All

First some background information. I recently purchased a 71 bronco with a 302. The previous owner was a co worker that bought it two+ years ago with dreams of fixing it up. My co worker has NO mechanial skills - doesn't even own a socket set. Well, it started leading coolant so he parked it. After a month or so he decided to start it up and get it out of the company parking lot - well she had a dead battery. He went and got a new battery and she now wouldnt start....

Two years later(never got it running and he never really tried) and it is mine for a seriously reduced price. Towed it back to my place, recharded the battery and start doing some debug. It has a MSD Coil and a CraneCams Fireball ignition system. I pulled the coil wire, grounded it and had the wife crank it over...

Well only one very little very yellow spark occured just as it my wife started to turn it over then nothing...nothing. Hmmm, coils are cheap so I go and buy the coil mentioned above. Same thing...Then I pull out the multi meter and check the +/- on the coil - 6.5V when cranking. I think this is rather odd so I pull the wire from the ignition switch. Check that and it reads 9+V cranking. Connect the wire back up to the coil with the ignition system and bam - 6.5 volts.

The dude at the auto parts store says he thinks it is the resistor - fine (I dont believe him), grab a new one and start to try to find the old one. Look under the dash, in the engine compartment and cant find the old one...? So then I decide to stop and do some research.

So here I am. Any hints and/or tricks to diagnosis this problem would be appreciated. I don't see how I can have a 3+V drop when the coil/ignition system is connected. That seems strange, but you never know, as my user name implies....

thanks

badmechanic

 

What's the voltage at the relay?

 

one solution to chec kand see if it is a bad ignition switch is to wire the coil directly from the battery hot terminal to the hot side of the coil. it may start like this.

 

There is a resistor in the circuit, and it will drop it about 3 volts. But, you should have full voltage to the coil when cranking. (Your 9.5 volts is probably all you have in the system when the starter is trying to turn over). To get the full voltage to the coil the factory did it two ways. One way is from the starter solenoid on the fender. If it has two small terminals on top, one comes from the ignition key (start) and the other feeds 12 volt to the coil only when cranking. The other way is they had a seperate switch contact in the ignition switch, that bypassed the resistor in the harness.

Also, have you verified you have fuel? Look down the carb throat and push the throttle back. You should see a strong stream of gas when the throttle is pushed back.

 

Thanks for the tip....I just got back from the garage.

Found the wiring diagram on bronco.com and checked the correct side of the Starting Motor Relay. It read 10 volts.

Checked the coil again (with my electronic ignition hooked up) - 5.85 volts. This is measured between the primary + and primary - terminal. Between the primary + terminal and ground it is 6.8 volts.

Fuel? Yes I was giving her a couple squirts of starting fluid as well.

I didn't have a chance to check the bad switch suggestion above but will try that tomorrow. Remember that when I pull the wire from the coil that comes from the relay (presumed as it is in the harness and under the dash)and not connected to the coil + ignition system it reads 9+ volts.

My building hypothesis though completely unfounded and from a "badmechanic" is that it is somehow related to the ignition system - CraneCams Fire Ball?

As always thanks for the help in this one!!!!!

badmechanic

 

since you obviously have a DC multimeter, you can easily test your coil by testing the resistance through the primanry wires. It should read around .7 Ohms, with .9-1.0 being the max. Any less and the coil will be losing too much secondary voltage. Don't know anything about Crane Fireball ignitions, I know that you can test the box by manually triggering the distributor's pickups, but I can't remember how to do it with points. If you can't find anything on it, go to MSDignition.com and look at their troubleshooting diagrams, all the Crane unit is is a more cheaply made MSD 6A, made in Japan, and all the wires are the same. TK

'77 F100, 302 (the aftermarket Prodigy), C4
Cadet Second Lieutenant John F. Daly III
South Carolina Corps of Cadets, The Citadel
The TorqueKing

 

You only have 10V at the starter relay?
What kind of shape are the battery cables in?
Sounds like the battery cables are toast.

Roger Lane

 

Checked the resistance of the coil this morning on the way to work - 1.0 ohms.....So I guess it is in the "max" spec.

Battery cables? They "look" newer, but I will take them off and clean up the contacts and measure the resistance this evening. I agree that this may be another issue, but do you think it is related to the root cause problem? Maybe it is the relay itself?

I would also like to thank everyone regarding this topic.

badmechanic

 

Fixed it -

Turned out to be the Crane Cams box.

Got a msd 6a and an ignitor kit. fired right up!

- bad mechanic