Any Ideas what this electrical item is & might be for?

[Moderator73]

This week I discovered this non-factory electrical component mounted under the ignition coil of my 1973 351C Q-code. I've had this car for 20+ years and never noticed it before. I'm totally stumped what it's supposed to do and even if it's needed. For background, I've removed my entire HVAC system and in the process of replacing it with a new Classic Auto Air system. So my project is doing a lot of additional "clean up" & "upgrades" in the engine bay before I get to installing the new system. I'm discovering all kinds of challenges with the vacuum lines being misrouted or plugged off, and emissions system components that don't seem to match any of the emissions calibrations in our manuals. But that's another issue.

For this Mystery electrical component, there are no identifying markings whatsoever on the black box. It has four (4) wires coming out of it, and the mounting plate looks to be "copper-like" assuming it serves as a grounding point.
1-Red wire was connected to the battery terminal side of the starter solenoid
2-Black wire with a female spade connector on the end but not attached to anything. It was taped over to provide insulation I'm assuming
3-Black wire with a male spade connector, also not connected to anything and taped over
4-White wire that is spliced into the Throttle Position/Anti-Dieseling solenoid wire (#640, Red w/Yellow Hash) near connector 401.

I've tried to lay this thing out on a yellow towel to help show an overview. The reason it's not out of the car is because of the white wire splice and I'm hesitant to cut it before I understand it.
The other pictures show different views of this mystery black "box", where it was mounted and the splice into the emissions system wire.

If any of you have ever seen something like this, I'd sure appreciate your ideas.

One other discovery that might be related to this, I also just found, is a degraded #37 Yellow wire with the 14GA fusible link that is connected to the battery side post on the starter solenoid. For the 20 years I've had this car, this cable has been taped over with 'yellow' tape. I took the tape off, and found severely burned insulation shown in the picture. At some past time this was obviously overloaded/shorted? I'm getting a new fusible link wire to repair this, but wondering if there is some relationship to this mystery component.

Thanks for taking a look at my project, and sharing any ideas you might have...
Bob

 

[cv_72mustang]

I'm guessing some sort of relay. Maybe normally 'off' while power is applied and 'on' when power is removed. Just a guess.
Workaround for Anti-Dieseling.

 

[Moderator73]

Thanks for your thoughts on this. The relay idea gives me somewhere to search. I really didn’t even know where to start.
Bob

 

[rhigg1959]

My 73 Q code doesn't have that.

 

[Moderator73]

My 73 Q code doesn't have that.

Thanks for that. I was fairly certain it wasn't a factory item, but your input makes me certain. I've been chasing the suggested "relay" idea as some sort of universal type relay. Just can't get my head around what it would be controlling with only 2 of the 4 wires connected? Thanks again, Bob

 

[tony-muscle]

It seems to be tapped to the carburetor idle solenoid, which could be activated when the A/C compressor kicks in so I wonder if they added a gimmick to be activated when the A/C compressor turns on. The wires look to thin to be activating the compressor clutch. If you don't have A/C maybe they were using the A/C controls to activate something. Maybe NOS??

 

[Hemikiller]

Definitely not stock equipment. Holley makes a thermal choke delay that looks similar, but it's only a single wire connection. Guessing it's some sort of thermal control, maybe it was a low or high temperature defeat setup for the EGR or spark control system. Where did the disconnected the wires reach to?

As far as the idle solenoid, that's an emissions device, it not related to the AC system. It was necessary due to the run on (dieseling) problems these cars had with the emissions tune they came with out of the factory. Retarded static timing required larger throttle openings which caused dieseling. The solenoid's purpose was to close the throttle when the ignition was turned off, restricting air flow.

 

[tony-muscle]

Definitely not stock equipment. Holley makes a thermal choke delay that looks similar, but it's only a single wire connection. Guessing it's some sort of thermal control, maybe it was a low or high temperature defeat setup for the EGR or spark control system. Where did the disconnected the wires reach to?

As far as the idle solenoid, that's an emissions device, it not related to the AC system. It was necessary due to the run on (dieseling) problems these cars had with the emissions tune they came with out of the factory. Retarded static timing required larger throttle openings which caused dieseling. The solenoid's purpose was to close the throttle when the ignition was turned off, restricting air flow.

When I got my car the solenoid was activated when the A/C compressor kicked in to increase idle. So it was probably someone's mod before me.

 

[Moderator73]

Definitely not stock equipment. Holley makes a thermal choke delay that looks similar, but it's only a single wire connection. Guessing it's some sort of thermal control, maybe it was a low or high temperature defeat setup for the EGR or spark control system. Where did the disconnected the wires reach to?

As far as the idle solenoid, that's an emissions device, it not related to the AC system. It was necessary due to the run on (dieseling) problems these cars had with the emissions tune they came with out of the factory. Retarded static timing required larger throttle openings which caused dieseling. The solenoid's purpose was to close the throttle when the ignition was turned off, restricting air flow.

