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71 "M" code factory tach car

 

The tach does not work, car runs. Going thru an upgrade on engine ignition as discussed in a different thread. PerTronix igniter III with matching coil and relay. What are the options as to the tach?

 

Rich

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Factory tach needs feed via resistance wire. Unresisted 12V feed will damage it.

Frank

1972 Mach 1, 351C-2V, wimbledon white, blue all vinyl luxury

born in Dearborn, grown in the district of San José, spent a lifetime in California, moved to Germany in 2010

Mustang_Mach1_wallpaper_300_150.jpg proud member of clublogozusammen.JPG.59fb4a10d15cfff9ec756235059135b8.JPG

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3 hours ago, Ryunker said:

71 "M" code factory tach car

 

The tach does not work, car runs. Going thru an upgrade on engine ignition as discussed in a different thread. PerTronix igniter III with matching coil and relay. What are the options as to the tach?

 

Rich

Talk to Rocketman on this forum. He converts the factory tach to work with aftermarket systems such as Pertronix, MSD, Holley EFI, etc.  https://www.rccinnovations.com/

Edited by rackerm

1973 H Code Convertible - Medium Copper Metallic - June 8, 1973, Built Ford Marketing Sales Vehicle

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57 minutes ago, Vicus said:

Factory tach needs feed via resistance wire. Unresisted 12V feed will damage it.

Frank

I've been running the factory tach with a bypassed resistor wire for years without issue.  I think the assumption is that it will give up the ghost at some point, but it has yet to happen.

Jason (71 Mach 1, 351C 4V, 4 Spd. Toploader, Grabber Blue)

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I'm making an assumption here as I really have no idea on this issue, but I'm thinking IF the pink resistance wire is still in the circuit and is powered (check with volt meter), the tach itself is somehow not getting voltage, again check with volt meter. I would think that as you are going to run your Pertronix directly off a relay, then you can separate the old coil connection from the system. Does that make sense? 

Geoff.

 I learn something new every day!

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Sounds like the  original resistance wire through the tacho has been bypassed. Isn't the resistor wire for the coil, not the tacho. As I understand it the tacho itself doesn't actually run the coil voltage through it as it is triggered by the voltage pulses from the points. I run MSD ignition with full 12V "through" the factory tacho and it works fine, accurate and steady. The resistance wire has been removed entirely.

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59 minutes ago, Aus73Mach1 said:

Sounds like the  original resistance wire through the tacho has been bypassed. Isn't the resistor wire for the coil, not the tacho. As I understand it the tacho itself doesn't actually run the coil voltage through it as it is triggered by the voltage pulses from the points. I run MSD ignition with full 12V "through" the factory tacho and it works fine, accurate and steady. The resistance wire has been removed entirely.

No, the factory tach is not triggered by voltage pulses, but by current pulses and it has to have the coil signal run through it.  The tach does not require the resistor wire (which is there to protect the points), but it may need a slight recalibration if you bypass the resistor wire.

Let me check your shorts!

http://midlifeharness.com

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As Midlife noted, the tach is current triggered, and reads the pulses resulting from the coil charging and discharging. The power for the coil flows through the tach, it is in-line or in "series" with the coil (+)

Igntion (+) -> tach -> resistor wire -> coil (+)

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.

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8 hours ago, Hemikiller said:

As Midlife noted, the tach is current triggered, and reads the pulses resulting from the coil charging and discharging. The power for the coil flows through the tach, it is in-line or in "series" with the coil (+)

Igntion (+) -> tach -> resistor wire -> coil (+)

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.

Awesome.

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On 9/25/2021 at 12:55 AM, midlife said:

No, the factory tach is not triggered by voltage pulses, but by current pulses and it has to have the coil signal run through it.  The tach does not require the resistor wire (which is there to protect the points), but it may need a slight recalibration if you bypass the resistor wire.

Midlife, I realize that the resistor wire was originally for protecting the points but I also thought that the purpose of the pink wire and 1.5 ohm coil was to keep the current lower in the starter circuit. This was done to protect the tach from eventually burning out.

I thought I read this somewhere, or is this not correct?

 

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The 1.5 ohm resistor is to protect the coil/points. During start the circuit has B+ (battery voltage). 

