73 Mach 1 where is the resister wire? color? What's the best bypass method?

[Paul M]

I've got a MSD box w/a Unilite distributor. I assumed I had a full 12 vt since this thing came with a Motorcraft electronic distributor. Starts and runs w/ a slight miss, then carbon fouls plugs. I'm leaning more towards the electronic side of this being the problem. I had 12+vt at the starter relay and assumed that I was ok. Now I'm 2nd guessing it. Maybe I should see more like 13.9 running. I've read 10 different stories on the net which wire to bypass along with where it is. I have a stock tach & read that was one place to bypass. The tach barely works at the moment. It reads 100 rpms when it's 900. Perhaps a tach adapter is needed. I can wait on that. One guy said back of tach to fuse block, another claimed it's from the ignition switch to the firewall. I assume from the ignition switch plug on the column mounted one. "I" terminal is supposed to be the resistance circuit after start. What's the best bypass method? I planned on 12/14 gauge with spades. I might have a 71 harness in it don't know if that was from a stock tach car. The difference being a pink wire no tach and a red with green stripe for tach. I see a red w/gn stripe at the connection for the column mounted ignition switch.

 

[midlife]

With a factory tach dash, the red/green spongy resistor wire runs from the tach to the back of the fuse box, 3rd row, pin just past the bolt hole on the side away from the large pin. It's rather hard to bypass given this pin placement, but the best way is to route a RUN-only line to the brown or red/green line on the back of that engine gauge feed plug that is shared with the resistor wire.

 

[Paul M]

Thanks for the reply. I ended up taking the dash apart to look behind that tach. Geeze they didn't leave much room to maneuver it. Run only? The MSD is grabbing the 12 from bat & signal from dist. So basically, I think your saying drop the red wire w stripe to distributor and wire direct to the brown or red/gr that is attached to the resistor wire plug. I assume either one of those 2 wires can handle the extra source. I'll look at it tomorrow.

 

[midlife]

Ummm...red-white wire is water temperature. There's a red/yellow wire that you can tap into that is the carb anti-stall throttle line.

 

[Paul M]

It may be the one w/ the yellow line. I'm hoping to get out there today.

 

[Hemikiller]

MSD doesn't use the factory coil (+) wire for anything more than to switch the box on. Mine is run off the "I" terminal on the starter relay. As Midlife noted, use the Red w/Yellow hash mark wire from the throttle position solenoid in the engine harness to power the Unilite and switch on the MSD box as shown below. This wire provides full system voltage in both Run and Start key positions.

You should have 12.6V on a fully charged battery, and the system should read between 13.6V and 14.6V while running.

You'll also want to verify you have a solid Battery (-) to chassis to engine block ground path. An intermittent ground path can cause all sorts of weird issues and especially with electronics. Unilite modules have a reputation for being very sensitive to bad wiring.

 

[mrgmhale]

The Red with Yellow Hash wire mentioned in prior posts will provide full battery voltage with the Ignition in the Run position. It is normally used (Circuit # 640) to power the Throttle Position Solenoid (TPS) for the carburetor for 1971 - 1973 Mustangs. I have a video showing where it is located on out 1973 Mustang Convertible. Not all Mustangs in 71-73 had TPS units, but the wiring included the lead that would have been used were the tPS there. Here is the video of the wire in question:




If you can't find that Red Yellow Hash wire you can use Circuit #63, the power circuit for the wiper motor. I cover that in the following video:




You are correct that the "I" terminal on the Starter Relay is the resistance wire voltage in Run position. It is full battery voltage while cranking, bypassing the resistance wire for the coil's positive primary terminal. An aside, full battery voltage while cranking is not 12 volts, as during cranking the voltage drops to 9.6 - 10.5 volts or so.

You mentioned carbon fouling plugs. That is not an ignition problem. It is excessive fuel in the air/fuel mixture. That can be caused by any number of problems. The more common problems include the choke coming off too late Either set incorrectly, a failed electric heater for electric choke, incorrect voltage for an aftermarket carburetor's electric choke, float level in fuel bowl is too high, idle mixture screws are set too rich, ruptured or leaking Power Valve is your carb had a Power Valve. Let me know what carburetor you are using and I will try to provide more info.

