I have a 78 F250 with a 460 and I need some advice on how to proceed with testing what the root cause of my elictrical problem is. There are 2 Red/Green larger gauge wires under the dash (one comes off the ignition switch) that are smokin', poppin', and sizzlin'! Also, just turning the key (not starting the engine) I can watch the Eng. Sol. fuse blow within seconds. So of course before I repair these wires I need to find the source of the problem first. I don't just want to start replacing components that are part of this circuit (i.e. starter relay, igintion switch, etc) as it can get pricey, but mainly I want to LEARN and UNDERSTAND my truck and why this stuff happens. Can anyone offer some advice as what component to check first and how to check it? Then the next, etc? Also, and this could be part of the problem, when the battery is hooked up, truck is off and just sitting, something is draining the battery - from a good 12.8 volts to about 10 volts within an hour or two, to about 7 volts in several hours. I have the Chilton's Manual for reference. Thanks everyone!

 

I'd recommend getting a reasonably priced multimeter - reads amps and volts. You can spend a bunch or a little to the same effect: Just figger out what's supposed to happen at each key position and then test what might be wrong. Plus, when you've got your wiring issue resolved, the cheap ones are great for telling you which AA's are really dead and which ones are not.

I'm certainly no expert, but do know that there's supposed to be about a 12V difference between the big wire that goes to the starter and the block/frame/ground when the key is turned "all the way". I'd start there.

Others come with far more experience than I do, but that's the first thing I'd try to establish. Sounds like there's a short or something somewhere, but I'm not a color-coding guru.

Another thing to think about is what happens when that key is in crank position: On my truck - '75 vintage - the key supplies voltage. On other makes and models at least, it'll supply ground.

 

Yikes. First I would try and find out what is draining the battery. Remove the negative battery cable, and put a test light in series with the cable and the negative battery post. Since you have a battery drain, the light should turn on as current passes through it. Remove fuses one at a time until the light goes out; this will help you identify what circuit is the culprit.

RED with GREEN is the main source of power when the key is in RUN. It runs the coil, ignition module, voltage regulator, and other items. If it's smoking when the key is on, there's a short to ground somewhere. Unfortunately there's no straightforward way to find the short other than carefully tracing the wiring. Look for frayed insulation. It's also possible that one of the items that's powered by the circuit has failed and is responsible for the short. There is nothing obvious that jumps out at me.

I'm not positive, but I believe the engine solenoid refers to the idle stop solenoid on the carburetor. If equipped, this is used to set the curb idle speed on the carburetor.

Any chance a previous owner has hacked up the wiring harness? This is by far the most common problem for multiple hard-to-find problems like this.

 

When you find the short with the power leads under the dash the next thing I'd do is replace those wires completely. as old as they are the smoking makes me think that it's a good time to get rid of them.

Look at what is going on with the voltage regulator/diode that's hooked up to the alternator as well. I've blown a few fuses having this mis-wired as well. If it's been replaced or if the alternator was replaced recently then this could be part of the problem.

A test light will really help you get this stuff done more quickly, but Jason is right about the multi-tester.

Overall my way of diagnosing electrical issues is as follows:

1.) figure out what I want at the point of the problem (you want the wires to quit smoking)
2.) figure out what you need to get at the point of the problem (the power leads not to go straight to ground i.e. a short)
3.) Use my test light at the point of the problem and work my way back to the fuse panel/battery until I get what I want.
4.) Pull new wire from the point where I have what I want to the point that I need it

For you step three is going to include figuring out what the wires are supposed to do (fmc might have it, check the back of your chilton's for final confirmation) Then figure out where they're going bad for you. You're looking for power on one spot and no power in another where there should be some. By grounding a test light then poking it into the wire along the circuit path, you should be able to find the place where it stops being a hot lead. If this happens at the point that the wire changes color, or on the backside of the resistance in the circuit (light bulb,horn,motor, etc) then the resistor in the circuit is your problem and not the wire itself.

Step 4 for you is going to include running new wire from where the red/green wire is to where it goes in the stock configuration or replacing the broken resistor. This should bypass the short or the hack and get you back to running.

Sometimes you won't be able to visibly see what is going wrong with the harness, and at the end of the day you don't care. You only want 12v at some places, ground at others, and resistance in between the two. Good luck.

 

something ive just gone through, be aware that the voltage regulator, alternator, ignition system, starter solenoid, battery, even the dimmer switch all are wired up together somehow, fords are crazy... multimeter and circuit tester are gold on these trucks

 

Thanks everyone for all the advice. I was gone for the better part of the weekend, but I also have today off so I can dig in and start testing. I know how to use and read a multi-meter, but my biggest problem is not knowing the exact function of each ignition component and what it should test at. Wish me luck! If anyone has any more suggestions let me know. Thanks again.

 

Here is a great reference for you that has helped many other users before as well. Try it out, it may give you an upper-hand on your wiring issues.


1978 F-Series Wiring Diagram & Gauge Cluster Diagram:

https://www.ford-trucks.com/forums/9...r-diagram.html

 

Wow! This is a lot better than the Chilton's Manual diagram. Thanks, this will help tremendously!