Since the primary ignition circuit goes through the tach the tach is designed to have that resistor. Without it the tach will read high (and eventually die). If you have a tach reading consistently low you may have an external ballast resistor as well as the OEM 'pink' wire, or one of those MOPAR dual ballast ceramic external resistors that have 1.5 ohms on one side and 5.0 ohms on the other.

 

Bob

 

Tachs, Voltmeters, Headlight kits, Wiper delays and more at

http://www.rccinnovations.com/Images/smlogo.jpg

Rocketman's Classic Cougar (and Mustang) Innovations, LLC

 

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16 minutes ago, TheRktmn said:

The 1.5 ohm resistor is to protect the coil/points. During start the circuit has B+ (battery voltage). 

Since the primary ignition circuit goes through the tach the tach is designed to have that resistor. Without it the tach will read high (and eventually die). If you have a tach reading consistently low you may have an external ballast resistor as well as the OEM 'pink' wire, or one of those MOPAR dual ballast ceramic external resistors that have 1.5 ohms on one side and 5.0 ohms on the other.

 

I'm curious about the inevitable failure, since as I mentioned I've been running the factory tach with a bypassed resistor wire for years without issue.  We all agree the factory tach is nothing more than a glorified current meter.  The more current goes through it, the higher it reads.  Bypassing the resistor wire simply removes some resistance, thereby allowing more current to flow through it.  From my observations, the tach reads approximately 150-200 rpms higher with the resistor wire bypassed.

So my question is, how is bypassing the resistor wire dangerous to the tach at all?  Stepping on the gas pedal, driving around, even adjusting the idle higher, would all have the same effect of subjecting the tach to a current increase.  My point is, all tachs will probably fail at some point.  But I don't believe the resistance wire bypass does anything to shorten its lifespan.

Jason (71 Mach 1, 351C 4V, 4 Spd. Toploader, Grabber Blue)

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6 hours ago, trillizo_y_uno said:

I'm curious about the inevitable failure, since as I mentioned I've been running the factory tach with a bypassed resistor wire for years without issue.  We all agree the factory tach is nothing more than a glorified current meter.  The more current goes through it, the higher it reads.  Bypassing the resistor wire simply removes some resistance, thereby allowing more current to flow through it.  From my observations, the tach reads approximately 150-200 rpms higher with the resistor wire bypassed.

So my question is, how is bypassing the resistor wire dangerous to the tach at all?  Stepping on the gas pedal, driving around, even adjusting the idle higher, would all have the same effect of subjecting the tach to a current increase.  My point is, all tachs will probably fail at some point.  But I don't believe the resistance wire bypass does anything to shorten its lifespan.

I agree with you and disagree with TheRckmn that it will cause to tach to fail prematurely. 

Let me check your shorts!

http://midlifeharness.com

cactus.jpg.92e5d9d8700abc0ed60c8ccb3426248e.jpg

 

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  • 2 weeks later...

Hi all. I've been reading this thread and trying to fix a problem. My factory tach reads too low. I seem to remember swapping out the tach and having the same problem. I'm running a Mallory Unilite distributor. Does anyone know if that is a common problem with Unilite distributors? If the tach is okay, what could be the problem and how do I fix it? Thanks!

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1F03M, Maroon,Blk Int,Wht Top,A/C,Rim Blow,Posi

1F03M, Maroon,Wht/Wht,PW

1F03M, GrbrLime,Wht/Wht,A/C,CA Car

1F03M, Drk Green,Grn/Blk,4-Spd,Tach,R/A,Tilt,Rim Blow,PW

1F03M, Drk Grn,Blk/Blk,4-Spd,Tach,A/C,Tilt,Rim Blow,Mgnms

1F03M, Black,Wht,Wht,Tach,Posi

1F03M, Brown,Ginger,Blk,4-Spd,R/A,Tach

1F03M, Pewter,Blk,Blk,4-Spd,A/C,R/A,Tach,Mgnms

2F03R, GrabberBlue,Wht,Wht,Dcr Grp,Tach

2F03R, Brgt Blue,Wht,Wht,Dcr Grp,Tach

2F03Q, Red,Wht,Wht,Rim Blow,A/C,Tach,Conv.Grp,DcrGrp,Stripes,Mgnms

2F03Q, Rd,Rd,Blk,Rim Blow,A/C,Tach

2F03Q, Brgt Lime(Orig. Gld Glw),Blk,Blk,A/C,P/W,Tilt,Conv Grp,Dcr Grp

2F03Q, Grbr Blue,(Orig. Brt Blue)Wht, Wht,4-Spd,Tach,A/C

3F03Q, (2) Gold Glow(one Blk int,one Ginger int),both 4-Spd,Tach,Dcr Grp,Stripes,TuTone Hood,Frgd Whls

