MSD 6al installation and factory tach operation

[Padrino 5]

Hi all,

69 Mach 351W.

Im sure this has been asked before but I did a search and couldn’t find the info I needed. Closest thread was about resistance wires. 

After some restoration work, I switched to a Pertronix igniter distributor. I think that caused the factory tach to quit working. 

Just installed the MSD 6al . 

I knew I’d need a tach adapter so I did some research and concluded I needed the 8910.

Hooked up the 8910 and went back to a no-start situation. Unhooked it, and it started. 

Called MSD and the guy told me to remove the ballast resistor/resistance wire to the tach. 

Before I do that, I want to check with you guys to make sure it’s safe. I don’t want to remove that wire and cause a fire or fry something. 

If I should bypass the factory resistance wire, please explain the reason why it won’t be unsafe to do so. My guess is the 8910 compensates for that, but not positive so want to make sure.

 

[1969_Mach1 333]

If I recall correctly the original tach in these cars were current triggered and not voltage triggered.  Last time I needed an MSD tach adapter with an MSD 6ALN ignition box it was the MSD tach adapter part number 8920.  The 6ALN is simply the NASCAR approved version of the 6AL but with waterproof double potted electronics and with water proof connectors.  Unfortunately the 8920 is the more expensive model tach adapter.

MSD tech could never tell if the resister wire should be used when connecting the original tach to their 8920 adapter.  I tried it  both ways, tach seemed to operate the same either way.  So I ultimately did not use the resister wire to connect the tach to the 8920 adapter.

You can double check, but I think the 8910 tach adapter is only for voltage triggered tachs.  The 8920 adapter can be wired for either voltage or current triggered tachs. 

[Rsanter 152]

Do a search on the web

somone has already solved this.

i was looking at it as I will be adding a factory tach to my car and I have a pertronics and MSD

[Padrino 5]

19 hours ago, 1969_Mach1 said:

If I recall correctly the original tach in these cars were current triggered and not voltage triggered.  Last time I needed an MSD tach adapter with an MSD 6ALN ignition box it was the MSD tach adapter part number 8920.  The 6ALN is simply the NASCAR approved version of the 6AL but with waterproof double potted electronics and with water proof connectors.  Unfortunately the 8920 is the more expensive model tach adapter.

MSD tech could never tell if the resister wire should be used when connecting the original tach to their 8920 adapter.  I tried it  both ways, tach seemed to operate the same either way.  So I ultimately did not use the resister wire to connect the tach to the 8920 adapter.

You can double check, but I think the 8910 tach adapter is only for voltage triggered tachs.  The 8920 adapter can be wired for either voltage or current triggered tachs. 

So the MSD tech said the 8910 is in systems with a ballast resistor, which I assume the resistor wire qualifies as. 

Ive searched all over the net and can’t find a conclusive answer to this.

[1969_Mach1 333]

But I believe the 8910 is for voltage triggered tachs and the original Ford Tachs in these cars were current triggered.

You're correct, there is no clear answer on which one to use.  When I went through this many years ago.  The MSD tech asked me how the tach was originally connected.  Which like you know was to the positive side of the coil.  Therefore it was current triggered and the tech told me I needed the 8920 adapter.  But your reasoning  for the 8910 sounds just as valid.  Here are the instructions for the 8920.  See if it looks like something that might work. http://documents.msdperformance.com/8920.pdf  I don't know how the Pertronix affects everything.

[Padrino 5]

9 hours ago, 1969_Mach1 said:

But I believe the 8910 is for voltage triggered tachs and the original Ford Tachs in these cars were current triggered.

You're correct, there is no clear answer on which one to use.  When I went through this many years ago.  The MSD tech asked me how the tach was originally connected.  Which like you know was to the positive side of the coil.  Therefore it was current triggered and the tech told me I needed the 8920 adapter.  But your reasoning  for the 8910 sounds just as valid.  Here are the instructions for the 8920.  See if it looks like something that might work. http://documents.msdperformance.com/8920.pdf  I don't know how the Pertronix affects everything.

Ok, that makes sense. I didn’t know the difference of current vs voltage triggered. How would a voltage triggered tach be hooked up? 

[1969_Mach1 333]

The voltage triggered tachs were merely connected to the negative side of the coil along with the wire to the distributor.  So current doesn't flow through a voltage triggered tach to power the ignition system.  I used the 8920 for about 10 years then the movement in the tach got sticky.  Since it wasn't the original instrument panel for my 69 Mach 1 I sold it, reinstalled the original instrument panel and added an Autometer tach simply clamped to the steering column.  I like the look of the original tach better but I will say the Autometer tach works much better than the original tach ever did.

Your other option is to have your original tach retrofit with modern internal parts.  It will then simply plug into the tach terminal on the MSD ignition box.  Cost is a little more than that 8920 tach adapter.  But most original tachs, if still working, are getting towards the end of their life and most of the internal parts are no longer available new.