GT500 Tach to MSD tach adapter

[Bob Gaines]

There should not be two tach leads coming through the firewall. If you have a second wire separate from the OEM three wire engine feed harness that wire has been added by someone.

I have tracked down the two "tach" leads that are forward of the firewall. One was part of the three wire connector that had the temp gauge and a black wire that was dead headed, which I suspect was done at Shelby since they did not use the factory Ford oil pressure indicator. The other tach lead came separately thru the firewall and was the only wire exiting at that point. Both test 12V. I can reverse where they are connected and the tach behaves the same, goes to the peg and drops back. Further reading and some of your posts indicate that if the tach fails, your ignition is dead. This car starts with the tach leads connected in either configuration. I looked under the dash as best I could without removing the seat and see a three way wire connector that has leads suspiciously looking like they are part of the tach wiring. I am guessing that the tach has been bypassed as part of the ignition by this configuration. I should have added in the first mail that the tach WAS working and jsut recently failed. So, now the question changes to how do you know if your tach is toast? Should I consider rewiring to the original thru the tach for ignition or is that really subject to failure? Last, just throw in the towel and have the tach redone to more modern specs?--yeah probably what I should do but need a coax to spend the $$. 
As an aside, some of your posts have indicated problems with MSD ignition systems. I have gone the route of throwing parts at my MSD system and it is not pleasant. I live in Maryland where the summers are very humid and that may be a contributing factor. I just replaced the magnetic pick up in my current , 2nd, distributor. Car ran, then stopped cold, just like a failed 6A box. This time the box checked out OK, the coil checked OK, opened the cap of the distributor and it was part filled with water. Condensate I suspect. Have never had that problem with a distributor before. Any more problems with this system and I will switch to no MSD box and a Pertronix distributor. Just my 2 cents.
Thanks again for all your suggestions.
Charlie D

+1 from the factory you had the oil pressure wire which was snipped by SA in the case of 67.The temperature sender wire and the coil wire.Anything else is PO added .

[The Going Thing]

There are three wires in that feed. Coil power, Coolant Temperature Sensor and of course the Oil Pressure Sensor lead.
If you have a Pertronix ignitor you likely DO have another individual lead. It's the proper way to do it. it would be the black lead on 67-8 cars BEFORE the tach, not after that, it would be pulled from.

[charlie D]

Bob I wish I could blame a PO, but we have had this car since new. So I can only regret the days when I was too busy working to pay much attention to the work that was done on it to keep it going. Now, I am paying for that lack of attention!

[mikeh]

I have MSD 6AL & distributors on 3 vehicles (one of them for around 30 years). Had a pickup coil in distributor go bad once, but zero other problems. I really like to be able to change timing curve without removing distributor, plus rev limiter etc as Royce mentioned. Rev limiter is a plus when you run your as stuff hard as I do!    Not sure if 67 & 69 tach wiring is the same, but 2 of mine (69 GT350 & 69 Boss 429) are used with factory tachs & I wired without butchering factory wiring. Been 15-20 years ago but I think I have notes/schematics about how I did it if you are interested.

fwiw Mike

[The Going Thing]

They don't require "butchering" by any means. The Pedometer that is offered allows a plug and play fix for the issue. The Pertronix III also has a rev limiter and multi-spark capability. It all fits under the factory cap.  I see no need for externally adjustable advance. If you were racing in different places or changing cams and gears, perhaps. I am using the original 67 GT500 mechanical advance distributor and had it rebuilt and the curve set.  Despite the brightwork, the car retains the Shelby/Ford look.

[charlie D]

Follow up to my original post. I pulled the tach and sent it to Rocketman as one of you suggested. He did the conversion promptly. The tach face was in good condition so only the internals were converted. I was able to clean up many wiring connections that had been previously done to try to make the tach work without a tach adapter and later when the tach adapter was installed. With the Rocketman conversion, there is no need for the tach adapter and the wiring for the MSD 6A is straightforward. Tach works fine, and it feels good to know the wiring behind the dash is now well done. I suspect this summer will tell me if the MSD distributor is prone to condensation pooling and subsequent failure. Thanks to all for the advice and guidance.
Charlie D

[The Going Thing]

Great to hear you have everything squared away.  There is so much misinformation out there about the Pertonix in particular. I didn't want to deal with the box for the  6AL.  It is relatively easy to hide from all but the sharpest eyes.  I didn't want to covert my tach to Bob's solid state tach mechanism.

I am going to try and do a write up with pictures for the less mechanically inclined. The Pertonix II is really a nice upgrade over dual points.