MSD install no tach
 

So I am totally stumped.

I changed from a 2.4 (with a working tach) to a 3.0 and installed a MSD with new blaster coil. Wired it up according to the schematics. At first I tried to wire it without a tach adapter but no tach. Bought the correct tach adapter for the magnetic pickup and still no tach.

I checked to make sure that the tach was wired correctly and that the red wire going to the tach has power and that the black/purple wire was securely connected.

I also checked continuity of the black/purple wire from the engine compartment to the tach.

Any ideas? Could I have done something to mess up the tach? How would I test it to see if it is bad?

This is on a 1972T

Thanks in advance

Did you confirm the tach wire is in the correct location in the 14 pin connector from the engine harness for the '72?

With the MSD, I don't believe the tach signal goes through the 14 pin connector (on mine it doesn't). You also need ground on the tach. You need to snip the correct wire(s) in the MSD box. You need the correct tach adapter (usually 20).

I am not sure about the 14 pin connector. I just ran the purple/black wire to the tach adapter (it is a 20).

I snipped the correct wire on the MSD box

Is there a place on the tach to attach a ground wire?

Common problem. Your 2.4 tach likes a low-level signal. The MSD does provide such a signal but I think it is on a separate output port on the side of the box. In my experience, an adapter should not be needed. If the tach gets accidentally hooked up to other places (like the coil, or an adapter that pushes a high level pulse out) it will be instantly damaged.

I did hook it to the coil initially.

Is there a way to check if the tach is still functional?

Why not describe each connection you made and what you changed on the 3.0 engine harness.
Are you running the 3.0 dizzy with the green wire? It was an original 6 pin connection correct?
The purple/black tach signal wire in the 3.0 harness runs to a different terminal in the 14 pin connector than the '72 does. The 3.0 has it on pin 12 and the 72 has the tach wire from the engine harness on pin 9.

The '72 pin 12 is ground for the alternator , so if you connected the tach output to the purple black wire on the 6 pin CDI connector, it is going directly to ground.

Eww, that could be bad juju. I thought you tried the tach output on the MSD box first. If you are using the -20, didn't you connect it to the box tach output? Connect the purple/black directly to the box tach output and report back.

It really would be helpful to know how everything is hooked up for the 2.4 to 3.0 conversion and the MSD installation.
Can only guess what you did otherwise.

I used the harness that was on the 3.0. Plugging it right into the harness.

Everything worked perfectly except the tach.

Do I need to connect it to a different wire from the harness? What color is the tach wire on a 3.0 harness?

MSD box
Big red to starter
Black to ground
Small red to power by key
Purple and green to distributor
Red and black wires to coil
White is not used

Tach adapter
White to tach connection on side of MSD box
Red to keyed power
Purple to purple/ black wire

Move the purple and black wire in the female 14 pin connector of the 3.0 engine harness from position 12 to position 9.
Just swap the green white in position 9 with it or move the green white to the empty position 7.
The green white wire is from the oil pressure idiot light sender of the 3.0 that you don't need anyways.
The position numbers are on the back of the connector.

The purple/black wire is #9 coming out of the harness.

The purple/black wire is in the female #9 position.

Test the tach wire from the back of the gauge to the output of the tach adapter for continuity with a meter. If it is good, then as someone else mentioned you may have a bad tach if you connected it to the coil.
(Long jumper from meter leads to reach back of tach wire required)

Did that and have continuity

Most 911 tacho 'drive' signals just pull the TD signal wire to ground. Grounding the wire momentarily (or scrape it across a ground to create 'pulses') should cause it to bounce at least. Ignition on obviously.

Well, if you have continuity from the tach adapter purple wire all the way to the tach, and the car runs normally, then I think it is time to test the tach or the MSD unit and adapter.
Do you have access to a working tach you can borrow or the instruments and ability to test the tach?
If not, send it out.

I wouldn't put a another working tacho in the car if the install blew the first one.

Pull tacho, it should have 12V and ground and the input TD wire. Check the first two with a meter, then just ground the input wire, needle should bounce.

I pulled it and checked the 12V wire and it is good.

I will pull it again and check the ground and tach wires

No access to another tach.

Where is the recommended place to send it for a checkup?

I have read that North Hollywood in CA is very good.
(Have them install a voltmeter in the gauge while you are at it...) :)

Try the test of pulse grounding as mentioned by Johnny?

MSD Wiring................
 

When I put an MSD 6A in my SC, this is how I wired it:

CDI Plug......................................MSD Wires
1 System Ground ......................................Black coil wire
2 Tachometer Signal..................................Black wire from left end of MSD unit (Tach )
3 Signal Grd (Green wire shield) Grd............Purple wire
4 Coil Hot +12 volts...................................Orange coil wire
5 Switched +12 volts..................................Small red wire
6 Signal (Green wire center conductor) Pos...Green wire

Heavy red wire +12 volts (wire to starter terminal
Heavy black wire Grd. (wire to chassis and engine)


I did not use the White MSD wire. Hope this helps

There are two diagrams one is:

"Wiring a Current Triggered Tach to Points/Amplifier or Magnetic Pickup" where the purple (violet) wire is not used and the red wire from the tach adapter is the trigger wire (to the tach)

The second says "Wiring with a Magnetic Pickup Distributor and a Voltage Triggered Tach". on this one the red wire from the adapter goes to keyed power and the purple goes to the tach.

I have it wired like the second one. Red to keyed power and purple to tach.

From the tach adapter both show black to ground and white to tach output on the MSD box

Is this correct or should I be using the first setup?

Anyone?

Tom, why don't you try momentarily grounding the TD wire as per my previous post. Simple and free! Let me know what it does,

What wire is the TD wire?

Wire the adapter the other way and see what happens?
"What is the worse that could happen" is our district Engineer's favorite saying! :)

TD, tacho drive. Apart from 12V and ground, it is the only other wire on the back of the tacho. It is clearly marked 'TD' on my SC but not sure about the 2.4

Ok, it is marked different on your car but it's the purple and black wire.

I assume that is the purple wire. I disconnected it at the engine and grounded it.

Nothing happened. The tach didn't move at all.

Provided you had the igntion on I think your tacho is broken.

Reading back through the thread, you had a 50/50 chance of frying it by connecting it to the coil. if you connected it to the '+' terminal then I'm afraid you put around 350V into it which isn't very friendly.

The ignition was on.

Guess I will pull it and send it out for repair.