So I have this 1977 F100 Supercab. The mechanical side of things are running great. It really doesn't show its age, gets good mileage considering it's a 351w, etc. Electrical is all jacked up. It came with a jerry-rigged ignition coil from an 80's Thunderbird, an aftermarket CD player sorta wedged into the dash, a rewired horn that is now a button under the middle of the dash, an on/off switch wired to the ignition switch, and a 4/6/8cyl universal tach zip tied to the steering column. The project I tackled while I was home last was getting that damn tach and CD player out of my cab, because I believe(haven't gotten around to having a parts store test my alternator yet) that the CD player was steadily draining my battery while I wasn't able to drive the truck much. So basically I go about it with my newbie common sense. I can remove the CD player, cap the bare wire ends for if I decide to put a different radio in, snip the two wires that were running to the tach, put those two together, because I am guessing it is measuring RPM's based on how quickly it gets 8 pulses from whatever electrical component it is wired to, and not lose any essential function of the truck. Well, I did that. Truck wouldn't fire up. Would turn over, but that's all I got. Disconnected the two leftover wires from the tach, and it turned over, fired, and then was silent. If I kept the key in the start position, the starter would just spin. It was such that if I had the wires touching, I could turn the engine over, then while it was turning, disconnect the wires, and the engine would fire what sounded like once per cylinder, and then quit immediately to let the starter spin on its own.

I tried several different solutions, including messing with getting the tach and CD player wired back up. Turns out there was a fuse burned out. Since I hadn't checked if it was already burned out by the time I started messing about, I bought a box of fuses and started trying to track down what was causing the fuse to blow. No luck, every combination I tried I watched as I turned over the engine, and never saw it blow. I've done some research that points at the ignition coil being something to check with the tach, and see if the tach wiring goes to that, and if I need to bypass anything or what.

At this point, I need to narrrow down my troubleshooting as much as I can, because I'm moving, but the truck isn't here with me, I basically need to go, get it running again, load it up with furniture, and get back to where I am now preferably in 24 hours or less. When 6 of those hours are gonna be taken up with driving, I need this as streamlined as possible. So literally anything to point me in the right direction is useful.

Thanks so much,
Snarge

 

The fuse that blows is the one in the red square. The fuse block doesn't seem to be the usual one for a 77 F100 with a 351w motor. But I'm a newbie, so I might very well be dead wrong.

 

I'm going to *edit* your post

 

HIO SILVER is one of the better electrical guys on the site, maybe he will be along to help out as well

 

If your ignition system was from an '80s car, it's a Duraspark II electronic ignition system, which is a great design - unlike the GM systems, it keeps the electronics out of the top of the distributor where there's extra heat and high-voltage flying around to cause problems, and provides a nice strong spark for an OEM system. With an OEM Motorcraft ignition module, it's very reliable too. Electronic ignitions don't use points; they have a pickup coil (hall effect sensor) in the bottom of the distributor, and an electronic ignition module to charge/discharge the coil at the right times. The tach is definitely wired into this system.

The Duraspark module is wired like this:

• Orange & Purple wires from ignition module to pickup in distributor for position signal
• Black wire to ground at distributor
• Green wire: timing signal from module to coil negative terminal
• Red wire: +12V when ignition switch is in "run" position
• White wire: +12V when ignition switch is in "start" position

There's also a red/green wire feeding the positive terminal on the coil, which runs through a piece of resistor wire from the ignition switch to provide ~9V to the coil, and usually also has a brown wire to the starter solenoid "I" terminal spliced in to provide +12V during cranking.


Typically, aftermarket tachometers will have three wires: a red power wire that gets +12V from the ignition switch, a black ground wire, and a green wire that goes to the negative side of the coil/ignition module.

Depending on which wires you shorted together, different things could've happened:

• Red to green would've sent +12V to the negative side of the coil (it's already got +12V to the other side) ... This would result in zero voltage across the coil during cranking, and thus no spark, except *maybe* a weak one when you went back through the run position and there was momentarily 3V in the wrong direction across it.
• Red to black would short out the power supply to the tach and blow a fuse immediately at ignition on, as long as the PO was smart enough to include a fuse to begin with, but shouldn't affect the rest of the ignition system.
• Black to green would keep the coil grounded constantly, which would make it get really hot (maybe damage it depending on how long the ignition was on), and fire one strong spark right when you turn the ignition off. Same symptom as if you had a bad pickup coil in the bottom of the distributor.

