Sparky’s got me. It’s the coil. Why?

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Vinnie

Project manager "Project AmsterFoose"
7173 Mustang Supporter Member
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Location
Amsterdam, The Netherlands
My Car
1973 Mustang Grande 351C 2V, built on the very last production day (July 6, 1973) for Grande's.
Hey guys,

My fuse nr 4 kept blowing today after I had a nice run. I came back, turned it off, gave the car a clean and when I wanted to drive it inside, ignition was gone. Upon closer inspection fuse nr 4 was blown again.

I replaced it twice but it blows as soon as the key hits ON.

In the past I replaced the turning signal wiring and I have added 1 wire to operate the solenoid on my pmgr starter. Both work fine.

My Haynes book says fuse 4 is for: indicator lights, oil pressure, engine water temp, dual brake system, seat belt warning lamp and buzzer, emision control solenoids, throttle solenoid positioner, electronic spark control.

I have no idea where to start investigating this issue. I kind of know about all those things the fuse is for but I have no idea where thewires for them are.

If I want to tackle this issue, where should I start? What would be the things to look into first?

Any pointers would be appreciated! In the meantime I’m going to study some schematics I think...

Thanks for any helpful ideas!

Vincent.

 
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Disconnect your dash cluster and start the car (assuming you have no tachometer). The oil pressure, brake proportioning valve, and temperature bulbs in the dash may be shorting to ground. If the fuse still blows, disconnect the blue square plug for your seat belt indicator lamps in the engine compartment, near the NSS/BU switch. Those wires may be pinched and shorted to ground. The other three purposes/wires are rarely used today.

 
I would start with the wires in the engine compartment, easier to get to.

Did you repurpose any of the wires, like using the throttle position solenoid wire for electric choke?

 
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There was an electric choke in use when I bought the car (still is), I I don’t even know what that solenoid is or where it is located, let alone which wire was connected...

Here’s Haynes on nr 4 btw:

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Disconnect your dash cluster and start the car (assuming you have no tachometer). The oil pressure, brake proportioning valve, and temperature bulbs in the dash may be shorting to ground. If the fuse still blows, disconnect the blue square plug for your seat belt indicator lamps in the engine compartment, near the NSS/BU switch. Those wires may be pinched and shorted to ground. The other three purposes/wires are rarely used today.

I got brand new cluster printboard and I replaced a bunch of bulbs with led versions a few years ago.

What/where is the nss/bu switch?

 
I would start with the wires in the engine compartment, easier to get to.

Did you repurpose any of the wires, like using the throttle position solenoid wire for electric choke?
Vinnie,

what Don is saying makes pretty good sense.

I would hook up a circuit breaker up in place of that fuse and start targeting the loads of that circuit.

you have more solenoids on that line than meets the eye.

get the ones in the engine compartment first.

then the warning buzzers.

after the major players are eliminated it comes down isolating the circuit (opening up splices)

no one wants to do that on a car that is almost 50 years old.

Boilermaster

 
I have ordered a circuit breaker to do further testing.

Where abouts should I find that blue connector?

Also, could this somehow be temperature related? All the times the fuse blew was when the engine was warmed up...

 
If you have a 1971, you may not have a blue connector, as it was introduced mid-way through production. It is located in the engine compartment, near the neutral safety switch/backup plug, the engine gauge feed (oil, coil, water) plug, PRNDL lamp plug near the firewall on the back side of the engine.

 
Before start doing any dashboard sport or get headaches reading diagrams, i'd put back the fuse and connect to a voltmeter at this #4 start the car, and observe if voltage is constant or fluctuates with rpms.

 
Before start doing any dashboard sport or get headaches reading diagrams, i'd put back the fuse and connect to a voltmeter at this #4 start the car, and observe if voltage is constant or fluctuates with rpms.
I put back the fuse 3 times last saturday and it blew as soon as the key hits the ON position. So that would show exactly 1 (not so helpful) fluctuation I'm afraid ;-)

When I get my automatic fuse (can be reset) I'll first see if it also blows with a cold engine (I hope it does coz it helps reproduction and as a software engineer I looooove reproductable problems haha). After this I'll start working down a list of things I can unplug, starting with that magical blue connector :)

This fuse is also for "engine water temperature". As I've only seen it blow with the engine warmed up I'm thinking the temp warning light/sensor might be (part of) the problem. However, I really don't want that to come on so quick coz that would indicate other heating issues (or a crappy sensor...)

To be continued...

 
Easy enough to determine if it's the temperature switch, whether or not you have the idiot light. Just disconnect the wire from the switch/sending unit. Both apply a ground when activated. If that's the problem follow Midlife's recommendation. In any case, it'll be a process of elimination.

 
Easy enough to determine if it's the temperature switch, whether or not you have the idiot light. Just disconnect the wire from the switch/sending unit. Both apply a ground when activated. If that's the problem follow Midlife's recommendation. In any case, it'll be a process of elimination.

If I understand the wiring diagram correctly, the same goes for the oil pressure and dual brake warning lights. Right?

Where is the dual brake switch located though?

 
Easy enough to determine if it's the temperature switch, whether or not you have the idiot light. Just disconnect the wire from the switch/sending unit. Both apply a ground when activated. If that's the problem follow Midlife's recommendation. In any case, it'll be a process of elimination.

If I understand the wiring diagram correctly, the same goes for the oil pressure and dual brake warning lights. Right?

Where is the dual brake switch located though?
On top of the brake proportioning valve, located on the driver's side fender close to the master cylinder.

 
Right. I put in the breaker and started pulling off the water temp, oil pressure and dual brake senders. All 3 had no effect. I turn the key in the ON position and 1 second later the breaker pops.

On to the blue plug. Is it almost 2” long with 2 wires coming out on each end? Looks VERY big to just connect 2 wires? Anyway, one side of the bottom part looks burned and melted and crispy. And it’s tied together with a tie-rib. I didn’t have time to investigate further. It’s hard to reach, does it come apart easily? Can the plug be replaced?

 
Is there an easy way to disconnect the cluster? Last time I worked on it I removed the steering wheel and took the cluster out...
I loosened 4 bolts, some were nuts, under the dash to the point that I could lower the column and maneuver the dash out, that is of course after removing the dash bezel and dash pad, and then using your best language to remove the plug and speedo cable. I also had the driver's seat out for another reason..............good luck!

 
Question about the blue connector:

2 wires from the connector's bottom part go to a bigger loom going towards the tranny and 2 wires from the connector's upper part go up to a loom going through the firewall. Are the latter wires the ones going to the warning light/buzzer?

The reason I'm asking is because it is the lower part of the connector that looks half melted so disconnecting it possibly leaves the "short" active? I'd have to actually cut the connector off then and replace it with what? Any kind of connector?

 
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Simply disconnect the blue connectors. If the short is in the section going down to the tranny, then you've temporarily solved your problem. If the short is in the section going back towards the main harness and firewall grommet, then that piece needs refurbishment and/or replacement.

 
If the wires in that connector are shorted to each other they will not blow the fuse, but will turn the seatbelt warning light and buzzer on continously. So if either the buzzer or light are shorted that will blow your fuse. Make sure the connector isn't touching anything.

Another possibility is the throttle solenoid wire. If your carburetor doesn't have one and was originally equipped with one the wire may be loose in your engine compartment. One of the joys of trying to figure out what a previous owner did that may be causing a problem. Systematic elimination until the problem circuit is found is the most practical way.

 
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