Fox Msd Box Install

Farrell5.0

Not sure what "DP" means, but know's he likes it!
15 Year Member
Feb 15, 2012
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Vancouver, WA
Not 100% if this should be talk or tech, my mistake if it's in the incorrect forum.

I installed an msd6al-2 box in my car and encountered something that made me scratch my head. I bought the tfi adapter harness to simplify things. I followed instructions and diagram step by step. Orange to orange, black to black, white to white, etc. Then I got to the gray wire that goes to "tach output", had to google search where to wire that to.

My findings said to splice to negative side of coil, the yellow/tan wire for 1992 year, so I did just that. Checked all my connections, go to crank the car, no start. Disconnected the harness and still no start, which was odd since it started just fine before box install. I check for spark with box harness connected and disconnected, no dice.

I realize the only thing I hadn't checked was the gray wire, so I disconnected it from the yellow/tan wire and it fired right up with the harness connected, and my tach even works.

My questions are, how do I know my box is actually doing it's job if it's not getting a tach signal, yet the box led is steady? Also, why would my car not start with the gray wire connected to negative side of the coil when this works for other people with the same year model?
 
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I believe the only thing the gray wire is worthy of is the rev limiter feature for the MSD box. Is this an option you really want the MSD box to control? Did you splice the gray wire into the coil wire or did you cut the coil wire and wire it only to the MSD gray wire?
 
Hmmm, I guess I'll have to look back at the instructions to verify. I spliced the gray wire in to the coil wire. I have a megasquirt with easily controllable rev limits but I figured using the one on the msd box wouldn't hurt, either.
 
I did a search and found another post similar to yours....Seems to me you need a Tach adapter to make it work. :shrug: If I were you I'd leave it up to MS to control it, just my $.02

Anyways, the tach stopped working, so I had the idea of disconnecting it at the tach and connecting the tach output wire directly to the tach. As you know, the same green wire on the negative side of the coil is what signaled the tach. Connecting the tach output wire like this will cause no spark or an erratic tach.

So I pulled the cluster and snipped the green wire on the back and ran the tach output wire to it, and it worked! Accurate too, matched perfectly with the 2 step rpm. Only problem is, now it cuts out at 4000RPM. I'm coming up with ideas with diodes and resistors to try and get it to work properly. I mean, what does the 8910 tach adapter really do?
 
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I also never bother with the $30 harness, All I do is use $.30 of wire and 2 insulated and 2 non insulated male connectors as a splice for the connector from the main harness and 2 insulated female connectors for the connectors going into the coil. All you need to do is unplug the wires and plug the stock connector back into the coil if needed.. It also makes a cleaner install with less wires.

Its the diagram on the bottom of page 50
 

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I also never bother with the $30 harness, All I do is use $.30 of wire and 2 insulated and 2 non insulated male connectors as a splice for the connector from the main harness and 2 insulated female connectors for the connectors going into the coil. All you need to do is unplug the wires and plug the stock connector back into the coil if needed.. It also makes a cleaner install with less wires.

Its the diagram on the bottom of page 50
That looks simple enough. Too late now, though haha
 
I was under the assumption that the tach out put was only used if you used an aftermarket tach?? I wired one of these in my buddies coupe and it works fine without that hooked up. Tested the 2 step on it and all with no issue.
 
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I was under the assumption that the tach out put was only used if you used an aftermarket tach?? I wired one of these in my buddies coupe and it works fine without that hooked up. Tested the 2 step on it and all with no issue.
My tach worked fine without it as well but wasn't sure if my box was actually doing it's job at that point. They don't really specify if the grey wire is for aftermarket tachs only. They just mention that some factory tachs need an adapter. :shrug:
 
My tach worked fine without it as well but wasn't sure if my box was actually doing it's job at that point. They don't really specify if the grey wire is for aftermarket tachs only. They just mention that some factory tachs need an adapter. :shrug:
The box deff does its job without that tach output hooked up . I've ran mine for 2 years already like that and I have bypassed the box a few times to troubleshoot and I noticed a difference right away.
 
I was under the assumption that the tach out put was only used if you used an aftermarket tach?? I wired one of these in my buddies coupe and it works fine without that hooked up. Tested the 2 step on it and all with no issue.
Correct, Tach output needs to go an aftermarket Tach. Seems like I remember the msd box fires the positive side of the coil versus the negative like the stock electronics. Thus the reason for a tach output that some setups may need since most tachs use a ground pulse.
 
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I was under the assumption that the tach out put was only used if you used an aftermarket tach?? I wired one of these in my buddies coupe and it works fine without that hooked up. Tested the 2 step on it and all with no issue.

+1.

I use my tach output for the shift light only.

Joe
 
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