Howdy all,

My 1977 F-100 cut off on me while going down the road the other day and I've just had a chance to look into it today. It's a 351M with a C6 and stock Duraspark ignition. The ignition module has the blue plastic insert and was apparently replaced once before by the previous owner - it's a Standard Ignition product.

I've verified that I have 12V to the + terminal on the coil when the ignition is on, but am getting no spark at all while cranking. I've checked the resistance in the coil windings and it tests good, so now I'm on to the pick-up in the distributor and the ignition module itself. Unfortunately all I have is a Hayne's manual and it's useless at this point. Can anyone tell me what wires to probe and what I should be looking for on the pick-up and ignition module?

My thanks in advance for any and all responses.

Bruce in South Carolina
1977 F-100 Ranger XLT
1984 Volvo 245 Turbo - it's a Ford now!

 

The best way IMHO is to get another ignition module and try it, even if the module doesnt fix the problem it sure is nice to have a spare when the other one dies on the road, cheap insurance.
Are you not getting any spark at the coil wire itself or just at the plug wires? Could it be as simple as the coil button in the centre of the distributor cap? Or even the coil wire itself?
I thought there was only 12 volts to the coil + when the key was at start and it dropped to 8 or so when the key was in run due to the resistor wire?
I am reading the diagram and it looks like if you disconnect the wire on the - of the coil and connect a meter you should have a pulsating ground when the engine is cranking. You probably wont be able to see this on a digital meter though. Of course this is my interpreation Not that it proves anything if you dont get the ground as it still could be the module or the pickup or the wires.
To check the pickup in appears you should be able to measure from orange/yellow wire to the pink/blue wire and get some kind of very low resistance, as you are basically measuring a coil of wire which is the pickup windings. Also you should be able to measure from the distributor base to the black/green wire and get a ground reading. All these measuremnets are from the connector near the ignition module.
The only way I know of checking the ignition module is by trying a new one.
I hope this helps more than it confuses.

Sparky

 

Just another thought. Is the distributor rotor button turning when you crank the engine? It has been known to happen that the pin on the distributor gear shears. Then you have no rotation of the distributor. Thus no spark to the plugs. Quick check to do and not that hard of a fix if it is the problem.

Sparky

 

Sparky,

Thanks very much for your help!! I agree 100% that the easiest way to test the module is by replacing it with a known good unit, but I don't have another on hand and am reluctant to just buy one until I'm sure it's at fault. That plus the fact I'm trying to rationalize replacing it with and MSD if it is indeed bad...

I'm testing for spark at the coil wire (pulled off the cap) so that rules out the distributor, rotor, wires, etc. as the cause of the problem. I've tested the coil wire too, and while it could stand to be replaced, the resistance is not sufficient to cause this problem. I am indeed getting 12V at the coil + terminal - I think the 8V thing you are thinking of is when you have a ballast resistor with points. Good thought on the sheared pin, but the rotor is indeed rotating.

I do have an analog meter as well as my digital Fluke, so I'll check out the wires you indicated. I truly appreciate you taking the time to look at a diagram and write them down for me! Looks like a factory manual with wiring diagrams needs to go on my "must buy" list!

Thanks again!

Bruce in South Carolina
1977 F-100 Ranger XLT
1984 Volvo 245 Turbo - it's a Ford now!

 

Most autoparts places can test the ignition module for you. I know NAPA stores have this ability.

Mike

 

Bruce, Sparky is right, in that you should only have 7-8 volts at the coil + with the key on. (12v when starting) The DuraSpark system is resistored to cut down the coil voltage. If you have 12v with the key on, then someone has modified the wiring going from the switch to the coil. Over time, a constant 12v will overheat the coil, causing it to burn open. It might be worth another look at the (ignition) coil.

In checking the pick-up coil, normal resistance should be around 500 and 1500 ohms, measured between the two leads. The resistance between either of the two leads and ground should be infinite. (These values come from a DurasSpark equipped Jeep, so take them as a guide only)

My guess is it's a bad ignition module. Good luck.


________________________________________________
78 Bronco Ranger XLT, 351M, 4 sp, Trailer Special

 

>...If you have 12v with the key on, then someone has modified
>the wiring going from the switch to the coil.

Interesting - I didn't know that - thought it was a points only thing. Where does the voltage step-down ouccur? Is it in the ignition module, or is there an inline resistor somewhere?

>In checking the pick-up coil, normal resistance should be
>around 500 and 1500 ohms...

Excellent - I'll check that out!

>My guess is it's a bad ignition module. Good luck.

Mine too, so I went ahead and ordered an MSD 6A...any excuse for upgrades ya' know...

Thanks very much for your help!

Bruce in South Carolina
1977 F-100 Ranger XLT
1984 Volvo 245 Turbo - it's a Ford now!

 

I have 7 pages of how to test Duraspark 2 ignitions, if you want them i'll scan them for you,and send them, complete with diagrams.

 

Any chance I can get a copy of those checks? Never can have enough info on these "Durasparkless" systems

Sparky

 

>I have 7 pages of how to test Duraspark 2 ignitions, if you
>want them i'll scan them for you,and send them, complete
>with diagrams.

That would be great! Thank you very much for offering them!

I've sent you an email off forum.

Bruce in South Carolina
1977 F-100 Ranger XLT
1984 Volvo 245 Turbo - it's a Ford now!

 

Bruce, the (pink) wire going from the switch to the firewall connector is the resistor wire. This circuit powers the coil only. It should read 7-8v with the key on, engine off. The stock Duraspark coil runs on this resistored voltage. If yours has a later model HEI coil, the square type, then it would run on 12v. The module takes 12v, only the coil runs with the lesser voltage.

The MSD is a good upgrade. MSD makes a wiring harness adapter to connect the MSD stuff to the DS distributor. Using this adapter makes installation a snap, everything plugs in, and no wire splicing is required.

 

 

No offense but my favorite method of checking a duraspark is to toss it in the lake. If it floats you can try it again.

They tend to have intermittant problems that don't show up on a test, just when you are far from home.

I'd upgrade to something more modern like Accel. I don't have good luck with Mallory.

Bear Tracks

 

i know this is an old thread but is it possible to geta copy of how to test the modules. they don't test them anymore.

 

Buy the cheapest one you can find and plug it in.
If it starts then you fixed it.
If it still doesn't taet when you do get it fixed you'll have a cheap spare.

A long time ago NAPA had a tester for them.