Question on msd 6-al ignition tach output

[dave harris]

The msd booklet states that the tach output is a 12 volt 20% duty cycle signal - but they don't state how many pulses per revolution - i've assumed it meant 1 pulse per rev, but my VDO tach seems to think that it's looking at a 4 cylinder (reads 2x rpm based on timing light). (it is quite possible that i've mis-configured the VDO tach, but before I tear the dash off, I would like to have some more information).

Does anyone know the pulse rate on this output?

Thanks

Dave

 

[Paul Thompson 'Hooligan']

Hi Dave -

The tach output on the MSD gives one pulse per firing. So on a V8, it gives 4 pulses per rotation of the crank.

I would think your tacho is incorrectly set so check the settings or any configuration switches.

Out of interest, the 6AL tach out is open collector output, so can also be used to trigger an EFi system with a 5v input range if required. Neat unit - despite the fact that some say it means 'My Sparks Duff'.

One last thing, if the system is being fitted to a motor that already has other rpm triggered devices, shift lights etc etc, IF they are connected to the coil, their inputs will need to be moved to the 6AL tach out too. The MSD will give multiple sparks at low revs and also drives the ignition coil with a higher than normal voltage. This in turn will give rise to false triggering or can blow-up the input stage of 'normal coil triggered' devices.

(Hope that makes sense.... seems a bit convoluted when I re-read it)

 

[dave harris]

Thanks Paul,

I agree with you that it's got to be the configuration switches on the VDO tach, but their chart showed some confusion setting possibilities so I wanted to make sure!

Thanks for the data - that will help! - now to tear the damn dash off - - - - again - - - @#!$.

Dave

 

[MReid]

VDO tachs are not directly compatible with the MSD tach output. You will need a tach adapter to make it work. This one if you are using the magnetic trigger (violet and green wires)

MSD Magnetic Pickup Tach Adapters: MSD-8920 - summitracing.com

Or this one if you are using the white wire as the trigger:

MSD Factory Ignition Module Tachometer Adapters: MSD-8910 - summitracing.com

 

[Randy V]

Mark is spot-on with his explaination for VDO compatibility with the rest of the "normal" world... You need the adapter.
I have a spare VDO tach now because I refused to buy the adapter - I replaced all the instruments with Autometer..

 

[dave harris]

Thanks Mark & Randy - - looks like an adapter is in my future - glad to know that before I tore up my dash.

Mark, do you know which switch settings on the rear of the VDO tach are correct for using the adapter (I currently have it set up for "v8) on the 3 little dip switches.

Thanks for all of your help.

Dave

 

[MReid]

Dave,

Leave it set for V8. The adapter will convert the standard MSD signal to one compatible with the VDO requirements.

 

[Jim Dewar]

Hi Dave, I did not need a tach adaptor (installed it and didn't help) using a 6AL and VDO Vision series tach. I called my VDO distributor and his answer was to flip dip switches (not energized) until I got the right combination (the instructions were worthless). I did just that and it worked! I did not record the settings but could remove my tach to retrieve for you if necessary.

 

[dave harris]

Hey Jim,

That's good news! - - the switch settings would be helpful, but I know how hard it is to take my dash out and I don't think you would enjoy that experience just to read switches for me! besides, there's only 8 combinations, and I already know one that's crap!

my guess is that since the "v8" setting is exactly opposite of the "4 cylinder" setting and my tack is reading 2x actual rpm - i'll give the 4 cyl setting the first try!

i'll try the switch flip as soon as i feel like pulling that damn dash!

Thanks again everyone!

Dave

 

[Jim Dewar]

Well Dave, I've just come from under my dash!!!!! The 3 dips are all set "on". This is using a 6AL box with the small Ford MSD distributor. I called a friend (the VDO distributor) to ask who to call at VDO for the answer and he said(laughing)"they call me"! Then said you "just flip switches til it works"! You will notice there is no such combination in the instructions.
Good Luck!!!!!!

 

[MReid]

Thanks, Jim, that helps me also. VDO must have a different series or new engineering, as they never used to work with MSD boxes when connected directly to the tach output. Always up for learning something new!

 

[dave harris]

thanks again! - glad you were so "flexable" - and could read the switches.

I'm using the 6al box and the msd small ford dist as well - same VDO gauges too!

I'll give it a try this coming weekend!

Dave