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W G WALKER
01-25-2006, 04:43 PM
I installed a MSD 5 series unit part number 5200, and blaster 2 coil on my stock 1970 MGB (negative ground, 12 volt electrical system with points distributer). The problem is the tach is now unstable and reads about 1700 rpm high and erratic. The limited data I have available tells me the tachometer in this year MGB is triggered by a current pulse from the ignition system. I also checked the wiring diagrams for this car and found the current goes from the IGN switch, through the tach, to the positive side of the coil.

Will any of the MSD adaptors correct this, or can you recommend another fix ?

msdtechsupport1
01-26-2006, 09:42 AM
Try running the red wire from the MSD 5 to a separate switched 12 volt source. The tach might be picking up some interference form the coil positive.

Keep me posted on this one.

W G WALKER
01-27-2006, 06:59 AM
As you suggested I connected the red wire on the MSD 5 to a different power source. This produced exactly the same result with the tach as when it was powered from the positive side of the coil.

I next tried using a Chrysler Dual Ballast Resister as depicted on page 138 of the 2006 MSD catalog. The car would not start, or run with it installed.

I also tried using the Blaster 2 coil with out the MSD 5 to see if the tach would read correct. The result here was the car would start but would misfire and not continue running for long. The tach reading was close this way but still a little erratic.

I then installed the OEM coil and the car ran fine and the tach indicated correctly. I have now re-installed the MSD 5 and Blaster 2 coil. The car runs good but the tach problem is still there.

Any more ideas ??

msdtechsupport1
01-30-2006, 09:55 AM
In that case, you'll need to use PN 8920. This is a tach adapter that will tie into the white wire on the MSD 5, and to the tach itself. Once you have the adapter in-hand, hit me up and I'll go over the wiring with you.

W G WALKER
02-06-2006, 08:02 AM
As you suggested I purchased tach adapter PN 8920. There is no wiring diagram that shows this adapter being used with the MSD 5.

The tach wiring is directly from the ignition switch through the tach, to the positive side of the coil. The car runs good with the MSD 5 wired as per the diagram, (with the red wire to the positive side of the coil, the orange wire to the negative side, and the white wire to the points). How does the adapter wire into this ?

msdtechsupport1
02-06-2006, 09:19 AM
Try this..

Take the white wire from the 8920, and run it to the negative side of the coil. Take the black wire and ground it. Connect the red wire from the 8920 to the tach wire. The voilet wire will not be used.

That should do it.

W G WALKER
02-06-2006, 10:40 AM
The tach wire on the MGB supplies the power to the positive side of the coil.

Your instructions will have me connect the white wire from the 8920 to the negative side of the coil, the red wire from the 8920 to the positive side of the coil, and the black wire to ground.

Is this correct ??

I thought I should verify this before I chance damaging something.

msdtechsupport1
02-06-2006, 03:12 PM
No, you'll need to run a separete switched 12 volt wire to the positive side of the coil. The red wire from the adapter will go to the tach wire. Do not connect it to the coil. Hope that helps.

W G WALKER
02-08-2006, 08:34 AM
I connected the 8920 exactly as you described using a separate switched 12 volt wire to the positive side of the coil. Except for a slight increase in indicated RPM there was no change in the tach indication. (Engine is idling at 600 RPM and tach is erratic and reading 2500 RPM before the 8920 was installed the tach read 2000 RPM at idle).

Any more ideas ??

I copied a technical description of the tach operation from the manual maybe it will help.

“The MGB tachometer is based on the principle of the monostable multivibrator, which has one stable state and one quasistable state. The circuit is normally in the stable state, producing no output. However, when a triggering current pulse from the ignition system is received, the circuit transitions to the quasistable state for a given time before returning again to the stable state. In this way, each ignition pulse produces a clean pulse of fixed duration that is fed to the gauge mechanism. The more such fixed duration pulses the gauge receives per second, the higher it reads. This design requires that the ignition coil current passes through the tachometer on its way to the coil.”

msdtechsupport3
02-08-2006, 03:10 PM
If you connect the MSD 5 according to Figure 2 of the instructions, using a different 12V switched source to the coil and bypassing the tach altogether. Will the vehicle run?

W G WALKER
02-09-2006, 08:04 AM
The vehicle runs ok when I connect the MSD 5 according to Figure 2, and use a different 12V switched source to the coil.

msdtechsupport3
02-09-2006, 09:11 AM
OK now that the ignition portion is working, leave that part alone.

Now, for the tach circuit, connect the tach wire that was orignially connected to the +coil terminal and connect it to the RED wire of the Tach Adapter. Ground the BLACK wire of the tach adapter to a good, clean ground (preferrably to the battery). Join the WHITE wire on the Tach Adapter with the wire attached to the WHITE wire of the MSD 5. The VIOLET wire will not be used. Maintain the separate switched 12V source for each the tach and MSD 5.

This should allow the tach to operate properly.

W G WALKER
02-14-2006, 09:18 AM
This last fix seems to work. The tach and everything else is working properly so far. Thanks for the assistance.

msdtechsupport3
02-14-2006, 10:35 AM
Thanks for the reply.