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-   -   MSD tach feedback issues (http://www.clubcobra.com/forums/small-block-talk/114734-msd-tach-feedback-issues.html)

Xavier 03-11-2012 02:58 PM
MSD tach feedback issues
 
My Tach does nothing. I have a msd 6al ignition box that supplies tach feed to a tach that was just checked by the mfg to be fine, no problems supposedly. The msd 6al is running the car fine, but after checking everything and a very simple tach hook up, tach does not work. If I am running a msd ignition box, can I still use the negative side of the coil to give tach feedback? Just rather get the tach feed somewhere else than to replace the ignition box.

Thanks.

Tom Wells 03-11-2012 03:14 PM
Xavier,

You don't say whether the tach was working and quit, or not.

If it never worked, the probability is high that you need one of these adapters: MSD Ignition Products

The type of ignition you have will dictate which adapter you need.

Tom

Xavier 03-11-2012 03:32 PM
The tach was working at one time a long ime, then stopped, then startedfor a few dys, then stopped. The only real test to see if the ignition box is the problem is using an oscilloscope. Rather just hook up to something I have than to add another product that can fail.

Tom Wells 03-12-2012 05:41 AM
Xavier,

OK - if it did work at one time here's some logic for you:

1) MSD box - MSD says it's OK

2) So check the wire from the MSD to the tach (and its end connections) for continuity. If the wire is OK,

3) It's possibly the connection at the tach or the tach itself...

4) If the tach has a separate 12V supply and ground, check those too.

Hope this helps,

Tom

vatdevil 03-12-2012 12:07 PM
Tach on my SPF works up to 4K and then drops to zero. When I tried the MSD tach adapter, the tach reads zero all the time. Not really helping you here.......

Xavier 03-12-2012 01:12 PM
Tom I have did all the trouble shooting.

Tach wiring is fine - 3 wires Ground (okay) hot (okay) tach feed (okay)
Had Autometer check out the tach (okay)
DO NOT want to take out the MSD box, a pain in the butt. Can check for 2 to 5v coming from the MSD box to the tach feed (okay) but this can actually read correctly and still be a problem because the true tool you need is an oscilloscope (don't have one).

So, no need for the lesson in trouble shooting - just wanted to know if connecting a tach to the coil while still running a MSD box will do something harmful to the tach. MSD box works otherwise.

I can try it myself to see, but did not want to burn up a good tach trying, or worse yet, something else.

Tom Wells 03-12-2012 02:50 PM
Xavier,

Don't know the answer to that one.

Perhaps this MSD document will help you solve the problem: MSD Ignition

Tom

DougD 03-16-2012 10:16 AM
Quote:
Originally Posted by Xavier (Post 1180581)
Tom I have did all the trouble shooting.

Tach wiring is fine - 3 wires Ground (okay) hot (okay) tach feed (okay)
Had Autometer check out the tach (okay)
DO NOT want to take out the MSD box, a pain in the butt. Can check for 2 to 5v coming from the MSD box to the tach feed (okay) but this can actually read correctly and still be a problem because the true tool you need is an oscilloscope (don't have one).

So, no need for the lesson in trouble shooting - just wanted to know if connecting a tach to the coil while still running a MSD box will do something harmful to the tach. MSD box works otherwise.

I can try it myself to see, but did not want to burn up a good tach trying, or worse yet, something else.
I can't see that working correctly.

MSD = Multiple Spark Discharge. Multiple sparks = multiple tach signals = screwy tach readings. They wouldn't provide a tach signal wire if it was OK to hook it up to the coil.

mreid 03-16-2012 10:23 AM
If you hook it up to the coil, you will fry the tach. You said it is Autometer, what model tach is it? Almost every Autometer will work with MSD without an adapter (Autogage I know is an exception). Sounds like the msd box to me.

Tom Wells 03-16-2012 01:43 PM
Xavier,

Here's some cut & paste from MSD:

Quote:
Aftermarket Tachometers----White Wire Trigger----Magnetic Trigger Connector
Autogage----------------------8910-------------------8920
Autometer---------------------None-------------------None
Classic-------------------------8910-------------------8920
From this table you can see MSD says no adapter is required for the Autometer tach.

Further:

Quote:
Note #1: There are basically two types of tachometers: Current Triggered Tachometers and Voltage Triggered Tachometers. These (sic) type of tachometer that is on the vehicle may be recognized by the way it is connected to the ignition coil. The drawings below show two wiring diagrams to help in determining which type of tachometer is on the vehicle.
You'll need to use the link in my post above to see the diagrams supplied by MSD.

The major caveat is given as:

Quote:
Note #4: If the Tachometer is connected to the positive side of the ignition coil, do not attempt to connect this tachometer to the tach ouput terminal on the MSD 6 series unit.
So you may want to ask MSD about your specific application before wiring anything up, although it appears OK as long as you don't use both tach voltage sources.

I tend to agree with mried. The MSD box may have a problem.

Let us know how you proceed!

Tom


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