Factory tach inop

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Rich
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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
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/

 
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trillizo_y_uno

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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.

 
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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? 

 

Aus73Mach1

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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.

 

midlife

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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.
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.

 

Hemikiller

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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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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.

 

SteveO_71

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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?

 

midlife

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I have not read that, or am aware of that issue.

 
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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.

 

trillizo_y_uno

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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.
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.

 

midlife

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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. 

 
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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!

 

Don C

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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.

 

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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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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.

 
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