Wow...you are a gold mine of information. I think you've solved this puzzle for me. Your knowledge that Holley made a thermal choke delay that looks similar put me on a path to look those up. Here's a link for others if they are interested in what you were talking about:

https://www.speedwaymotors.com/Holley-45-267-Choke-Delay-Kit,143985.html

I've looked for other versions of a thermal delay that might have 4-wires but come up empty today. You asked where he two disconnected wires reach too, and it would have been one of a couple places: to the carburettor choke or to where I'm guessing the Spark Delay valve Bypass Solenoid would be. I don't have this solenoid on my 351-4v. Not sure if it's removed or was never intended to be there on this car because the electrical dwgs show three different variations of this ckt# 369 Brown-Orange Hash wire at connector C-1212. The 3 listed engine variations show 1: 250 CID w/Auto, 2: 250 CID engines, & 3: 351-2v engines, none of which are a 351-4v.

So, thinking this electric component probably isn't an emissions related item, I'm pretty sure it was choke related. That got me looking at Electronic Choke conversion kits and I see that almost all of them have two electric spade connectors on the choke plate, which these two disconnected wires would/could mate up perfectly. So I think you've really helped me. When I first got this car 20 years ago, it ran terribly especially cold. It had been in my family for the previous 20 years and I know the carburettor was something that was getting "rebuilt" every year it seemed like. So the first thing I did was get a new/refurbished one from Pony Carbs (back in the day when our guy in Las Cruces, NM was still with us) and it's run great ever since. I had Ford install the new carb at the time & now wondering if the old carb had a modified electric/electronic choke plate. With the new one being factory correct, it's connected to the factory harness electric choke connection.

Thank you so much for putting me on this path of understanding what this thing is/was, and most likely what it's function was. I'm very happy to eliminate it now. This forum is such a wonderful resource thanks to you and our other members.

Bob

 

[Moderator73]

Just to wrap up this thread, I wanted to share what I learned from another member on our forum that most likely explains my 14ga Fusible Link wire overheating/loading. From my pictures of the burnt wire, he saw that my Starter Relay and Junction Block were wired incorrectly. He shared this drawing that apparently is posted somewhere in our forum threads but I've never seen. It's a fantastic drawing that is so much more understandable compared to our Wiring Diagrams.

I was helped so much by this drawing and wanted to post it again hoping it might help someone else.

It's titled "1973 Mustang Overcurrent Protection with Tach and Amp Gauge Harness". The detail of the Alternator, Voltage Regulator, Starter Solenoid & Junction Block wiring connections leave nothing to the imagination.

Thank you all for your help & contributions!
Bob

 

[Bill Kanode]

This week I discovered this non-factory electrical component mounted under the ignition coil of my 1973 351C Q-code. I've had this car for 20+ years and never noticed it before. I'm totally stumped what it's supposed to do and even if it's needed. For background, I've removed my entire HVAC system and in the process of replacing it with a new Classic Auto Air system. So my project is doing a lot of additional "clean up" & "upgrades" in the engine bay before I get to installing the new system. I'm discovering all kinds of challenges with the vacuum lines being misrouted or plugged off, and emissions system components that don't seem to match any of the emissions calibrations in our manuals. But that's another issue.

For this Mystery electrical component, there are no identifying markings whatsoever on the black box. It has four (4) wires coming out of it, and the mounting plate looks to be "copper-like" assuming it serves as a grounding point.
1-Red wire was connected to the battery terminal side of the starter solenoid
2-Black wire with a female spade connector on the end but not attached to anything. It was taped over to provide insulation I'm assuming
3-Black wire with a male spade connector, also not connected to anything and taped over
4-White wire that is spliced into the Throttle Position/Anti-Dieseling solenoid wire (#640, Red w/Yellow Hash) near connector 401.

I've tried to lay this thing out on a yellow towel to help show an overview. The reason it's not out of the car is because of the white wire splice and I'm hesitant to cut it before I understand it.
The other pictures show different views of this mystery black "box", where it was mounted and the splice into the emissions system wire.

If any of you have ever seen something like this, I'd sure appreciate your ideas.

One other discovery that might be related to this, I also just found, is a degraded #37 Yellow wire with the 14GA fusible link that is connected to the battery side post on the starter solenoid. For the 20 years I've had this car, this cable has been taped over with 'yellow' tape. I took the tape off, and found severely burned insulation shown in the picture. At some past time this was obviously overloaded/shorted? I'm getting a new fusible link wire to repair this, but wondering if there is some relationship to this mystery component.

Thanks for taking a look at my project, and sharing any ideas you might have...
Bob

40 years ago I had an item like this on my 1970 Corvette to provide power to an electric choke. The copper lug fit under a bolt which the unit provides voltage based on the engine temperature to open the choke gradually as the temperature increases.

 

[Don C]

Just to wrap up this thread, I wanted to share what I learned from another member on our forum that most likely explains my 14ga Fusible Link wire overheating/loading. From my pictures of the burnt wire, he saw that my Starter Relay and Junction Block were wired incorrectly. He shared this drawing that apparently is posted somewhere in our forum threads but I've never seen. It's a fantastic drawing that is so much more understandable compared to our Wiring Diagrams.

I was helped so much by this drawing and wanted to post it again hoping it might help someone else.

It's titled "1973 Mustang Overcurrent Protection with Tach and Amp Gauge Harness". The detail of the Alternator, Voltage Regulator, Starter Solenoid & Junction Block wiring connections leave nothing to the imagination.

Thank you all for your help & contributions!
Bob

Very nice, thanks for sharing

 

[Steve McMahan]

You can always count on @71fast electrical diagrams. I wouldn’t be driving my 73 coupe with 87 EFI drivetrain is he hadn’t help me. Just saying.