MSD 6AL Tach Issue

[Louisieg55]

Hey guys,

Looking for some input. I’ve been searching but I keep going down a lot of rabbit holes ��, Last time I did that I ran new fuel lines, rebuild a bunch of parts, put in a new ignition system lol.

Car:
1975 911
MSD 6AL (triggered off green/violet mag pickup)
Pertronix Ignitor II
MSD Blaster coil
Stock tach
MSD 8920 tach adapter installed (Kinda) not working

Wiring on the 8920 is:
• Red → same switched 12V feed as MSD and Pertronix
• Black → chassis ground
• White → MSD white wire
• Violet → black/purple tach wire

If I ground the black/purple wire, the tach jumps — so the gauge itself seems good.

Problem:
Tach is still completely dead.

I’ve read the 8920 can be sensitive to weak 12V and may not like sharing the same switched feed as the MSD + Ignitor II. My next step is to isolate the 8920 red wire and power it off a clean external 12V source to confirm if that’s the issue.

Question on the tach wire (see picture):

Is this the correct black/purple? It sits in a 2-prong connector. On the engine-harness side it is black/purple that turns into two red wires going into the plug. On the body/gauge side it splits into two white wires plus one red wire. Grounding the gauge-side black/purple makes the tach bounce.

Before I go further, does this setup look correct for a mid-year running an MSD off the mag pickup? And does anyone here run a different tach adapter or a different wiring method with the 6AL on a ’74–’77?

I took the car for a quick spin around the block today, the little guy been bugging me to take the "Race" care out, nothing crazy since I can’t see RPM, but the 6AL is set for 6K right now so I’m not too worried. Feels like it’s running okay overall.

Thanks,
Louis

[Bob Ashlock]

You should not need that adapter. If tach is original to the car, it will trigger directly from the MSD Tachometer output signal. That is the GRAY wire coming out of the MSD. Connect it to the black/violet wire and make sure that wire doesn't have anything else 'hanging' on it. In other words, the black/violet should go directly to the tach, nowhere else. It is difficult to understand how that adapter was connected, hopefully it's higher voltage pulses did not hurt your tach.

[Louisieg55]

So I think I had the MSD 6AL wired wrong originally. I was using the mag pickup wires (violet/green). I removed those completely from the circuit.

It’s now wired using the MSD white wire going to the black wire from the Pertronix Ignitor II. My understanding is that the Ignitor II is not a mag-style pickup, it behaves like a points-style trigger. Please correct me if I’m wrong on that. It was running when I had it wired the other way

So now my wiring is (basic layout):

• Pertronix black → MSD white (trigger)
• Pertronix red → switched 12V (same as msd) 10 amp fuse
• MSD RED→ Switched 12v (Same as pertonix) 10 amp fuse
• MSD orange → coil +
• MSD black → coil –
• MSD heavy red → battery 12V
• MSD heavy black → ground
• MSD violet/green → not used

I am now using the MSD gray wire to trigger the tach. The tach appears to be the stock VDO unit. If I ground the black/purple tach wire, the gauge jumps, so I’m 99% sure I have the correct tach wire. This is spliced right into the plug, I did not unplug. Its still has the black/purple wire that goes to the (2) white wires and the red wires?

I removed the tach adapter entirely from the system.

Does the above wiring look correct?

Am I missing something else?

[A930Rocket]

See my notes from the 911 Turbo forum years ago.

J&S Safeguard with MSD

[A930Rocket]

I don' tremember what tach adapter I used, but it a square black plastic box.

MSD Installation Instructions On A Porsche 911


You'll need the tach adapter if you want to keep the stock tach.

Here's some question and answer info I got from Steve Weiner who owns Rennsport Systems:

QUESTION: I'm installing an MSD 6AL in my '87 930 and have a question about the wiring. Here goes:

1- The large red wire goes to the battery… EDIT. DO NOT USE THE POS WIRE AT THE STATER.
2- The large black wire goes to ground.
3- The small orange wire goes to the Positive side of the coil.
4- The small black wire goes to the Negative side of the coil.
5- The small red wire goes to switched Pos.
5- The white wire is used to trigger a MSD shift light.
6- The violet wire?
7- The green wire?

Since the Turbo uses a magnetic pick up type distributor, my question is which wire does the MSD violet wire connect to and which wire does the MSD green wire connect to coming out of the distributor? Or does it even matter which one I connect to?

ANSWER: You are right in that the violet & green wires conduct the trigger voltage to the MSD. This is phase sensitive and must be connected correctly otherwise the car will not start or run poorly. Connect the green wire to the center conductor and violet wire to the shield.

I didn't want to cut any of my harness up, so I unplugged the wiring harness off the CD and connected my wires there (the ones from the distributor).

NOTES: In addition, to make the tach work correctly, the tach adapter is used and:
1- The red wire is to switched Pos.
2- The black wire is to ground.
3- The white wire goes to the tach output on the MSD unit itself.
4- The violet wire is run to the tach input (the purple and black wire by the CD unit).


EDIT. OTHERS HAVE SAID THE TACH ADAPTER DOES NOT NEED TO BE USED.




04-21-2008 11:13 PM

[A930Rocket]

[Louisieg55]

I guess the questions is do I run my current system how its wired now with using the white wire or go back to using the green/violet wire.

Then from there I can go back to trouble shooting the tach.