Best thing in the short term might be to put it back the way it was before you messed with it in order to get it home, then figure out what you need to modify at your leisure.

 

Ok I'm gonna re-edit my own.

So I have this 1977 F100 Supercab. The mechanical side of things are running great. It really doesn't show its age, gets good mileage considering it's a 351w, etc.Start with keeping yourself reminded of this.. This is good news! Yes. I am reminded of that every day that the wiring isn't jinky. If I could get the wiring squared away, I would be happy to slap a 4bbl and a new exhaust on it and move on in the running resto.

Electrical is all jacked up. It came with a jerry-rigged ignition coil from an 80's ThunderbirdSince the original ignition is missing I suggest going with a HEI conversion dizzy. It is a 1 wire 12v activated distributor
I actually put an OEM coil in, as well as a new cap, rotor and plug wires. That was the first thing I did after putting a Redhead steering box in (I'm just no good with wiring)

an aftermarket CD player sorta wedged into the dash Pull that and all its offending wires because a jerry-wired CD player can burn your truck down.. It'll be better to start over there anyways It seemed like it took the original wiring and just spliced it into a new system. I think the PO knew what he was doing, but just didn't wanna do anything the RIGHT way, or maybe LMC killed his father or something.

a rewired horn that is now a button under the middle of the dash, does the truck have the original steering column? Yes, but supposedly the horn contacts corroded, so that was rewired. I'm not even gonna mess with it, it passes inspection. I'll have a lot of the less convenient, but still working things updated once I get a chance to go into a shop with about $1500 towards getting up to daily driver condition.

an on/off switch wired to the ignition switch, This is likely a result of the cobbled ignition.. Get that HEI I know, updating the entire ignition system is on the to do list, between the keep-it-running repairs and the aforementioned 1.5k shop trip.

4/6/8cyl universal tach zip tied to the steering column. don't waste your time with this now, the hei dizzy has a male spade tach terminal Yeah, I don't wanna deal with the tach at all. It's an automatic C4, so unless I pick up one of those fancy aftermarket clusters with a tach already on it, I have no real need for it. I have a good ear for engines, my manual Kia doesn't need a tach at all.

The project I tackled while I was home last was getting that damn tach and CD player out of my cab, because I believe(haven't gotten around to having a parts store test my alternator yet) that the CD player was steadily draining my battery while I wasn't able to drive the truck much. So basically I go about it with my newbie common sense.good plan, disregard my previous post Wilco

I can remove the CD player, cap the bare wire ends for if I decide to put a different radio in, snip the two wires that were running to the tach, put those two together, because I am guessing it is measuring RPM's based on how quickly it gets 8 pulses from whatever electrical component it is wired to, and not lose any essential function of the truck.Does the tach work at all? It didn't.

Well, I did that. Truck wouldn't fire up. Would turn over, but that's all I got. Disconnected the two leftover wires from the tach, and it turned over, fired, and then was silent. If I kept the key in the start position, the starter would just spin. It was such that if I had the wires touching, I could turn the engine over, then while it was turning, disconnect the wires, and the engine would fire what sounded like once per cylinder, and then quit immediately to let the starter spin on its own.one of the wires running to the tach is obviously a signal wire for the ignition
So a signal wire would be indicating how fast the engine is turning, yes? Would opening/closing that circuit affect the ignition???

I tried several different solutions, including messing with getting the tach and CD player wired back up. Turns out there was a fuse burned out. Since I hadn't checked if it was already burned out by the time I started messing about, I bought a box of fuses and started trying to track down what was causing the fuse to blow. No luck, every combination I tried I watched as I turned over the engine, and never saw it blow. I've done some research that points at the ignition coil being something to check with the tach, and see if the tach wiring goes to that, and if I need to bypass anything or what.Again I would pull ALL the CD player wires that were added out because they are non essential.. It will be hard to help diagnose this considering another ignition has been grafted so I will keep pushing the HEI idea because you will go through hell trying to "accidentally" figure out which wires got crossed Basically it is down to two OEM-looking 14 or 16 ga wires in red, and one in black that came from the CD player. I noticed that the cigarette lighter wasn't connected. Surely those couldn't be from that?

