I have a 78 f100 with a 351m in it. I recently pulled the old distributor out and have replaced it with an HEI distributor. I set the timing and everything, and now I just need to hook up the electrical part. It has two tabs on it that say "tach" and "bat". My truck doesn't have a tach on the dash. How do I hook it up? Thanks

 

Read the instructions or search the internet lately?

http://static.speedwaymotors.com/pdf/447-1071.pdf

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Yeah, sorry. My distributor did not come with instructions. The link does not help, because my question is what wire do I hook up to the "bat" terminal on the distributor? And also, does there need to be a wire on the "tach" terminal, even though I have no tach? I'm sorry if I'm an inconvenience

 

So I figured it out. The wire from the old cool goes onto the terminal. I'm still not sure if the tach should be hooked up though

 

I meant old coil, not cool

 

The following is how your original ignition was setup. If you already knew this stuff, skip it as I typed most of it before actually reading your post completely. On the other hand, it is good background.


The BAT terminal is powered from the ignition switch through the ballast resistor (and the S terminal on the started solenoid) on a point ignition and the ignition switch via a resistance wire on an electronic ignition. It's also called the + terminal.


The TACH terminal connects to the distributor on a point ignition and the module on an electronic ignition. It's also called the - terminal.


Since I don't know the rules for posting links to other forums, google "1978 f100 wiring diagram" and pick the hit that has .net/technical and 1973-1979 Ford Truck Wiring...


Pick your year from the list. Click on page 3. It shows the ignition in the center of the drawing. It can be very confusing to read as it shows electronic ignitions for 6 and 8 cylinder engines. Notice that there are numbers along the top edge of the drawing. The coil is in section 14. It has 3 connections. I'm going to use the wire numbers here, I'm hoping that you'll figure out the colors and that the C numbers are connectors.


Wire 16 comes from a source of power and connects to the BAT terminal on the coil. It actually comes from 2 sources. The S terminal on the starter relay and the ignition switch. That happens father up the harness so you should only see one wire at the coil.


Wire 232 runs to the ignition module and may connect to a radio noise interference suppression capacitor. On my '74 390, there is a short wire with connector just floating in the air, a tachometer can be connected to it. The module pulses this wire to ground to create the spark.


I think you can guess where the 3rd, all black wire on the coil goes. ;-)


A google for "ford hei distributor wiring" images came up with a bunch of hits. The very first one looks promising but since I don't know which version you are running I've no idea if it's the right one.

 

Okay, thank you very much. That is a very detailed reply, and I appreciate it. I think that since my HEI distributor has the coil built in, that I don't need the original coil, correct? Also, I hooked up the wire that was labeled "bat" on the old coil to the bat terminal on the new distributor, then checked for spark, and I have spark now.

 

You are welcome. I operate on the principal that if I teach you to fish rather than open the can of tuna for you, I can go back to being lazy. ;-)


Assuming that you have an all-in-one HEI, you probably have it hooked up mostly correctly.


But...


According to the installation manual at Speedway, the BAT terminal goes to a 12 volt switched source and the wire originally connected to the BAT terminal on the factory coil is less than 12 volts due to the resistance wire. The only time it sees a full 12 volts (or battery voltage, what ever that may be) is when the key is in the start position, that's what the wire from the S terminal on the solenoid is for. The HEI distributor may be able to run fine on the 8 or 9 volts available on that wire, and it may not cause the resistance wire to get hotter than designed. I do not know, I've never owned a HEI distributor.


I'd search this forum (73-79) for HEI and wiring or hook up or something like that to see what others have done.

 

Not a bad principal haha. And okay that makes sense. Is there a way to remove the resistor? And I'm assuming it is okay to leave the tach terminal alone?

 

Since you have added an item that requires 12v, both in Run and START, you will need two wires that go to the BATT post on the HEI. One will be a line that will tap off the RUN circuit. You may have to add this wire. The other is easier as it comes off the "I" post of the starter solenoid. This provides voltage when the key is in the start position.

 

Okay, how do I wire the one from the run circuit? I already have the one from the start circuit

 

Are you handy with a volt meter? If so, see if there is an extra tap on the fuse bock that only has power when the key is in RUN position.

 

What 84espy didn't point out is that I called out the S terminal instead of the I terminal on the solenoid. He is correct and I goofed. The S is for Start, the I is for the ignition resistor bypass.


It looks like wire 732 at the old ignition module is hot in RUN. Look for it on the diagram near the coil.


I wouldn't mess with the resistance wire itself, it's buried in a harness and not obvious. You could use the '79 diagram to take a look at the ignition switch to find it's source and run a new wire. That's what I would do. The resistance is far enough into the harness that you can cut the wire a few inches from the switch and connect there or if you are careful and good at soldering, you can remove a half inch of insulation and splice your new wire into the old. If you do that, clip the end of the old wire where it used to connect to the coil, fold over a half inch and heat shrink it so it can't short out just as you are pulling into the passing lane to get around that old diesel Mercedes.

 

Yes, if you do not have a tach. If you plan to run a tach, you may want to pull the wire to have it in place for future install. Make sure it is protected against shorting.

 

Bypass the ballast resistor...you don't need it anymore w HEI.