Electrical troubleshooting and wiring education 101

[sargentrs]

I'm starting this thread for our members to share their knowledge about electrical troubleshooting, reading wiring diagrams and tips and tricks we've learned over the years. This is not meant for posting to solve any issues you may be having but for general knowledge and training. If you have a problem you are trying to work through, please start another thread about your specific issue and let's not clutter this up with a lot of questions and answers.

I'll start this off by posting links to the tech section for a quick hop over to the main site while you're studying on this topic.

Wiring schematics: http://www.fordification.com/tech/schematics_h.htm
Pictorial wiring diagrams: http://www.fordification.com/tech/schematics_i.htm

1972 Ford F-100 thru F-350 Master Wiring Diagram: http://www.fordification.com/tech/wiring_1972master.htm This diagram is specific to 1972 models but is the most comprehensive and detailed diagram containing almost all options including A/C.

Feel free to post any resources you may be familiar with that may help our members to solve the many electrical gremlins we encounter with our wonderful bump sides!

[sargentrs]

An excellent article by our founding father (thanks, Keith!) on Charging System Troubleshooting.

http://www.fordification.com/tech/charging.htm

[Ranchero50]

When troubleshooting DC wire breaks, if you read no voltage above the load (light, fans etc) or switch then you have an open above it (fuse, fusible link or wiring). If you read full voltage below a load, then you have an open in the ground circuit or below a switch, an open to the load or below in the ground circuit.

[sargentrs]

Another great tip from Ranchero50 from another thread. Thanks, Jamie!

With the points closed you should read 0 volts from coil - to batt - and somewhere close to that from coil + to batt - With points open you should read batt + from coil - and the same from coil +. The key is voltage from coil - to batt - should be 0 when closed and batt + when open.

There's a long resistor wire that keeps the wiring from burning when the points are closed with the key on. It'll drop the voltage between the coil primary windings and the resistor wire depending on the resistance of each at coil +.

[Thipdar]

Make sure your map is appropriate for your trip...

1970 F100-250, the wiring diagram shows how the idiot lights are implemented.
1970 F350, the wiring diagram shows how the gauges are implemented.
There are differences at the Voltage Regulator, Alternator and Instrument Cluster.
Attempting to diagnose problems from the wrong diagram could easily lead you astray.
If you have an F250 with gauges, it will make more sense to look at the F350 diagram!

[Thipdar]

This was written for a 1970, but should be a useful guide for years that are close to 1970.
Some wire colors may be different.
1970 Was the first year that Ford implemented the Dual Brake Warning system, probably due to federal law.


For 1968, the wiring diagram for F100-F350 Ignition, Charging, Starting and Gauges:

According to this schematic:
1) In the "OFF" position, +12VDC power comes from the + terminal of the Starter Motor Relay via Wire #37, through the "Circled A" connector, and becomes Wire #21. No other pins on the switch connector are electrically active.
2) In the "ACCY ONLY" position, power comes from the "P" pin and is applied to the "A" pin (Wire #16. Red/Green stripe), which feeds, through a one-pin connector, to Wire #16A (Pink Resistor Wire). At the "Circled D" connector, Wire #16A connects with Wire #262, which feeds through the "Circled B" connector to one of the small studs on the Starter Motor Relay; on the other side of the "Circled D" Connector, wires #16A and 262 are connected to Wire #16 (Red/Green stripe), which goes to the Coil. The other two wires on the "Circled D" connector come from the Water Temperature Sender and the Oil Pressure Sender.
3) In the "ACCY & COIL" position, everything in step 2) applies, but also: power comes from the "P" pin and is applied to the "C" pin, which feeds Wires #98 and 297. Wire #98 connects to Wire #904 at the "Circled C" connector; Wire #904 goes through the Instrument Cluster Connector and feeds the Charge Indicator Lamp. On the other side of the "Circled C" connector, Wire #904 goes, via a two-pin connector, to one of the pins on the Voltage Regulator Connector.

In my opinion, this is wrong. I believe that the schematic should indicate that the "ACCY ONLY" position should provide power to the "A" pin, but that the "A" pin should feed wires #98 and 297, while the "ACCY & COIL" position should provide power to the "C" pin, but the "C" pin should provide power to Wires #16 and 16A. Essentially, it seems like these functions have been reversed, and I have no explanation as to why this happened. The appropriate parts of the schematic for 1970 seem to make more sense.

4) In the "COIL START & PROVE OUT" position (a 'momentary-closure switch', which means that when you release the switch, it reverts to the position in step 3)), everything in step 3) applies (including everything in step 2) before it, regardless of whether steps 2) and 3) are functionally reversed), but also: Power comes from the "P" pin and is applied to the "S" pin (which activates the Starter Motor Relay); at the same time, ground is provided to the "PO" pins, which don't seem to be connected to anything.
5) When the switch returns from the "COIL START & PROVE OUT" position, ground is removed from the "PO" pins and power is removed from the "S" pin, which shuts off the Starter Motor Relay and makes the Starter Motor drop out of the circuit and stop trying to start the engine.
6) When the Ignition Switch is turned from the “ACCY & COIL” position to the “OFF” position, power is removed from the accessories and from the Ignition Coil’s field; without a field present, the Coil cannot produce spark for the Distributor.

"Large Yellow" wire is Wire #21, the main source of +12VDC power for the switch, connects to pin "P".
"Large Black w/ Green stripe" wire is Wire #297, the wire that provides power to the brake light fuse and to the Instrument Cluster Voltage Regulator, connects to pin "C".
"Smaller Red w/ Blue/Violet stripe" wire is Wire #32, the wire that goes to the Neutral Safety Switch (if an automatic transmission) and then to the Starter Motor Relay, connects to pin "S"
"Smaller Violet or Purple w/ no stripe" - we'll get back to this in a moment.
"Larger Pink Ballast..." wire is Wire #16A, while the "smaller red wire with green stripe" is wire #16. The schematic shows them connected to pin "A".
The remaining (Violet) wire, not shown on the color code chart, must be Wire #98, which is connected to pin "C".

Issues, notes, cautions, etc.:
A) The pig-tail that I got had wires that were all the same length. If I were building them, I'd stagger the lengths by about 3/4 of an inch or so... it's a little awkward to have all the butt splices in the same place.
B) The apparent swap in functionality between steps 2) and 3) bothers me. If you are doing anything in this area, please make absolutely sure you understand the schematic as it is and what I suggest the schematic should be. This also carries over to the wire # vs. colors vs. functions described later.
C) Modifications to eliminate the breaker points apparently involve changes to Wires #16, 16A and 262. Without being certain of what the aftermarket suppliers actually require, changing from "Wire #16 feeds Wire #16A, which feeds Wire #262" (which is shown on the schematic) over to "Wires # 16 and 16A both connect to pin "A" and then go off to feed other things" could make sense. Cutting the Pink wire to eliminate it from the circuit might also disable functions which are farther 'downstream'. Putting both wires into the same pin would then bypass the cut wire and allow for the 'downstream functions' to happen as needed.
D) In general, the schematics that are available for this vintage of truck have some subtle differences, based on whether they have full gauges or just idiot lights. This may or may not matter for your diagnostic needs.