At this point, I need to narrrow down my troubleshooting as much as I can, because I'm moving, but the truck isn't here with me, I basically need to go, get it running again, load it up with furniture, and get back to where I am now preferably in 24 hours or less. When 6 of those hours are gonna be taken up with driving, I need this as streamlined as possible. So literally anything to point me in the right direction is useful. Since its gotta be done quick lemme see how I would go about it...

1- I would make sure the "on off" switch is getting power and make sure that signal has a good connection wherever it is running..
99% sure it is. I drove it on errands, parked it, and started yanking on wires. I traced a wire right up to something that goes right into the battery, so I am assuming that it is basically a fancy battery disconnect, where basically just the ignition system was hooked up, because when the CD player was connected, I kept it turned off, but whenever I disconnected and reconnected the battery, it'd turn back on. So I think that the ignition toggle switch didn't act as a true battery disconnect, but certain systems were tied in. For instance, I don't remember if this happened before the battery went dead and was jumped, but I know that after running errands, I couldn't turn on the windshield wipers or lights without the ignition toggle in the on position. But that could be something with the alternator, where I was able to run errands on the charge from the jump, but then the battery got down below a point where the accessories would work. That toggle seems to be the one bit of non-original wiring that seems to have been done right with the correct terminals and no electrical tape splicing.
2- trace those tach wires and figure out how they cobbled them into the ignition I did a little tracing, but it's all pretty Greek to me. I can get photos and send them your way, but I am not sure that I will get much info from figuring out where they go.

3- Make sure they wired the CD player as a 'seperate' system.. meaning: make sure it had its own power source
So from the three wires that go into the dash, I can't easily trace them. Can I take a circuit tester or a voltmeter to them to figure out any of that?

Does your truck have points, did I read that right? Yeah, it was original except for that Thunderbird coil and some plug cables. I'm kinda grateful, because it's more analog, and thus, easier for me to understand.

 

Here is a folder of photos of the truck. The interior was gutted by owner before PO. The engine bay is mostly the same, save for the ignition coil, the wires, and the fact that I swapped out the steering gear. Planning on adding an electric radiator fan, and then eventually getting a much better radiator, assuming I can also update the AC exchanger at the same time (AC blows tepid farts, but until I get things in better order, it's not high on the priority list at all. Something about a blower motor being dead. The motor is on the list of stuff to get a quote on for the $1.5k trip)

 

Here is a folder of photos of the truck. The interior was gutted by owner before PO. The engine bay is mostly the same, save for the ignition coil, the wires, and the fact that I swapped out the steering gear. Planning on adding an electric radiator fan, and then eventually getting a much better radiator, assuming I can also update the AC exchanger at the same time (AC blows tepid farts, but until I get things in better order, it's not high on the priority list at all. Something about a blower motor being dead. The motor is on the list of stuff to get a quote on for the $1.5k trip)
https://drive.google.com/open?id=0Bx...REE&authuser=0

 

Hm. I was seeing a fuse blow a couple times, but couldn't trace what it was. But I only pulled two wires from the tach. Both black, I think one spliced into a green at some point. Hm... Hold on. I remember a red wire into one of the poles of a fuse on my fuse block... Putting that on the list to trace... What if the PO hadn't put in a fuse? Is there something to look for? And it was just the coil off the Tbird. The rest of the ignition was left mostly unmolested.
To clarify, the tach has been removed by me, didn't label the two wires because I assumed connecting them would close whatever circuit the tach was a part of, and prevent exactly what is happening.

And I have been trying to do that, I looked up the wiring schematic for the aftermarket CD player, and have been trying to match up the two red and one black OEM wires that went to it, but no luck getting it to turn on. I didn't prioritize trying every combination on the CD player, because I figured the circuit being open shouldn't affect the operation, since flipping my ignition toggle doesn't affect it, only disconnecting and reconnecting the battery, I assumed it was isolated at least from the ignition system. So I focused more on getting the two wires coming from the tach re-wired correctly.

 

Thanks, I'll get a link sent his way ASAP.

