Sparky’s got me. It’s the coil. Why?

[midlife]

The fact that the poster indicated that some of the wire's insulation looks burned suggests a short to ground, which will blow the fuse.

 

[Vinnie]

I will try and disconnect the blue connector as well as tidy up some loose ends next time I’m in my garage.

Thanks so far!

 

[Don C]

While you're at it, I would try to figure out what burned the connector. That may lead you in the direction of the short.

 

[Vinnie]

Well, tonight I went through a process of elimination and it’s the ignition coil that’s given up. The symptoms match with everything I know about coils and it explains why it took a bit of time before it would break down completely.

The coil’s copper wire’s coating must’ve been damaged somewhere.

This Pertronix firethrower coil (chromed version) was newly installed 18 months ago and the car hasn’t been driven a lot. I have it mounted at the front of the pass. side valve cover so it does get pretty warm.

Question 1: Is this location problematic for coils?

Question 2: What is the original location of the coil with a 351C engine?

Thanks!

 

[73pony]

from your description that should be the stock location of the coil. Heat does kill coils.

 

[Don C]

You can look at the Ford pictures here, on the cover page and on page 19, in their 351C Brochure for the original position:

https://www.7173mustangs.com/thread-ford-351c-4-v-marketing-manual

I have seen them mount to the front of the heads, also. I repositioned mine to the front of the passenger side shock tower, to reduce heat and vibrations.

 

[Vinnie]

While you're at it, I would try to figure out what burned the connector. That may lead you in the direction of the short.

The only explanation I can think of is that the connector didn't hold and the bottom part hung against the exhaust. That also explains why there is a tie wrap around it.

 

[Vinnie]

Something else I noticed: From the engine feed (cable coming from the firewall on drivers' side) I found 2 wires having +12 volts on them with the key n the ON position. One of these wires was hooked up to my electric choke and the other one connected to the + on my ignition coil. Strangely enough there is (soon: was) also a wire running between the + on the choke and the coil thus connecting those 2 positive leads. My car used to have airconditioning, could one of the leads be from that?

I think I'll be removing the wire connecting the 2 positive leads.

Thoughts?

 

[Fabrice]

Glad you found your gremlin!

Not sure from your 2 replies to understand what went wrong exactly. Its the connector that was against the exhaust that shorted all or the coil that shorted?

One of your feed might be for the compressor I think. Got same situation (AC removed) and have 2 extra 12v wires feeds, I haven't removed them, just relocated/closed/secured and kept them in case of.

 

[Vinnie]

Glad you found your gremlin!

Not sure from your 2 replies to understand what went wrong exactly. Its the connector that was against the exhaust that shorted all or the coil that shorted?

One of your feed might be for the compressor I think. Got same situation (AC removed) and have 2 extra 12v wires feeds, I haven't removed them, just relocated/closed/secured and kept them in case of.

The short is caused inside the coil. The burned connector has nothing to with the short it seems. It was a possible cause but it's not anymore.

A new coil (same; pertronix flamethrower II, chrome) is hopefully coming in this week

 

[Fabrice]

Ah ok.

If you cut it open, post some picts...

 

[Vinnie]

Hey folks, sparkies in particular!

In the photo below the connector at the red arrow is +12V. The blue one is not in use. Both come from the very end of the engine harness together with the wire for the water temp sending unit:

That red connector was connected to the postive lead on the coil. Is that what it is for?

There is also a +12V coming out of the engine harness at the front of the drivers side valve cover. Anybody know what that was for? Currently that powers my electric choke (and WAS also connected to the positive lead on the coil.

A subtle diff between these two feeds is that the red connector is not using the fuse that keeps blowing and the other one is using it.

So which one should be hooked up to the coil? What are they both for?

Thanks!

Vincent.

 

[rvrtrash]

I'm not an expert, and I'm going off memory, but I believe the 3 wires of the engine feed harness are Coil, Water Temp and Oil Pressure. Those connectors are crimp connectors, not factory connectors. Also, the coil wire is a resistance wire in the under dash harness that cuts the voltage to the coil to around 8 volts when it's powered on in the "Run" position. Pertronix coils call for 12V all the time, and they recommend a separate power source or the installation of a relay on the factory circuit. It sounds like a previous owner was using the power for the air conditioning compressor to provide the full 12V to the coil, and also tapping off it for the electric choke.

Steve

 

[midlife]

Based upon your description, the red plug should go to the + side of the coil, as it is not fused. The blue plug, which is fused, appears to be a black/green hash wire, if true, is for the AC clutch. It's fuse is the top-right most fuse (ACC POWER), not the one just below it which is fused RUN-ONLY power. I see multiple lines spliced off of the coil line; where do they go?

For now, forget about the power to the carb choke and get the car running without blowing fuses.

 

[Vinnie]

I'm not an expert, and I'm going off memory, but I believe the 3 wires of the engine feed harness are Coil, Water Temp and Oil Pressure. Those connectors are crimp connectors, not factory connectors. Also, the coil wire is a resistance wire in the under dash harness that cuts the voltage to the coil to around 8 volts when it's powered on in the "Run" position. Pertronix coils call for 12V all the time, and they recommend a separate power source or the installation of a relay on the factory circuit. It sounds like a previous owner was using the power for the air conditioning compressor to provide the full 12V to the coil, and also tapping off it for the electric choke.

Steve

Tomorrow when I hook up my new coil I’ll check the voltage, good point!

My oil pressure wire hooks up with the harness close to the back of the engine, drivers side, and looks stock.

 

[Vinnie]

Based upon your description, the red plug should go to the + side of the coil, as it is not fused. The blue plug, which is fused, appears to be a black/green hash wire, if true, is for the AC clutch. It's fuse is the top-right most fuse (ACC POWER), not the one just below it which is fused RUN-ONLY power. I see multiple lines spliced off of the coil line; where do they go?

For now, forget about the power to the carb choke and get the car running without blowing fuses.

OK I’ll hook red up to the coil. The blue plug is not connected to anything. I don’t know what it’s for. Those multiple lines are the blue plug and water temp.

 

[Vinnie]

Right. The red plug gives +12V so the people who put in the previous pertronix prolly removed the resistance wire.

I put in the new coil and re-created the previous situation and no fuses were blown so that took care of that.

Still, I dropped the extra +12V wire from the choke to the coil. I can’t understand why that was there. Everything worked fine that way.

Thanks all who contributed to this thread with helpful ideas/knowledge

Cheers!

PS: I measured the resistance of the coils and the primaries and secundaries of the old and new coil were the same. But one is broken. Strange?

 

[Don C]

The coil resistor wire will only show reduced voltage if there is a load on it. Older analog voltmeters will load it enough to show some reduction, digital meters won't load it at all.

People that don't understand how loads on a resistor reduce voltages think it is OK to use the coil wire to power the electric choke. The additional load reduces the voltage even further, usually leading to performance issues with the coil, as well as causing the choke to take too long to open.

 

[Don C]

Did you check the primary (+) side to the coil case?

 

[Vinnie]

Did you check the primary (+) side to the coil case?

No I measured not connected. I’ll measure the + of the coil next time I’m there sometime this week. Hopefully it will still give me 12V...