Factory tach inop

Help Support 7173Mustangs.com:

knn

Well-known member
Joined
Aug 27, 2017
Messages
113
Reaction score
13
Location
San Francisco, CA
My Car
1971 Mach 1 (302)
This is a great thread everyone!  I'm sure all of us have (or will) struggle with this at one time or another.

I have a Pertronix III on our 71, but after reading so many horror stories about nuking your tach, I've never had the guts to run it with the tach connected.  But reading through this, I have hope.

  • Since the tach is inline and basically a current meter, does it introduce resistance in the ignition circuit?  My Pertronix doesn't like anything but full voltage (misses at idles and/or stalls when put in reverse)
  • My motor is an older 302.  It doesn't have an anti-dieseling solenoid.  Any ideas where I find the wiring for the anti-dieseling solenoid?  I have the normal 302 engine wiring harness.  Pictures would be great!!
  • Lastly, from what I've read, the Pertronix III (not I or II) is a multi-spark ignition.  Doesn't this create extra current pulse that will throw off the tach (read high)?

Having @TheRktmn convert my tach is my fallback on getting this all working.  At the moment, we don't want to run any new wires into the engine compartment (we're going for the sleeper, factory-looking build).

 

SteveO_71

Well-known member
Joined
Apr 8, 2015
Messages
476
Reaction score
42
Location
Upstate NY
My Car
71 Mach 1, 351c 4v, C6 trans, grabber blue with white interior
FWIW, you don't need to bypass the resistor wire to run the Pertronix, nor spend $35 on their $2 relay. The 71-73s have a hot lead in Start and Run right at the engine - the anti dieseling solenoid wire feed. It's red w/yellow hash marks. You can power the Pertronix unit itself from there, and leave the coil (+) circuit alone.
Would you happen to know the schematic wire number of the red w/yellow hash marks wire that is mentioned?

 

Hemikiller

Tech Advisors
Staff member
Moderator
Joined
Jul 12, 2010
Messages
3,562
Reaction score
770
Location
Killingworth, CT
My Car
71 Mach 1
65 coupe
SteveO_71 said:
Would you happen to know what wire number in the schematic, the red w/yellow hash marks is ?
Circuit 640, powered by the 14A #4 fuse. 

The 71s have it as a separate bullet type connector at the firewall, which will connect to a jumper lead to the solenoid on the carb. 72-73 will have other plug styles that connect to the engine gauge feed harness. The repop harnesses use a red w/yellow stripe wire. 

71_TPS_WIRING.JPG

72_gauge_harness.JPG

 
Last edited by a moderator:

knn

Well-known member
Joined
Aug 27, 2017
Messages
113
Reaction score
13
Location
San Francisco, CA
My Car
1971 Mach 1 (302)
Thanks @Hemikiller!   That was perfect.  You gave me all the details I need on how to make it look original

 
Joined
Dec 29, 2011
Messages
563
Reaction score
74
Location
Birmingham, AL
So, I checked my coil. It's an MSD Blaster 3. Can't remember why I ended up with that one with a Mallory Unilite, but the car does run good. Here are the specs:





Brand


MSD




Peak Current


140 mA




Primary Resistance


.7 OHMs




Product Type


Coil




Secondary Resistance


4.5K OHMs






 So could the coil be the cause for my tach under- reading? If so, can someone recommend a coil that can give the car the most juice while not interfering with the tach?

Thanks!

Kevin. 

 

Don C

Fords Forever
Joined
Jan 1, 2012
Messages
8,111
Reaction score
868
Location
Springfield, OR
My Car
1971 Mustang Sportroof M code
The point ignition systems used in cars with 12 volt ignition systems had to have the current draw reduced across the points to prevent arcing, pitting and early destruction of the contacts on the points. In order to reduce the current resistance was introduced into the ignition system. This was accomplished by either an external ballast resistor or a resistor wire in the circuit. Ignition coils are also available with built in resistance, which do not require a ballast resistor or resistor wire.

The total resistance used, that reduces the arcing and pitting on the points and also provide an ignition system that supplies enough voltage to create a spark in the spark plug gap, is around 3 ohms.

The currents of the various resistances used in ballasts and coils are shown in this table. The engine stopped condition is with the points closed or negative side of the coil grounded:

Ignition Resistance.jpg

[SIZE=11pt]Because the tachometer in older Ford cars[/SIZE][SIZE=11pt] is also inlin[/SIZE][SIZE=11pt]e in the ignition cir[/SIZE][SIZE=11pt]c[/SIZE][SIZE=11pt]uit the current flowing through the igni[/SIZE][SIZE=11pt]tion als[/SIZE][SIZE=11pt]o flows through the tachometer. The tachometer has a small transformer that the ignition system current flows through and the pulses created by the coil turning on and off creates a small pulsed voltage that the circuitry in the tachometer converts into a variable voltage that moves the tachometer needle. This small transformer was designed to function with the 2.7 to 3 ohms found in the typical older Ford ignitions and more importantly, the current draw. A high current draw may destroy the transformer.[/SIZE] Tach Wiring-c.jpg

While the electronic aftermarket ignitions don’t have points, and can be designed to run on battery voltage, those with factory tachometers have to be aware of the loads placed on that little transformer. There are two ways to get around this limitation and not fry the transformer. One is to get the MSD adapter, Part No. 8920. The other is to get Rocketman to convert your tachometer to a modern three wire design, eliminating the need for adapters or worries about if your system will function properly.

