow...but MSD's site says the SS Coil primary resistance is 0.4 ohms. Throw 12.5 V across that sucker and you get a current draw of 31.3 A. 12-gauge wire seems to be rated up to 40 A, so that should work.

Regarding the 4-pin SPST Bosch relay that you linked...they got one rated for a higher amperage?

I'd hook up the old coil positive wire to pin 86, and ground pin 85.

I'd then run 12-ga wire from the alternator to pin 30, and 12-ga wire from pin 87 to the Blaster coil positive terminal.

Sound about right?

 

http://www.waytekwire.com/bosch-automotive-relays.htm



Yes there are higher amp relays... but you are really looking at a ~ 50% duty cycle square wave which will reduce the amp requirements. Think of and old point IGN system with the points opening and closing... point is... 30 amp relay will work fine.

Your wiring sounds good to me.

The reason I like the ALT stud as the power source is that it has a fuseable link in line to help protect the IGN system.

On a side note... I have always found fmc400 posts to be right on the money... always solid advice.
Jim

 

Can't thank y'all enough!

 

Got a 12V/40A (Autozone didn't have a 30A on hand) relay and some 12 gauge wire this AM. Ran the wiring as described above, except I didn't use the alternator output since I don't have the front-end of the engine buttoned up yet. Ran it to the battery positive terminal instead. Once the alternator is mounted and turning again, I will switch it over.

Battery voltage: 12.70 V
Closed-circuit relay voltage: 12.04 V
Closed-circuit coil voltage: 12.4 V

Frickin' outstanding. I love FTE!

Now if I can just get my header problem fixed...

 

Have you looked at PMGR starters. There is a slight wiring change but it is very easy. Not sure why the big block style cost so much more.
Jim

High Torque Mini Starter Installation. PMGR Starter. - FordMuscle


Ford Starter 351 - Starter

CVR Gear Reduction Mini Starter Ford 351M 400 429 460 | eBay

New Powermaster 2-Bolt 351M/400/429/460 Ford Starter | eBay

Pro Comp Ford 302 351 400 High Torque Starter Motor New | eBay