Following. I have had the same question for years. This weekend I'll try and add some information.

 

Adding a data point - the 68 Cougar wiring diagrams show the base car wiring, which was speedo/fuel/temp gauges only, with oil and alternator warning lights. Base Cougar and Mustang w/tach share the same alternator harnesses. WCCC has the service diagrams on their site for a free download.

Free Resources at West Coast Classic Cougar :: The Definitive 1967 - 1973 Mercury Cougar Parts Source

Free Resources at The Definitive 1967 - 1973 Mercury Cougar Parts Source

secure.cougarpartscatalog.com

 

Adding a data point - the 68 Cougar wiring diagrams show the base car wiring, which was speedo/fuel/temp gauges only, with oil and alternator warning lights. Base Cougar and Mustang w/tach share the same alternator harnesses. WCCC has the service diagrams on their site for a free download.

Thanks Hemikiller, this may be our best source. Unfortunately the wire diagrams are in "branched- highway" format. All the wires go into a highway and you have to guess where they come out. Since this is on multiple pages, it takes a lot of effort, and unless you already know how it is connected, they aren't of much use. But fortunately I was able to condense the charging circuit for the 68 Cougar below. The 67 Cougar was incomplete, but they appear to have added the information left off the 67 onto the 68 drawing. And as usual, there are obvious mistakes on the diagrams. Once all of this is accounted for, it appears that the 67 and 68 Cougars have the same charging systems.
Hopefully this lines up with the Mustang:

 

Here's the wiring diagram with tach for a '69, it might help a bit.

 

I done some more research, including downloading WCCC’s free wiring diagrams and looking into my car’s wiring colors.

First, WCCC’s diagrams are from the factory. But they are clearly not correct. Surprised? Following the wires, one side of the Alt light (#30C, black / green strip) goes to power and the other side (#643, yellow / black strip) goes to the instrument panel connector, then to the four-pin connector in the engine compartment, and then it becomes #32 (red / blue stripe) which is the starter wire to the solenoid! Oops. I think #643 is connected to #904 (green / red stripe) at the terminal pin.

@Mach1 Driver 's diagram seems correct, but the wire colors are different than my Mustang. See photo.

One other missing wire is the one going to the ‘PO’ connection on the ignition switch that grounds the Alt and Brake dash lights, so you see that the bulb works when starting the car.

 

I done some more research, including downloading WCCC’s free wiring diagrams and looking into my car’s wiring colors.

First, WCCC’s diagrams are from the factory. But they are clearly not correct. Surprised? Following the wires, one side of the Alt light (#30C, black / green strip) goes to power and the other side (#643, yellow / black strip) goes to the instrument panel connector, then to the four-pin connector in the engine compartment, and then it becomes #32 (red / blue stripe) which is the starter wire to the solenoid! Oops. I think #643 is connected to #904 (green / red stripe) at the terminal pin.

@Mach1 Driver 's diagram seems correct, but the wire colors are different than my Mustang. See photo.

One other missing wire is the one going to the ‘PO’ connection on the ignition switch that grounds the Alt and Brake dash lights, so you see that the bulb works when starting the car.


View attachment 944198

Thanks much!!
Your description of the wire diagram mistake is exactly what I found. The diagrams are rife with that sort of thing, and trying to figure that out on a "branched highway diagram" is definitely no fun. So there are two differences between your car and my 68 Cougar diagram: 1) The field wire from the regulator to the alternator is orange instead of white, and 2) You said: "...goes to the instrument panel connector, then to the four-pin connector in the engine compartment...". The diagrams appear to show the 4 pin plug in the cabin, but you're saying it is in the engine bay, so then it passes through a grommet (with 15 ohm wire 904) in the firewall to the cabin- is that correct? This differs from my 69, where they pass though the firewall disconnect plug to go from the engine bay to the cabin, so I have to rely on pictures or descriptions like yours to identify this detail. I assume the 4 pin connector is also in the engine compartment where 37 BLK/Y passes through the firewall grommet?

 

Like many, I wanted to upgrade my 68 Mustang GT J code fastback to a factory tach. I found a tach cluster and bought every harness needed (alternator, headlight, under dash and even a new cluster harness. Good news, the tach works fine, bad news is the alternator light doesn't work and the car won't charge. I changed the alternator, regulator and even the ignition switch to no avail. When the cluster was out, I upgraded to LED bulbs and in trouble shooting (reading these forums) realized the alternator light needs to be a 1445 bulb. I had read the bulb should provide the 15 ohms of resistance but the one I put in only measured about 5 ohms. Going to buy a new bulb at the parts store but if it's also less than 15 ohms, I'm at a dead end. Any thought from this group would be greatly appreciated.