 

Keep throwing elbows at it! If we were close I'd surely help you out, somethings wrong with me, I LOVE electrical gremlins

 

Rubbing salt but here is a visual difference in the ignition setups

 

I actually put an OEM coil in, as well as a new cap, rotor and plug wires. That was the first thing I did after putting a Redhead steering box in (I'm just no good with wiring)

Good, well we know that is correct.. If you can put a steering box in you can make it through this

It seemed like it took the original wiring and just spliced it into a new system. I think the PO knew what he was doing, but just didn't wanna do anything the RIGHT way, or maybe LMC killed his father or something.

good, keep the humor!
If I can't bring myself to at least grin at the silliness of something, I walk away, scrub hands, eat chow, and sit on the couch with a dirty mag(LMC) until the catalog gets me to the point where I can't wait to go back out and take another swing at it.

I know, updating the entire ignition system is on the to do list, between the keep-it-running repairs and the aforementioned 1.5k shop trip.

I don't know why there is so much animosity towards the HEI dizzy on the forums here. Mine has been a STRONG trouble free distributor for me and the fact that it REPLACES the duraspark box, coil, and points in your case for 75 bucks is a no brainer.. Ehh, tossing pebbles at pebbles I guess.. That's pretty sweet. I might see if I can't find some similar systems, do some comparison. Might be a nice way to simplify.

So a signal wire would be indicating how fast the engine is turning, yes? Would opening/closing that circuit affect the ignition???

Yes, there's a 12v switched on, Ground, Illumination and The tach signal which comes from the coil.. its possible you broke this connection (depending on how they spliced in)
Oh boy.... So let's start with the tach signal, since there was just the one set of wires found. I assume I just need to reconnect them to the tach to solve this one

Basically it is down to two OEM-looking 14 or 16 ga wires in red, and one in black that came from the CD player. I noticed that the cigarette lighter wasn't connected. Surely those couldn't be from that?

Yes, Those are likely the original cigarette lighter and CD player wires Then I am gonna look up reconnecting them, and see if I can get current from it. Would be nice if I could charge my phone from it on these longer trips. No more battery packs in the truck!!!

99% sure it is. I drove it on errands, parked it, and started yanking on wires. I traced a wire right up to something that goes right into the battery, so I am assuming that it is basically a fancy battery disconnect, where basically just the ignition system was hooked up, because when the CD player was connected, I kept it turned off, but whenever I disconnected and reconnected the battery, it'd turn back on. So I think that the ignition toggle switch didn't act as a true battery disconnect, but certain systems were tied in. For instance, I don't remember if this happened before the battery went dead and was jumped, but I know that after running errands, I couldn't turn on the windshield wipers or lights without the ignition toggle in the on position. But that could be something with the alternator, where I was able to run errands on the charge from the jump, but then the battery got down below a point where the accessories would work. That toggle seems to be the one bit of non-original wiring that seems to have been done right with the correct terminals and no electrical tape splicing

okay as long as it continues to operate all those accessories then I'm sure it is still receiving and sending power correctly
So I'm thinking it's a circuit that is open in the realm of ignition that needs to be closed, or something needs to be added to alter the current somehow to avoid blowing fuses.... Gonna ignore the wires that came from the CD player except for wiring up the cig lighter. Is that just one wire maybe???

So from the three wires that go into the dash, I can't easily trace them. Can I take a circuit tester or a voltmeter to them to figure out any of that?

Lets just assume that if they did come off the back of the CD player then they are not part of your ignition woes

Yeah, it was original except for that Thunderbird coil and some plug cables. I'm kinda grateful, because it's more analog, and thus, easier for me to understand.

Make sure those points and condenser are new.. Start simple right

I know the points are new, same with the rotor, but the condenser not so much. The other thing I am thinking is the alternator might just be dead. I think I am gonna pull that real quick and take it in for a quick diagnostic first thing, maybe have a friend work on tracing wires and circuit testing while I do that. Divide and conquer + save time.


Ok, well I gotta get to bed, got some stuff related to the apartment to deal with in the morning. Thanks for all the help.

 

You'll find that dam wire and laugh about this for years

 

A slight wrinkle in that schematic. It shows a newer style setup where the ignition switch sends the coil full voltage during start. Whereas our trucks that full voltage comes from the Starter Solenoid/Relay I terminal.