Jim Florance

Hello everyone:
I just finished the installation of a MSD6AL ignition system. I didn't want to be bothered with the red MSD accessory unit required to make the 6AL work with my Porsche tach, so I pulled the tach and had it modified by a local, high quality speedo and instrumentation shop to allow it to accept the MSD waveform signal. After about 6 hours of beating my head against the car in frustration trying to get the tach to work, I tested the black and purple wire from the engine compartment to the tach and found it wasn't carrying the MSD tach signal. When jumped directly from the 6AL to the tachometer and it worked fine. I pulled the multiple pin plug off the bracket where the Bosch CD unit was originally located. I isolated the black and purple tachometer signal wire. I then attached a spade connector to that wire, and attached the Tach sending unit wire directly to it, bypassing the electronics in the engine compartment completely. By doing that, the tachometer now works great. What is in that set of electronics that could have failed and caused the disruption of the MSD tachometer sending unit's signal to the tach? Is there anything that I may have compromised by bypassing that part of the circuit? Everything seems to work just fine now, and if my shortcut won't hurt anything I'd just as soon leave it as I now have an operational tachometer. If any one else has any experience with this sort of thing, I really want to hear from you.
Thanks, Jim

hmintz

Jim, you didn't say what year 911 you put the MSD box in, but I recently put one in my 76. I, too, was concerned about whether my tach would work with the MSD signal, especially since I planned on a Pertronix II points eliminator also. I decided NOT to use the MSD tach signal, since my tach worked without it for 20 years. I simply used the points (Pertronix) output, same as always. Tach works fine. On my 76, there is nothing but the purple and black wire between the points output and the tach. No active electronics, so I don't know why your tach wouldn't work at the front end of the wire, except for a bad connection or a break.

Howard
76 911S Targa
53 Nash

mo_gearhead

Old thread but I have a similar problem now. (1969 2.0)

Installed rebuilt engine with old Permatune box (had MSD unit with tach. adapter ordered). Car ran and tach. worked. MSD arrived. Installed it per instructions (IE: Do NOT attach anything to the coil terminals except the two MSD wires!) So I ran the tach. wire to the MSD output terminal. Car runs great but ...no tach! Tried unhooking tach. adapter...no change. Engine runs great, but no tach.

On some site I read that running the tach. wire back to the coil wire while using a MSD box could FRY it (the tach.)??? But the above posting has me wondering if that would indeed work? Anyone???

PorscheMD

I have installed several MSD units on 911's. I have NEVER had to use a tach adapter. I have always plugged the black/purple wire directly into the the tach output on the MSD unit. It has always worked. I pick up the blk/pur wire from the plug that would plug into the bosch CD unit.

911pcars

Various factory tachs will work with aftermarket MSD, Crane, etc. Some won't, some need resistor, some need transformer, etc. I don't have the info but it's somewhere here or at Pelican's site.

Sherwood

LaughaC

My purple/black tach wire is tied into one of the wires coming from my Pertronix (points replacement) system and MSD box. This was an alternative posted on Rennlist or Pelican, for which I am thankful.

I never really figured out how the Permatune had the voltage flowing back and forth.

mo_gearhead

Well, when it worked (with the Permatune) my black / purple wire for the tach. was attached to the positive coil post. It runs from there, into the wiring harness that heads back into the tunnel, so I assume it goes directly up to the tach.?

So should I attempt to put it back on the coil + post (in contradiction to the MSD info.)???

LaughaC

Next week when I get home I can post the instructions I followed. I thought it was the negative side of the coil, so it might be best to wait until I find the document on my home PC.

mo_gearhead

Thanks Chris, I will wait. I do not want to ruin a perfectly good tach. that I know was working.

mo_gearhead

Chris? Anyone?? Hello???

Ok everyone, this is your last chance to save MY TACH! (Holding a pistol to the tach. gauge). I want an answer by ....ok ...midnight, Saturday ...Saturday the 7th.

Otherwise the tach. gets it! (meaning, hooking it back up to the coil post!)

red911 Targa

Dear Mo,

I had the same issue, and I changed the ground to the motor, from the chassis and I had a tach.

Good luck, Tom

mo_gearhead

Got the tach working on my '69.

On another BB a fellow told me how to wire it, ignoring the MSD pickup point, and putting the diode in the circuit.