Junction block fender apron - purpose? 1973 Mustang special wiring...

7173Mustangs.com

Help Support 7173Mustangs.com:

Joined
Jul 10, 2011
Messages
1,970
Reaction score
474
Location
Germany, Southwest, Black Forest
My Car
1973 Ford Mustang Mach 1 T5 Q-Code 4-Speed
At the moment I restore the wiring of my once disappeared hood lamp out of 3 original harnesses. By tracing the original harness and it's routing (15A669 Assy) I discovered a second wire in parallel, yellow, which is winded together with electrical tape, all original. So there is something special going on. By studying all possible wiring diagrams from 71-73 and my car, I came to the conclusion, that this is a separate 37 Y wire, which comes directly from the heated backlite relay. All other wiring was clear before, because my car has also Convenience group, Tach and Gauges. Studying the 1973 wiring diagrams they say that the circuit breaker of the 15A669 assy should be connected to the positive post of the starter relay. The yellow wire from the heated backlite could be attached either to the same positive post or to the junction block. 

Now my question is: in the past it was the common view, that a car which has Convenience group, Heated Backlite and more AND Gauges, the black wire with the 20 AMP circuit breaker has to be mounted on the junction block. Otherwise on the relay post. The original wiring diagrams for 1973 say something different. So what should I do? Follow the '73 wiring schematic? I know the '73 Mustangs, more the later ones, are different animals. So the above said sounds some kimd of logical.  But what's the purpose of the junction block? Is it nothing more than an outsourced positive post for the starter relay? That said if the positive post of the starter relay would be long enough I could also fit every wire from the junction block to this connection? 

Any help would be appreciated! 

 

midlife

Shorts checker
Joined
Jan 24, 2012
Messages
4,160
Reaction score
584
Location
Tucson, AZ
My Car
No Mustangs at the moment.
The junction block is only used for factory tach dash configurations.  The convenience harness, backlight, power windows, hood lamp have nothing to do with mandating the junction block.  The purpose of the junction block is to provide the ammeter with a one line showing the alternator output and the other line attached to the battery side of the starter solenoid to show the battery output/input.

 
Joined
Jul 10, 2011
Messages
1,970
Reaction score
474
Location
Germany, Southwest, Black Forest
My Car
1973 Ford Mustang Mach 1 T5 Q-Code 4-Speed
The junction block is only used for factory tach dash configurations.  The convenience harness, backlight, power windows, hood lamp have nothing to do with mandating the junction block.  The purpose of the junction block is to provide the ammeter with a one line showing the alternator output and the other line attached to the battery side of the starter solenoid to show the battery output/input.
Ok, that sounds reasonable, however the consens here in the last years was sometimes something other... 

But, that said, the conclusion would be to attach the circuit breaker of the 15A669 assy and the separate 37 Y wire, which comes directly from the heated backlite relay, to the battery side of the starter solenoid, right?!? 

 
Joined
Jul 10, 2011
Messages
1,970
Reaction score
474
Location
Germany, Southwest, Black Forest
My Car
1973 Ford Mustang Mach 1 T5 Q-Code 4-Speed
And an additional question: why is it said that when a car has gauges and Convenience group the circuit breaker has to be mounted on the junction block? How does the convenience wire in this case operate with the ammeter connection on the junction block?

 

midlife

Shorts checker
Joined
Jan 24, 2012
Messages
4,160
Reaction score
584
Location
Tucson, AZ
My Car
No Mustangs at the moment.
David, I don't see the connector from the battery, so I can't tell if your wiring at the starter solenoid is reversed or not (it actually can work by swapping the starter and the battery connections). 

And an additional question: why is it said that when a car has gauges and Convenience group the circuit breaker has to be mounted on the junction block? How does the convenience wire in this case operate with the ammeter connection on the junction block?
It doesn't, technically.  The convenience wire simply needs a hot connection and a circuit breaker to provide power.  When you have a tach dash and the isolation post, there are two leads on the main headlight harness that are hot: one goes to the starter solenoid and the other to the isolation post where it is married to the alternator output line.  Inside the headlight harness, the two main leads are tied together so either one can be tapped for use as a hot point. 

If an isolation post was used with a standard dash and the Convenience group, then the isolation post was simply used with a jumper wire from the starter solenoid to de-clutter all the wires around the starter solenoid.  I don't think Ford did this, as it would require two extra parts that are superfluous and the bean-counters would have a conniption!

 
Joined
Jul 10, 2011
Messages
1,970
Reaction score
474
Location
Germany, Southwest, Black Forest
My Car
1973 Ford Mustang Mach 1 T5 Q-Code 4-Speed
This is picture of my 1973 Mach 1 with gauges, convince group, Power Windows & rear window defroster.

View attachment 53155

View attachment 53156

View attachment 53157
David, thank you very much for the pictures - I have quite the same options and that makes clear, that the convenience wire and the yellow rear defrost wire were taped together on their route to the starter solenoid. And the rear defrost wire has its fusable link, good to know. Interesting about the big black integrated rubber wire clips on the black cable which is not taped together with the yellow one at this points. My black wire showed no traces of the big rubber chunks on it's way. So I assume it could be mounted with special cable ties as on picture in the assembly manual shows, too. All very special on these late '73 Mustangs... 

 
Joined
Jul 10, 2011
Messages
1,970
Reaction score
474
Location
Germany, Southwest, Black Forest
My Car
1973 Ford Mustang Mach 1 T5 Q-Code 4-Speed
David, I don't see the connector from the battery, so I can't tell if your wiring at the starter solenoid is reversed or not (it actually can work by swapping the starter and the battery connections). 

It doesn't, technically.  The convenience wire simply needs a hot connection and a circuit breaker to provide power.  When you have a tach dash and the isolation post, there are two leads on the main headlight harness that are hot: one goes to the starter solenoid and the other to the isolation post where it is married to the alternator output line.  Inside the headlight harness, the two main leads are tied together so either one can be tapped for use as a hot point. 

If an isolation post was used with a standard dash and the Convenience group, then the isolation post was simply used with a jumper wire from the starter solenoid to de-clutter all the wires around the starter solenoid.  I don't think Ford did this, as it would require two extra parts that are superfluous and the bean-counters would have a conniption!
Randy, 

thank you very much for explaining the whole concept, I have my lesson learned, very cool! Now it all makes sense  :thumb:

 
Top