 

You could install a 3G alternator and eliminate the regulator. I did this on my 70 and the alternator light does not light up as brightly as it should. But the light does work and the charging system works perfectly.

 

This is the closest I could find. A 1895 is 3.78w, 12v, .315a, 38ohms. There was nothing specific listed under Mustang in the shop manual.

 

View attachment 971218
This is the closest I could find. A 1895 is 3.78w, 12v, .315a, 38ohms. There was nothing specific listed under Mustang in the shop manual.

About right. The Sylvania 1895 produces 25 lumens which is just a shade under 2 candelas, with 1 candela = 12.66 lumens.

Just remember to add a bridge resistor across the bulb to provide an alternate current path in case the bulb burns out so the regulator will still receive juice.

 

maybe this 68 diag will help.
but have a 67 diag somwhere, be close

 

Thanks for the replies. CJ Pony Parts shows the bulb to be 1895 so that will be my next step. Should I not assume that a new harness (specifically for a 68 with factory tach) would include 15 ohm "bridge" resistor? If not, it's looks easy enough to add a 15 ohm resistor across the two wires (on the cluster harness) that go to the bulb socket.

 

The resistor may be part of the instrument cluster. I have never seen a picture of it or heard much about it. Maybe someone has an actual picture of it.

 

I just did this mod shortly ago. Simple to add a bridge parallel circuit with the 15 ohm resistor behind the cluster, then you are covered. If the ALT light burns out, the resister excites the alternator and it continues to charge.
I installed LED's in the dash bulbs, and was going to do same for the ALT light and then for once my common sense kicked in and said, hey, WAIT! You need the resistance of the incandescent bulb to make it all work. Duh....

 

I installed LED's in the dash bulbs, and was going to do same for the ALT light and then for once my common sense kicked in and said, hey, WAIT! You need the resistance of the incandescent bulb to make it all work. Duh....

You still CAN, but need to add resistance in series with bulb, too. So, stick a 15 Ohm resistor in series with the bulb and a 30 Ohm in parallel.

 

Thanks Woodchuck, that's good, I'll keep that in mind. Unfortunately I just put my cluster all back together last week, and covered the car for winter sleeping. Not eager to take it all out again.

 

Just bought a package of 1895 bulbs (2). I measured the resistance and both read 5 ohms, right out of the package so I wasn't surprised when the light didn't light and battery didn't charge. The next step will be to add the resistors, a 15 ohm in series and a 30 ohm in parallel with the bulb but where does one buy these particular value resistors?

 

I'm guessing those must be LED bulbs? An incandescent 1895 should have around 40 Ohms resistance. Regardless of the low resistance, it should have lit with the key "on" and engine not running. Make sure you use LED's that are NOT "polarity-sensitive" as, being a diode, they may only permit current flow in one direction.

 

You don't need to be so scientific with light bulb Ohm readings. All the regulator needs is a little bit of tungsten to heat up and it will turn on.

 

Sorry, definitely incandescent bulb and both measure 5 ohms. I only mention the value because most of the posts clearly state there needs to be at least 15 ohms

 

I'd check either your Ohmmeter accuracy or the bulb itself. A Sylvania 1895TP bulb is a 3.78W 12V 25 Lumen bulb. Ohms Law says that a 12V 3.78W bulb has a resistance of 38.0924 Ohms. That's a "law of nature". To have a 12V bulb with only 5 Ohms resistance would mean the bulb would generate 28.8W. That's enough to burn your finger.

 

Hi Bart. Certainly not here to debate the laws of nature but what I can say is the unpowered bulb was measured with 2 different calibrated meters. Not exactly sure but if that circuit goes through the voltage regulator on the back of the cluster, the voltage would be reduced to 5V. If you plug that number (5) into your ohms law calculator, the resistance value comes out to 6.6 ohms, much closer to what I'm measuring. I do appreciate your thoughts on this but from everything I've read, 15 ohms in series with the bulb and 30 ohms in parallel will be the next step. As much as it kills me to buy 2 resistors at less than a dollar each and pay $15 for shipping, this is small change when compared against the amount spent on adding the factory tach. I will definitely keep you posted.

 

The IVR only drives the gauges. The resistance of the bulb is irrespective of the circuit. If the bulb is rated at 12V and 3.78W then the resistance HAS to be 38.9524 Ohms. A change in the supply voltage will only affect the current load and wattage... the resistance RATING is a constant. The MEASURED resistance will increase with heat.