Connecting Tach to MSD

[ojh]

I'm working on a GT500 that has been fitted with MSD ignition, how did the original tach connect to the coil?  Was it connected to the positive or negative side of the coil?
Thanks, Oj

[Bob Gaines]

I'm working on a GT500 that has been fitted with MSD ignition, how did the original tach connect to the coil?  Was it connected to the positive or negative side of the coil?
Thanks, Oj

The post on the coil marked 'BAT"

[ojh]

That would be the +12v then, the tach would be a current triggered tach.  Thanks for that, I was guessing voltage triggered with the tach wire going to the ground side.
I appreciate you taking the time to help, Thanks
Oj

[Bob Gaines]

That would be the +12v then, the tach would be a current triggered tach.  Thanks for that, I was guessing voltage triggered with the tach wire going to the ground side.
I appreciate you taking the time to help, Thanks
Oj

When i did one maybe 15 years ago I remember having to get something called a MSD tach adapter. The tach worked after adding that. Don't press me for more details on the MSD system as there are others more versed and it has been too many years since I messed with it and i only vaguely remember . Also systems may have changed from when I did one and now.

[JD]

There are two MSD Tach adaptors be sure to get the right one for your car/set-up.

[zray]

I'm working on a GT500 that has been fitted with MSD ignition, how did the original tach connect to the coil?  Was it connected to the positive or negative side of the coil?
Thanks, Oj

The '65 / '66 GT350's original tachometers did not attach "directly" to either the negative or positive coil terminal. The tach was wired inline between the ignition switch and the coil + terminal.

Isn't the '67 wired the same way ?

The mds tach adaptor needed to make the tach read correctly is the 8920 . You need that if using an MSD box or a MSD ready-to-run distributor, which doesn't require an MSD box, but can be used with one as well.

Z

[Bob Gaines]

I'm working on a GT500 that has been fitted with MSD ignition, how did the original tach connect to the coil?  Was it connected to the positive or negative side of the coil?
Thanks, Oj

The '65 / '66 GT350's original tachometers did not attach "directly" to either the negative or positive coil terminal. The tach was wired inline between the ignition switch and the coil + terminal.

Isn't the '67 wired the same way ?

The mds tach adaptor needed to make the tach read correctly is the 8920 . You need that if using an MSD box or a MSD ready-to-run distributor, which doesn't require an MSD box, but can be used with one as well.

Z

The 65 GT350 tach was attached "directly to the coil terminal by way of separate run red wire outside the Ford harness to the coil.The 65 tach was done different then the 66/67 tachs that ran through the Ford wiring . Although they ran through the resister wire inline the Ford harness, the 66 and 67 tachs ended up connected to the coil Bat/+ terminal all of the same .

[zray]

Thanks for the clarification on the. '65's.

Z

[ojh]

Thanks for the responses, clarification and confirmations.  I feel a little more confidant connecting it now, I have the MSD #8920 to connect into the harness.  I'll ring out the circuit and wire the adaptor in this morning but can't fire the motor until I do some maintenance on the valve train. 
I'll report back with success or failure, Thanks again  Oj

[Bob Gaines]

Thanks for the clarification on the. '65's.

Z

No problem . 65 tach is the odd duck and easy to confuse the wiring given the typical Mustang Shelby wiring.

[ojh]

Why can't anything be simple?  I decided it was best to remove the dashpod and benchtest the signal rather than go thru a mess of work and not know where to troubleshoot if it didn't work.  I have the dashpod out and on my bench where I have a Sun504, I stuck a distributor in the machine, connected an MSD box and followed the instructions connecting the MSD 8920, it didn't work.  The problem might be the pigtails are swapped coming out of the tach, it has a Red wire and a Green wire.  I presumed the red to be +12v, the green to be tach signal, however, I see stamped into the tach mounting bracket adjacent to where the red wire exits it is stamped 'white red st' and adjacent to the green wire 'yellow black st' 
Can anybody confirm the Red wire is the 12v supply?
I tried continuity readings, Red to Ground, and got about 1 megaohm.  Geen to Ground is 0 ohms.
Again, what I have for schematics, and used, is the Mustang Electrical Assembly manual.
Thanks, Oj

[ojh]

Got it.  The tach Red is +12v, the tach Green will connect to the Violet from the MSD adaptor.  On this harness that would be wire #31, White w/Red tracer, in other words its replaced the oil temp gauge on a normal Mustang.
The tach reads a little low RPM wise. 
The adapter box connects as if it is a voltage triggered tach as if the original trigger came from the Negative side of the coil. 
I got brave and connected it every way possible and the above is the only way it worked, on the bright side I didn't burn it up so theres' that.
Is there an available schematic for these cars?
Thanks for the interest and help, Oj

[ojh]

I felt a followup necessary.  Careful as I was testing the tach before installing it into the car things can always go wrong.  I identified all the correct wires, verified voltage at the connector, rang out the entire circuit and connected it with confidance.  Except it didn't work.  I'd start the engine the tach reading would jump up and show idle RPm and then shut itself off.  I'd restart the car and it would do the same thing.  I pulled the dashpod out and all was correct.  I felt I had damaged the adaptor playing with the connections and ordered another one, I installed it a couple days ago and it did the same thing, it'd read briefly and shut itself off.  I knew the tach had power because it's supply was from the water temp gauge and it was reading proper.
I pulled the pod out and checked voltage at the water temp post and it was reading crazy - I had forgotten all about the rectifier!  I pulled the dashpod completely out and sure enough I had followed the original Electrical Assembly Manual and fed all the gauges with rectified voltage and the tach don't like it all!
I split the circuit at the rectifier and fed the tach from there, reinstalled the dash pod and it works like a champ!
Don't you just love these small victories!

[JD]

Congrat's - you conquered! and thanks for posting what it took to do so.

[1967 eight barrel]

I went with the Pertonix II. I didn't want the big box.  I only had to pull one 14Ga wire to provide 12V constant to the module. You really have to search for the wire.

                                                                            -Keith