 

[Paul M]

With a factory tach dash, the red/green spongy resistor wire runs from the tach to the back of the fuse box, 3rd row, pin just past the bolt hole on the side away from the large pin. It's rather hard to bypass given this pin placement, but the best way is to route a RUN-only line to the brown or red/green line on the back of that engine gauge feed plug that is shared with the resistor wire.

I had to get my eyes on it. You can never be sure of the info you get on the net lmao. I can see where it would be tough to r & r it. Ye it's possible just not practical. Those 2 wires are full 12vt in run? Ye I have the 71 harness that matched the 71-steering column. I found out the hard way that you have to have specific years of columns to match the wire harness. Now for the harness in the engine compartment it's most likely 73 or after market. I checked the wires and they don't resemble what's in the diagram. I am running the distributor on a red w/ w stripe and the temp wire is a booted red w/ w stripe (the longest one) Along with the brown that has a female 1/4" spade term (my guess the original elec choke on the resistive "I" circuit. On the passenger side of the taped together bundle is a blk round male to AC, a blk 1/4" spade term (not used) and a 1/4" spade connector not used ( I believe I need one of them for the back of that AC unit since it looks like a fitting is broke off on the back of it. I'm guessing the wire I'm using 2nd red and white that's not in the diagram is ok. It was 12.6 when I last checked. I'm curious what the running volts are. It's not going to be easy to check w/ the barrel connectors. If it's not @ 13.9 I'll grab one of those 2 sources that you mentioned. Any information I can get from a knowledgeable source is appreciated. Thanks. I'm also paranoid to foul my new $33 plugs. I've run through the carb and there wasn't any significant problems although I've leaned it out a bit.

 

[Paul M]

MSD doesn't use the factory coil (+) wire for anything more than to switch the box on. Mine is run off the "I" terminal on the starter relay. As Midlife noted, use the Red w/Yellow hash mark wire from the throttle position solenoid in the engine harness to power the Unilite and switch on the MSD box as shown below. This wire provides full system voltage in both Run and Start key positions.

You should have 12.6V on a fully charged battery, and the system should read between 13.6V and 14.6V while running.

You'll also want to verify you have a solid Battery (-) to chassis to engine block ground path. An intermittent ground path can cause all sorts of weird issues and especially with electronics. Unilite modules have a reputation for being very sensitive to bad wiring.


View attachment 67619

Thanks for the reply. I used this same diagram to wire it. It was very handy. I don't see the red wire w/ the hash marks. Which leads me to believe this is a oddball-late 73 or after mkt harness in the engine compartment. I have 2 red wires w/ w stripes, the longer one has a boot and is temp. Ye I checked for the 12.6 when I 1st assembled it. I'm trying to resolve this before fouling new plugs. I ran through the problem areas in the carb. It's fine even though I leaned it out a lil, not enough to see the buildup I saw. I'm going to configure a way to get a running voltage when I start it again. It's a barrel connector in a tight spot behind the distributor. Yes, I've been told by the MSD techs about the grounds when I called and had to send in by box to get rebuilt. They didn't like my positioning the 12vt source on my starter relay since it's borrowed by other items. So I ran a 12gage wire directly to the (trunk mount) batt this weekend. I've used the relays on other cars w/ no prob. Just going through everything.

 

[Paul M]

The Red with Yellow Hash wire mentioned in prior posts will provide full battery voltage with the Ignition in the Run position. It is normally used (Circuit # 640) to power the Throttle Position Solenoid (TPS) for the carburetor for 1971 - 1973 Mustangs. I have a video showing where it is located on out 1973 Mustang Convertible. Not all Mustangs in 71-73 had TPS units, but the wiring included the lead that would have been used were the tPS there. Here is the video of the wire in question:




If you can't find that Red Yellow Hash wire you can use Circuit #63, the power circuit for the wiper motor. I cover that in the following video:




You are correct that the "I" terminal on the Starter Relay is the resistance wire voltage in Run position. It is full battery voltage while cranking, bypassing the resistance wire for the coil's positive primary terminal. An aside, full battery voltage while cranking is not 12 volts, as during cranking the voltage drops to 9.6 - 10.5 volts or so.