Realistically, all that is needed for our cars is a properly functioning stock coil, which supply around 25,000 volts. Our cars, with a 0.35" gap and 10:1 compression, and not fouled with oil or carbon, require about 12,000 volts to fire the spark plugs. The wider gaps, that came about as car makers needed to improve the efficiency of the engines, required higher voltage coils for the wider spark plug gaps.

There is a lot of advertising about 30,000+ volt coils. In general these voltages are only produced in controlled or ideal conditions.  An interesting thing about coils is that they just produce enough voltage to bridge the spark plug gap. The total potential voltage is governed by the saturation current in the primary windings, the winding ratio between the primary and secondary windings, and the iron core. The primary side is charged when the points are closed (or connected to ground by an electronic ignition). As soon as the points are opened (or the ground removed) the magnetic field created by the primary windings collapses almost instantly through the iron core and secondary windings creating the high voltage necessary to bridge the spark plug gap. As soon as the spark is created the secondary field is discharged and the voltage does not get any higher.  So, if you have a 40,000 volt coil and your sparkplugs only require 15,000 volts to fire the remaining 25,000 volts is wasted, along with the energy required to charge the coil windings.

 
Last edited by a moderator:
Joined
Dec 29, 2011
Messages
563
Reaction score
74
Location
Birmingham, AL
Hi Don. So regarding the specs on the coil I posted above, the primary resistance is low (.7 OHMs). What is the significance of the secondary resistance (4.5K OHMs)? Rocketman said some coils have increased resistance that causes the tach to under-read. I'm still trying to figure out why my tach under-reads. It doesn't have any kind of resistor, as far as I know, other than the factory red resistor wire going to the tach. Thanks.

 

Don C

Fords Forever
Joined
Jan 1, 2012
Messages
8,111
Reaction score
868
Location
Springfield, OR
My Car
1971 Mustang Sportroof M code
The secondary resistance has no effect on the operation of the tachometer. It is the resistance of the secondary windings, which provide the high voltage pulses to the spark plugs.

 

Don C

Fords Forever
Joined
Jan 1, 2012
Messages
8,111
Reaction score
868
Location
Springfield, OR
My Car
1971 Mustang Sportroof M code
Have you checked to make sure that nothing else is connected to the positive terminal on the coil, or to the ignition wire (red with light green stripe), even at the starter solenoid?

 

Hemikiller

Tech Advisors
Staff member
Moderator
Joined
Jul 12, 2010
Messages
3,562
Reaction score
770
Location
Killingworth, CT
My Car
71 Mach 1
65 coupe
Hi Don. I do have the carb choke hooked up to that wire. 
Nothing should be on the coil (+) post but the Red w/Light Green stripe wire.  Take the choke wire off and see if you tach is more accurate. 

An electric choke should be hooked to the stator terminal of the alternator, or a relay triggered by the stator terminal. 

 

Don C

Fords Forever
Joined
Jan 1, 2012
Messages
8,111
Reaction score
868
Location
Springfield, OR
My Car
1971 Mustang Sportroof M code
Nothing should be on the coil (+) post but the Red w/Light Green stripe wire.  Take the choke wire off and see if you tach is more accurate. 

An electric choke should be hooked to the stator terminal of the alternator, or a relay triggered by the stator terminal. 
+1. The resistor in the choke not only adds to the load and current flowing through the tachometer, it can also act as an inductor, due to it being wound around a bimetallic spring, smoothing out the pulses that the tachometer senses. 

The advantage to it being connected to, or triggered by, the stator is the only time the choke receives power and begins opening is when the engine is running. If you try to start it on a cold morning, and it doesn't start right away, and it is connected to a key on source, the choke begins to open as soon as the key is turned on. This may add to the cold morning starting problem due to lack of enrichment and no fast idle.

 
Joined
Dec 29, 2011
Messages
563
Reaction score
74
Location
Birmingham, AL
Thanks Don. That makes a lot of sense. Do you know the color of the stator wire on the alternator? Also, regarding the voltage of the coil, if I'm running like a 45,000 volt coil and I really increase the gap of the spark plugs, wouldn't that cause a much hotter spark which would burn the gas more effectively? It would seem that there would be some kind of advantage to running a coil with more volts. 

 

Don C

Fords Forever
Joined
Jan 1, 2012
Messages
8,111
Reaction score
868
Location
Springfield, OR
My Car
1971 Mustang Sportroof M code
That depends on whether you have the original gauges, or not. With the gauges you will not have a wire connected to the stator terminal ln the alternator. Without gauges the stator wire is white with a black stripe.

You can see the differences in the file in this thread

Alternator and Regulator Wiring - Electrical, Lighting & Electronics - 7173Mustangs.com

You can also download the wiring diagrams for your car in the Wiki section of this forum.

 
Joined
Dec 29, 2011
Messages
563
Reaction score
74
Location
Birmingham, AL
Awesome. So mine has the factory gauges. It sounds like a fairly easy process to run a lead from the stator terminal to the choke. Thanks! Any thoughts on my question about widening the spark plug gap if I have a higher voltage coil?

 

Don C

Fords Forever
Joined
Jan 1, 2012
Messages
8,111
Reaction score
868
Location
Springfield, OR
My Car
1971 Mustang Sportroof M code
You might be able to see some benefit from a wider gap, but to do so you should find spark plugs that are designed for a wider gap and that are in the correct heat range. It will be difficult to widen the gap and keep the ground electrode square with the center electrode and across the center electrode.

Unless someone else has successfully done it, it will likely require some experimentation.

 
Top