Factory Tach in non tach car with MSD tach adapter

[TheDude]

Hey guys and Happy New Year!

I'm trying to get a factory tach in my non tach 72. I have a Mallory Hyfire VI-A ignition. Trying to see if i can get this working without sending it out to be converted as I already have an MSD 8920 tach adapter. Seems like it should work as I believe the tach signal is the same coming out of the ignition. 

I'm having a hard time understanding how to hook up the adapter to the tach itself. The instructions are here https://documents.holley.com/8920.pdf but it's just not clear to me how I need to hook up the adapter to the two wires on the factory tach. 

Can anyone point me in the right direction?

Thanks!

 

[TheRktmn]

The tach has two wires, one from the ignition switch and one to the coil + side.

The tach wire that originally went to the coil + terminal connects to the 8920 RED wire.

The 8920 BLACK wire goes to ground.

The 8920 WHITE wire goes to the Mallory tach signal port.

Unless you are running a crank trigger the 8920 PURPLE wire is not used.

 

[Carolina_Mountain_Mustangs]

Hey maybe I will try to get mine working today had same issue with their instructions. Not written for a simple guy like me to understand, lol.

 

[TheDude]

The tach has two wires, one from the ignition switch and one to the coil + side.

The tach wire that originally went to the coil + terminal connects to the 8920 RED wire.

The 8920 BLACK wire goes to ground.

The 8920 WHITE wire goes to the Mallory tach signal port.

Unless you are running a crank trigger the 8920 PURPLE wire is not used.

Thanks for the quick feedback.

So first question is, do you know which wire is which when looking at that the tach? Since I have the bare tach, I don't know which wire goes where. I have two black wires on a plug with a male and female side. Do you know which is which?

Sounds like you are saying one of those wires goes a switched power source and the other goes to the 8920 Red wire, is that right? That makes more sense, I was assuming one side was ground. 

Thanks again for the help.

 

[midlife]

The tach has two wires, one from the ignition switch and one to the coil + side.

The tach wire that originally went to the coil + terminal connects to the 8920 RED wire.

The 8920 BLACK wire goes to ground.

The 8920 WHITE wire goes to the Mallory tach signal port.

Unless you are running a crank trigger the 8920 PURPLE wire is not used.

Thanks for the quick feedback.

So first question is, do you know which wire is which when looking at that the tach? Since I have the bare tach, I don't know which wire goes where. I have two black wires on a plug with a male and female side. Do you know which is which?

Sounds like you are saying one of those wires goes a switched power source and the other goes to the 8920 Red wire, is that right? That makes more sense, I was assuming one side was ground. 

Thanks again for the help.

A factory tach has only two wires with a male/female plug on it.  The male side accepts the red/yellow or red/green wire; the female side goes to a pink or red/green thick resistor wire.  Female plugs are almost always outgoing signals; male plugs are almost always incoming signals.  The resistor wire signal passes through the fuse box connector and ends up as a red/green wire on either a 3 or 4 pin (depending upon year) engine gauge feed harness plug on the headlight harness.  The engine gauge feed takes that signal to the + side of the coil as a red/green wire.  2 black wires ending in a 2 pin male/female plug indicates someone has monkeyed with the tach itself and is re-wired.  All bets are off if it was done correctly.

A non-factory tach has 3 wires: ground, ACC or run-only power, and tach signal.  The tach signal needs to go to the negative side of the coil.

 

[TheDude]

Thanks for that. For the moment, I am just trying to hook this up leaning on the fender and not in the dash yet so nothing to do with existing wiring in the harness. I just want to make sure everything works before tearing the dash apart.

So from here, I have two wires on the tach, what does each go to? One I presume goes to the red wire on the tach adapter. Is that right? If so which wire on the tach should go to the tach adapter, male or female? The other wire, should that be a standard 12V switched + or is that one of the voltage regulated 5-6V sources like the water temp or something else entirely?

Thanks!

 

[midlife]

Thanks for that. For the moment, I am just trying to hook this up leaning on the fender and not in the dash yet so nothing to do with existing wiring in the harness. I just want to make sure everything works before tearing the dash apart.

So from here, I have two wires on the tach, what does each go to? One I presume goes to the red wire on the tach adapter. Is that right? If so which wire on the tach should go to the tach adapter, male or female? The other wire, should that be a standard 12V switched + or is that one of the voltage regulated 5-6V sources like the water temp or something else entirely?

Thanks!

One wire on the tach goes to the ignition switch, which is RUN-only; that line should hook up with the male lead coming out of the tach.  After passing through the tach, the female side of the tach plug should end up (eventually) going to the tach adapter.  The factory 2 wire tach is a current-sensing device, and must be in-line with the ignition switch and (usually) the + side of the coil.  There is no ground nor power to the tach; it is not at all like all other gauges.  Since you are using MSD, follow their instructions for how to hook everything up in the engine compartment.  The lead that normally goes to the + side of the coil no longer goes there, but goes instead to the tach adapter.  MSD provides its own wiring to the + side of the coil.

 

[Don C]

To answer your question about the voltages, at the tachometer connector the voltage is full battery voltage from the ignition switch. The resistor wire is between the tachometer and the firewall and reduces the voltage to the coil to around 8 volts when the engine is running. The actual voltage depends on if the points (or electronic ignition) are open or closed (grounded). The voltage reduction across the resistor wire is a function of resistance and load, with the load being the resistance of the primary windings in the ignition coil.

 

[TheDude]

OK, so looks like 12V switched to the male lead on the tach and red tach out from the 8920 goes to the female side. Thought I tried that but wasn't getting anywhere. I'll try again.

Thanks for the info and the feedback on voltages as well Don.

 

[TheDude]

Hmm, either still not doing something right, or the tach / tach adapter is broken. FWIW, I am pretty handy with electrical stuff in general, just haven't done much if anything with vintage gauges.

 

[Don C]

Does the engine run?

Have you verified that the tachometer is receiving 12 volts at the connector?

If so, there is a strong probability the tachometer is bad.

You should be able to test the adapter, it will produce ground pulses to the tachometer. Connect the positive lead of a voltmeter to the battery, and the negative lead to the output of the adapter, see if you're getting any fluctuating voltages.

You can also connect an ohmmeter to the two leads on the tachometer connector, you should get about a one ohm reading there.

 

[TheDude]

OK that makes sense. Yes, the engine runs. The testing like you suggested from the battery to the tach output of the adapter gets me steady voltage. So I'm thinking the adapter is shot

Tested the tach and getting around an ohm give or take so I have hope there. A buddy of mine has another spare 8920 that I'm going to try out this weekend.

Thanks for the help getting it this far.