You mentioned carbon fouling plugs. That is not an ignition problem. It is excessive fuel in the air/fuel mixture. That can be caused by any number of problems. The more common problems include the choke coming off too late Either set incorrectly, a failed electric heater for electric choke, incorrect voltage for an aftermarket carburetor's electric choke, float level in fuel bowl is too high, idle mixture screws are set too rich, ruptured or leaking Power Valve is your carb had a Power Valve. Let me know what carburetor you are using and I will try to provide more info.

Thanks for the reply, I actually used one of this guy's videos when I was trying to get 12vt's to the I and S terminals to begin with. I had to disassemble my neutral safety switch (round contact fell off put it back together w/a with a pc of spring) to get 12 vts to the connector behind the motor. Then the signal was weak, so I had to replace the ignition switch on the column. I must have a odd late 73 or aftermarket harness in the engine compartment, it's not diagram friendly. No red wire w/ hash marks 2 red w wt stripes. I was getting 12.6 when I assembled it. I need to configure a way to check the barrel connector of running vts on the one I'm using. I wanna see with my own eyes @ 13.9. Yes the 1st place I looked was the carb. It's a Holley and I've tinkered w/ em for years. I checked the floats, even checking for fluid & ended up turning the rear cw 1/2 a turn since it was just trickling out. Not enough to foul plugs. I checked the mixture screws and reset them more inward 1 1/2 turns out instead of the 1 3/4 turns out that it was. Not enough for fouling. No accelerator pump adj or leaks. The metering block was removed, and it is nice and dry behind the power valve. The choke was removed when I built the carb and the hole that goes into the body w/ no choke assy was plugged.

 

[Hemikiller]

Thanks for the reply. I used this same diagram to wire it. It was very handy. I don't have that red wire w/ the hash marks. Which leads me to believe this is a oddball-late 73

The wire you need is right there in your photo. Some GoJo hand cleaner will clean up your harness, which looks about as 50 year old factory as it gets.

 

[Paul M]

The wire you need is right there in your photo. Some GoJo hand cleaner will clean up your harness, which looks about as 50 year old factory as it gets.


View attachment 67649

Thanks for the reply. I was just on here typing all of this...lmao. It's not going to hurt to give it a try. I'll 1st check for the 12.6. Have any clue what the other red n white stripe wire is that I'm currently using. It's not in the diagrams I looked at. I'm curious if it's resistive while running. I guess If I hook up the other one and it runs better I'll know

 

[Hemikiller]

73 should have the below wires in the engine gauge feed harness.

Red w/white stripe - temperature sensor
White w/red stripe - oil pressure sensor
Red w/light green stripe - OE coil (+)
White w/black stripe - electric choke
Black w/yellow hash - AC clutch
Red w/yellow hash - Anti Dieseling solenoid

There may be others for various emissions items, which vary by the application, Those are usually brown with a colored stripe.

 

[Paul M]

73 should have the below wires in the engine gauge feed harness.

Red w/white stripe - temperature sensor
White w/red stripe - oil pressure sensor
Red w/light green stripe - OE coil (+)
White w/black stripe - electric choke
Black w/yellow hash - AC clutch
Red w/yellow hash - Anti Dieseling solenoid

There may be others for various emissions items, which vary by the application, Those are usually brown with a colored stripe.

Ye it was red w/ a faded line, I stripped back some of the tape and it's a light green stripe. So I've been running through the resistive " I" circuit. Geeze.. that would explain my plugs. I had the Motorcraft electronic distributor n module. I was assuming it ran on a full 12. I ran through 3 days of b.s. after talking w/ the 2 of you this morning, I'm pretty sure that was it. Now to put everything back together and try it. Thanks