I know this is an old thread, but I just wanted to chime in...
In theory, YES, you should be able to use the MSD 6A box with the GM TBI AND still have the ECM controlling the timing.
Not as straight forward as staying with the GM HEI setup, but it should work.
Two options to do this:
Use the GM TBI PCM, the GM 7 (or 8) pin module, AND the MSD 6A.
-Step 1, install the MSD 6A following MSD's directions EXCEPT do NOT connect the MSD box to the distributor!
-Step 2, install the GM TBI as directed in the EFI posts EXCEPT do NOT connect the leads from the "C" or "+" terminals on the GM 7 (or 8) pin module to the coil. The MSD is the ONLY thing that should be connected to the coil. Do not even connect that little silver beer can (condenser) as it will cause problems with the MSD -- eventually.
-Note: You MUST still connect a switched +12v source to the + terminal on the GM 7 (or 8) pin module.
-Step 3, connect the WHITE wire (ignition trigger) from the MSD box to the "C" terminal on the GM 7 (or 8) pin module. The magnetic pickup wires from the MSD will NOT be used.
-With this setup, the GM electronics will work just like they should, and the GM ECM will get the feedback it expects and as such, your spark will be properly controlled by the GM ECM. The MSD will simply be triggered by the coil - (the "C" pin) output of the GM module. This signal will appear to the MSD to be the same as that from a points ignition system and work just fine. Remember, the 6A box does not retard or advance your spark at all -- it just fires a fancy triple spark every time it gets a trigger and really doesn't care where the trigger comes from. The triple spark will NOT interfere with the GM ECM because the ECM is getting the normal feedback it is used to from the normal GM module it is used to. This eliminates the need to try an use the tach output from the MSD as well -- unless you want an aftermarket tachometer.... In which case the MSD tach output is where you should connect it.
-BIG NOTE: Do NOT install the GM module in the distributor if you choose to go this way. The extra EMI inside the cap, caused by the increased voltage used by the MSD, will likely cause problems with the GM module. Instead, you should install the module externally, and mount it to some sort of a heat sync. A hollowed out Duraspark II box has been said to work well.
-Another note: This method fixes the run-on problem noted in the MSD install manual as the MSD Box, even if it did stay on for 30 seconds or so after you turn off the key, would not fire as there would be no pulse from the GM electronics to trigger it.
OPTION 2: Ditch both the GM ECM and the GM 7 or 8 pin module.
-To do this, you would use a MegaSquirt ECM.
-Step 1, install the GM TBI as directed in the TBI posts / forums, EXCEPT don't install the GM 7 (or 8) pin module or the GM ECM.
-Step 2, install the MegaSquirt ECM as directed in the megasquirt forums EXCEPT don't connect the Megasquirt to the coil.
-Step 3, install the MSD box following the MSD install manual EXCEPT do NOT connect the MSD to the distributor.
-Step 4, connect the "ignition trigger" pin OR the "Coil -" pin from the MegaSquirt ECM to the WHITE "ignition trigger" wire on the MSD. Just like with Option 1, the magnetic pickup connection on the MSD will not be used.
-Step 5, connect the MSD "tach" output to the "tach" input on the MegaSquirt(or MicroSquirt) ECM as required (see the MS install forums).
-Note: This method also fixes the MSD run-on problem as the Megasquirt ECM also stops sending trigger pulses when the ignition switch is turned off.
-Another note: This method uses the built in timing control in the MS ECM to control spark timing, the feedback from the MSD box is acceptable to the MS without a filter (in most cases) and the MSD only provides 1 pulse per cylinder on this tach line, regardless of how many sparks are actually going to the cylinder..
-One more note: Do NOT allow the coil + and - wires from the MSD box to rest on, or come within 4 inches of the MS ECU -- unpredictable results can occur if you do...
-A note for BOTH methods: Do NOT use the stock coil + and - wires for the MSD setup. While they are fine for the GM HEI setup, they will cause unpredictable problems if you try to use them with the MSD. Use the twisted pair of wires supplied with the MSD for this purpose..
-A word of caution: Both the MSD 6A box and the GM 7 (or 8) pin modules have been known to spontaneously fail - -especially when home is a REALLY long walk away!! : o ) I highly encourage anyone trying either of these options to reuse or add connectors such that they can easily switch back and forth between the MSD being the spark generator and the GM module being the spark generator. This would allow quick road/trail-side troubleshooting should a problem occur. If one were to place a 2 pin connector on the wires coming from the "+" and "C" pins on the module, and another close to the coil where the "+" and "-" wires connect to it, then they could easily unplug the MSD from both ends and plug the GM directly to the coil without needing any tools -- this would make a quick fix in a pinch.. Remember, you can buy the Gm module in any parts store -- the MSD is a bit harder to come by..
Hope this helps somebody!!! : o )
Just my 2 Cents (Good grief!! Even my weedeater laughs at me now!! Silly gas prices!)
1986 Grand Wagoneer (225xxx)
1988 Comanche Pioneer (215xxx)