Topic: Tachometer Failure

[Building 3]

I have an original Rally Pac. My problem is that some of the time it works very well, other times it just shows 0 RPM.  I went for a 30 mile drive last weekend and it was on 0 the whole trip.  At other times it will respond to engine rpms but as if it was on a different scale:  idle shows at 200 rpms, revving might bring the RPMs to 2000 but that is about it.  Any thoughts?  Has anyone run into this sporadic problem? Thanks. 

[jwc66k]

Welcome to the club. I used to joke that the originality of a Rally Pac could be determined by the clock not working. Add the tach to the list.
Jim

[69cobrajetrugae2]

How is the tach grounded in the circuit?  Run a wire from the negative of battery to the body of the rally pac and look for a change.

[midlife]

The tach does not require a ground, sorry. 
The failure mode you describe indicates a fault at the transformer secondary outputs to the actual guts of the tach gauge.  When the tach fails but the car still continues to run just fine, the primary windings of the transformer are still working, allowing voltage and current to be transmitted through the tach. The transformer takes that voltage and either raises or lowers it so that it is suitable for the gauge mechanism itself.  Either the secondary windings, those contacts to the gauge mechanism, or the gauge itself has a bad connection (corrosion?) somewhere inside. 

[preaction]

Is it possible for a car with a Petronix installed to have a similar effect ?

[midlife]

Is it possible for a car with a Petronix installed to have a similar effect ?

Yes, depending upon which Petronix version is used, as it changes the resistance in the circuitry and thus the voltage/current relationship that the tach is looking for.

[Building 3]

Thanks for the responses.  It sounds like it is a corrosion or connection problem. I guess it needs another rebuild. Oh well... the car is as original as I can get it so I don't want to go with a reproduction. Yes, when I bought it the clock was not working, now the clock works, at least for now!

[rocket289k]

Thanks for the responses.  It sounds like it is a corrosion or connection problem. I guess it needs another rebuild. Oh well... the car is as original as I can get it so I don't want to go with a reproduction. Yes, when I bought it the clock was not working, now the clock works, at least for now!

Be careful who you choose to rebuild it (if you go that route).  A lot of folks either do not rebuild them anymore (can't source the parts) or convert it to a 3 wire installation (non-stock).  NOS tachs (new in box) still come up for sale from time to time at a somewhat reasonable price (although everyone has their own definition of reasonable).  I was faced with the same decision 3 years ago and it took about 18 months to find a tach that someone didn't try to charge $400+ for.

Ron

[69cobrajetrugae2]

Thank you Midlife for the left/right glove smack.

Well perhaps I deserved only half a smack since every electrical circuit requires a ground. Hot key into tach out tach in resistor wire to positive side of coil primary windings, then to secondary windings, then to spark plug, then to negative cable lug on engine, then to negative of battery.

Since the tach is an ammeter and the transformer samples the current flow and the synchronized "guts" raise and lower the tach needle to the pulse frequency of the coil, if the coil which is grounded to the spark plugs was removed from the circuit,  the current flow would cease and the tach would not work.

I incorrectly ASSUMED that the rally pac was an add on tach affair with a positive and a ground and a pulse signal from the distributor side of the coil.

[J_Speegle]

I incorrectly ASSUMED that the rally pac was an add on tach affair with a positive and a ground and a pulse signal from the distributor side of the coil.

Yes Ford tachs (and likely others I've just never had the please of running across) is an inline arrangement. Use to buy these from owners I would run across all the time. First time it was such a surprise (20 something kid at the time chasing down owners to buy stuff off their cars or the car) when I removed the rally pak and the car would not run 

Later ran across those same issues when the tach would fail and the car would not start    So different from the typical hot rodder after market style I had grown up with they caught me of guard

[69cobrajetrugae2]

The ignition switch has 12V in and in the on position 12V out to the fuse box and to the tach and or coil.

 In either a tach or non tach car is the resistor wire 62 inches long at 1.5 to 1.6 ohms? If this is true, does the resistor wire loop by the glove box and connect to the interior firewall connector then terminate halfway down the intake manifold to another connector which runs the temp battery side of coil and oil pressure gauge and starter relay for full 12V bypass for cold starting?  This wiring arraignment isn't clear to me and clarification would be greatly appreciated.

There are rumors of the ignition switch and or it's wiring burning which makes sense if an overcharging condition exists or normally due to the load across the resistor wire combined with a poor connection.  What would be the normal voltage range measured at the battery side of the coil to ground on a running engine at various RPM speeds and what would be the amperage as measured in series to the load at the same time?

[preaction]

I believe the resistor wire (pink) is a good bit shorter at 12" ?

[preaction]

Randy, is it possible to run a Petronix and have the factory tach work properly ?

[midlife]

The ignition switch has 12V in and in the on position 12V out to the fuse box and to the tach and or coil.

 In either a tach or non tach car is the resistor wire 62 inches long at 1.5 to 1.6 ohms? If this is true, does the resistor wire loop by the glove box and connect to the interior firewall connector then terminate halfway down the intake manifold to another connector which runs the temp battery side of coil and oil pressure gauge and starter relay for full 12V bypass for cold starting?  This wiring arraignment isn't clear to me and clarification would be greatly appreciated.

There are rumors of the ignition switch and or it's wiring burning which makes sense if an overcharging condition exists or normally due to the load across the resistor wire combined with a poor connection.  What would be the normal voltage range measured at the battery side of the coil to ground on a running engine at various RPM speeds and what would be the amperage as measured in series to the load at the same time?

The pink resistor wire loops through the underdash harness (I've never measured its length) and ends up at the underdash side of the firewall plug for the engine gauge feed harness.  This sticks out just a couple of inches from the firewall itself.  It is entirely self-contained within the underdash harness.  The wire from the starter solenoid mates with the pink resistor wire at the underdash side of that plug.  The purpose of this wire is to provide a full 12V when cranking rather than a much reduced voltage due to battery voltage losses plus pink resistor losses when cranking. 

Expected voltage at idle on the + side of the coil while running is typically 10.5-11.5 V, although 9V is often stated in the FSM's.  I've not measure current going to the coil at the battery side of the coil, but that would be interesting to do.  The problem with calculating it is that the resistance of the pink resistor wire changes (increases) with increasing current as it heats up, so the resistor wire acts as a self-limiting choke for current. 

[midlife]

Randy, is it possible to run a Petronix and have the factory tach work properly ?

Yes, but only if you use a Petronix 1 module.  I did so successfully on Midlife for 8 years without a problem.  It has a very slight effect on the factory tach, lowering RPM's by 1-200 rpms.