Bouncing Tach in 1975 911 With 6AL MSD Ignition - Solved
 

Bouncing Tach in 1975 911 With 6AL MSD Ignition - Solved

My OEM tach began randomly bouncing or surging. Generally this occurred after a few minutes of driving. The problem was made worse by using the turn signals. A typical bounce would be to 6,000 rpms while the engine was running steady at 3,000. There was never an impact on engine performance. The car ran perfectly other than this problem.

Some of the older Porsches have a tachometer which will not work directly with the tach output of the MSD. These older cars require an adaptor of some type. The MSD 8910 Tach Adaptor essentially acts as a signal amplifier for the timing signal going through the distributor. I am told but cannot confirm that the OEM tachometers which require an amplified signal are marked with a “1” and those that do not need an amplifier are marked “C”.

The 2.7 CIS engine in my car runs with a MSD 6AL Model 6420 electronic ignition. The PO had also installed a MSD 8910 Tach Adaptor. I run points (old school) and a MSD Blaster 2 coil. If you use a magnetic trigger (Pertronix) or Crane optical unit you might be running a MSD 8920 Tach Adaptor. In my original setup, the OEM tach was triggered by a shielded green wire to the points, amplified by the MSD 8910 tach adaptor, instead of being triggered from the tach output of the MSD 6 AL box.

After researching the experiences of other Pelicans I went through a diagnostic routine. At this point I have resolved the symptom but maybe not the cause. More work to follow.

Removed and reinstalled all plug wires - no change
Replaced the distributor cap and rotor - no change
Checked to make sure my plug wire set is compatible with MSD - OK
Replaced ground strap at battery - no change
Cleaned ground area at ground strap to battery - no change
Cleaned grounds behind instrument cluster/hood shocks - no change
Cleaned grounds in engine compartment behind fuel filter - no change
Cleaned the grounds at the rear of the tach - no change
Ran a new ground wire to the rear of the tach - no change
Replaced green shielded wire between distributor and MSD - no change
Rerouted the green shielded distributor to MSD trigger wire away from the plug/coil wires - no change
Replaced the voltage regulator - no change
Installed a digital volt meter to monitor voltage - voltage was within normal limits, with no obvious spikes
Installed a bypass wire from distributor to tachometer - no change
Checked the continuity of the purple/black signal wire from the engine compartment to the tach (the aforementioned green wire hooks up to this) - checked out fine

At this point I disconnected the MSD 8910 Tach Adaptor, which caused the OEM tach to stop working completely, so I knew the signal needed some boost.
I connected the OEM tach directly to the MSD 6 AL tach output - rather than to the green wire from the distributor - no tach response at all.
I connected a cheap ebay digital tach directly to the MSD tach output - worked perfectly - no bounce - now I knew the MSD 6 AL box was working well.

So at this point the problem was isolated to a poorly working OEM tach or a bad MSD 8910 tach adaptor or another problem, like a spiky alternator which is impacting the signal going to the OEM tach, causing it to bounce.

I installed a Ashlocktech Tach-Adapt, ($65 + shipping) which runs off a signal from the MSD tach output. No connection to the distributor is required. This product can be adjusted to provide the amplified signal that the early OEM tachs require. Some people also use it to make OEM tachs work when engines are swapped, 6 cylinder engines replacing 4s etc.

NOW MY OEM TACH WORKS PERFECTLY!!!

Things I have not done that Pelicans have mentioned in conjunction with this bouncing tach problem:

Install a solid rotor, with no resistor hidden under the epoxy top
Drill a hole(s) in the distributor cap to ventilate ozone from the distributor
Replace - regap points
Use an oscilloscope to check the waveform of power from the alternator for bad diodes or other issues
Replace the coil
Replace the MSD control unit
Replace the ignition/coil wires
Check the dwell of the distributor for random changes
Rebuild the distributor to original tolerances
Rebuild the OEM tach

I solved the symptom - ie the bouncing OEM tach, but am not sure of the cause. Likely the MSD 8910 tach adaptor could be randomly bad or the alternator is sending out a noisy signal, but that’s for another post. At the last tune-up (300 miles ago), the points were badly burned, which usually does not happen with a MSD system - maybe this is a clue? Some people posit that a bouncing tach is a diagnostic tool pointing to ignition troubles elsewhere. In this case, connecting directly to the MSD tach output rather than running off the distributor/points signal will foreclose a bouncing or erratic tach as a diagnostic tool.

Bob Ashlock did a great job updating the innards of my CDI unit on my '74. Almost looked too good to put in the car. It was like engine compartment jewelry!
Tony

Your description is correct, the earlier cars needed a higher voltage to run the tacho and Bosch provided a small adapter to do so. Porsche call this a 'speed switch' for some reason.

The early tacho signal is sensitive to points 'bounce' but also to pulse length. The duty of the signal required is around 5%. Essentially this means that you need clean points and that your dwell must be set correctly for a stable gauge.

I have discovered through testing that you can drive an early tacho directly from our CDI+ ignition simply by connecting the drive signal to the 12V Input on the tacho, the only other connection needed is the earth. Leave the ”1” input disconnected and remove the 12V feed wire. This may work on other ignitions but only if the duty is correct as I described above.