3F03Q,BluGlow,Blu,Wht,4Spd,A/C,Tach,DcrGrp,Stripes,TT,Frgd Whls

3F03Q, Copper,Gngr,Blk,4Spd,Tach,DcrGrp,Stripes,T-T,Frgd Whls

3F03Q, Trpl Wht,A/C,P/W,Tilt,Tach

3F03Q, Spcl Ordr Black,Blk,Wht,Tach

3F03Q, Spcl Order Purple,Wht,Wht,A/C,Tach,Tilt,P/W,Frgd Whls

And 10 more verts, 2 Boss 351s and 1 M-code Mach I! :D

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The distributor type should not have any effect on the tachometer. All the distributor does is turning the coil on and off. On and the outer windings charge up creating a magnetic force and off the magnetic charge collapses around the inner windings, creating the high voltage charge to the spark plugs.

However, because the stock tachometers are operated by the pulsed current flowing through the tachometer to the coil, only the coil positive terminal can be connected to the ignition wire. Aftermarket distributors must be powered separately, or in the case of some distributors and/or ignition boxes an adapter is required to simulate the pulsed current through the tachometer. MSD makes an adapter that also works for other systems, besides MSD boxes.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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Like Don noted, the only wire that should be on the (+) coil terminal is the factory lead. Unfortunately, Mallory, Pertronix and others like to have you power their breakerless units off that lead, which can cause the problem you are having. I suggest disconnecting the Unilite from the coil and powering it temporarily with a jumper lead direct from the battery (+) terminal.

If you find that the tach is now accurate, then you need to power the Mallory unit from another source that is hot in both Crank and Run. 

 

 

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Factory tach has always been broken. Car is at the new house under covers until My daughter and I move in May after she finished high school.

 

I believe my solution is "Rocketman" for the tach rework, and HemiKiller solution for the wire from the carb solenoid to feed the coil. I will be visiting the new house at the end of the month, if time allows looking for the solenoid wire will be at the top of the list. My engine harness I purchased aftermarket over a decade ago, do not have any solenoid on the aftermarket carb, so have really no clue if that " engine - the anti dieseling solenoid wire feed. It's red w/yellow hash marks" power wire is there. Sure hope it is.

 

This forum is great

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12 hours ago, kevken1959 said:

Hi all. I've been reading this thread and trying to fix a problem. My factory tach reads too low. I seem to remember swapping out the tach and having the same problem. I'm running a Mallory Unilite distributor. Does anyone know if that is a common problem with Unilite distributors? If the tach is okay, what could be the problem and how do I fix it? Thanks!

The tach will read low if you have too much resistance in the primary circuit.  Are you running the factory 'pink' wire along with an external ballast resistor, or an aftermarket 3.0 ohm '12V' coil?

A list of Pertronix coils and their resistance ratings is here:  https://pertronixbrands.com/collections/flame-thrower-canister-coils

Normally, running two resistors will effect performance, but high resistance is the main reason both factory tachs would read low.

 

Bob

 

Tachs, Voltmeters, Headlight kits, Wiper delays and more at

http://www.rccinnovations.com/Images/smlogo.jpg

Rocketman's Classic Cougar (and Mustang) Innovations, LLC

 

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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).

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On 9/26/2021 at 6:56 PM, Ryunker said:

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?

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27 minutes ago, 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.bdf74bf7fad5f6888567c732c9d11949.JPG

 

72_gauge_harness.thumb.JPG.ca0bd427c4d65d3276af3344fce88781.JPG

 

 

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Thank you everyone for your replies. I love this site too! Rocketman, I'll check on the coil and let you know. I definitely did use a coil I've never used before with this application.

1F03M, Maroon,Blk Int,Wht Top,A/C,Rim Blow,Posi

1F03M, Maroon,Wht/Wht,PW

1F03M, GrbrLime,Wht/Wht,A/C,CA Car

1F03M, Drk Green,Grn/Blk,4-Spd,Tach,R/A,Tilt,Rim Blow,PW

1F03M, Drk Grn,Blk/Blk,4-Spd,Tach,A/C,Tilt,Rim Blow,Mgnms

1F03M, Black,Wht,Wht,Tach,Posi

1F03M, Brown,Ginger,Blk,4-Spd,R/A,Tach

1F03M, Pewter,Blk,Blk,4-Spd,A/C,R/A,Tach,Mgnms

2F03R, GrabberBlue,Wht,Wht,Dcr Grp,Tach

2F03R, Brgt Blue,Wht,Wht,Dcr Grp,Tach

2F03Q, Red,Wht,Wht,Rim Blow,A/C,Tach,Conv.Grp,DcrGrp,Stripes,Mgnms

2F03Q, Rd,Rd,Blk,Rim Blow,A/C,Tach

2F03Q, Brgt Lime(Orig. Gld Glw),Blk,Blk,A/C,P/W,Tilt,Conv Grp,Dcr Grp

2F03Q, Grbr Blue,(Orig. Brt Blue)Wht, Wht,4-Spd,Tach,A/C

3F03Q, (2) Gold Glow(one Blk int,one Ginger int),both 4-Spd,Tach,Dcr Grp,Stripes,TuTone Hood,Frgd Whls

3F03Q,BluGlow,Blu,Wht,4Spd,A/C,Tach,DcrGrp,Stripes,TT,Frgd Whls

3F03Q, Copper,Gngr,Blk,4Spd,Tach,DcrGrp,Stripes,T-T,Frgd Whls

3F03Q, Trpl Wht,A/C,P/W,Tilt,Tach

3F03Q, Spcl Ordr Black,Blk,Wht,Tach

3F03Q, Spcl Order Purple,Wht,Wht,A/C,Tach,Tilt,P/W,Frgd Whls

And 10 more verts, 2 Boss 351s and 1 M-code Mach I! :D

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Thank you everyone for your replies. I love this site too! Rocketman, I'll check on the coil and let you know. I definitely did use a coil I've never used before with this application.

1F03M, Maroon,Blk Int,Wht Top,A/C,Rim Blow,Posi

1F03M, Maroon,Wht/Wht,PW

1F03M, GrbrLime,Wht/Wht,A/C,CA Car

1F03M, Drk Green,Grn/Blk,4-Spd,Tach,R/A,Tilt,Rim Blow,PW

1F03M, Drk Grn,Blk/Blk,4-Spd,Tach,A/C,Tilt,Rim Blow,Mgnms

1F03M, Black,Wht,Wht,Tach,Posi

1F03M, Brown,Ginger,Blk,4-Spd,R/A,Tach

1F03M, Pewter,Blk,Blk,4-Spd,A/C,R/A,Tach,Mgnms

2F03R, GrabberBlue,Wht,Wht,Dcr Grp,Tach

2F03R, Brgt Blue,Wht,Wht,Dcr Grp,Tach

2F03Q, Red,Wht,Wht,Rim Blow,A/C,Tach,Conv.Grp,DcrGrp,Stripes,Mgnms

2F03Q, Rd,Rd,Blk,Rim Blow,A/C,Tach

2F03Q, Brgt Lime(Orig. Gld Glw),Blk,Blk,A/C,P/W,Tilt,Conv Grp,Dcr Grp

2F03Q, Grbr Blue,(Orig. Brt Blue)Wht, Wht,4-Spd,Tach,A/C

3F03Q, (2) Gold Glow(one Blk int,one Ginger int),both 4-Spd,Tach,Dcr Grp,Stripes,TuTone Hood,Frgd Whls

3F03Q,BluGlow,Blu,Wht,4Spd,A/C,Tach,DcrGrp,Stripes,TT,Frgd Whls

3F03Q, Copper,Gngr,Blk,4Spd,Tach,DcrGrp,Stripes,T-T,Frgd Whls

3F03Q, Trpl Wht,A/C,P/W,Tilt,Tach

3F03Q, Spcl Ordr Black,Blk,Wht,Tach

3F03Q, Spcl Order Purple,Wht,Wht,A/C,Tach,Tilt,P/W,Frgd Whls

And 10 more verts, 2 Boss 351s and 1 M-code Mach I! :D

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