problem with factory tach after msd install

[gxnos]

I'm sure you guys have heard this before... but I am determined to get the factory tach to work without the 8910 tach adapter.

I have an 86 GT (TFI) with the new MSD 6AL-2. Love the 2 step!

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?

Has anybody successfully wired the MSD output to the factory tach and got it to read properly and in its full range?

[Hemlock]

I know this isn't what you want to hear but I was told by a Ford mechanic it can't be done so I spent the $50.

Robert

[81coupe]

By the time you put the cash into figuring it out, it might just be cheaper to just buy the tach adapter.

[theemustangman]

I know this isn't what you want to hear but I was told by a Ford mechanic it can't be done so I spent the $50.

Robert

Not entirely true, i called MSD and they told me all u need is a certain 1/2 watt resistor ( i cant remember the other part of the resistor) wired in line with the tach output wire, they said this is specific to being wired off the distributor not the Ignition box tach output and only for 79-85 mustangs.
Again this is according to them, i have not had the chance to test this since i had to send my distributor back to MSD for repair.

[gxnos]

i wish i could pop one of those tach adapters open to see whats inside. ill play with it more today, if i cant figure it out, ill buy it

[gxnos]

I tried all different kinds of configurations today. running the tach wire to the new coil neg (and pos), different configurations of resistors, diodes, capacitors between the tach output and tach wire. nothing worked. A different combination whether made it stop working, or made it worse. The best was with the tach output wired directly to the tach on the cluster.

Now that I think about it, I think the tach adapter amplifies the signal. Everything I was doing was bringing it down, which is why it got worse. I wish there was some more technical information readily available about the tach itself, the signal from the 6AL, and the adapter.

I guess ill just buy the damn adapter

[theemustangman]

The Tach output wire on my distributor failed to make my tach even read. I hooked it into the neg side of the coil and it read 2000 RPM because of the Multiple Spark discharge so it read it twice as high. to bad the tach doesnt have a 16 setting on it in addition to the 4-6-8 dial LOL

[fastback86]

I believe the tach output of the MSD box is a squarewave. What it takes to drive the factory tach I'm not sure. I would agree you probably need and amplifier of some sort. Maybe use the tach signal to drive an NPN transistor and the transistor to drive the factory tach. See attachment, may or may not work. May disrupt your smoke containment vessels!

[gxnos]

I believe the tach output of the MSD box is a squarewave. What it takes to drive the factory tach I'm not sure. I would agree you probably need and amplifier of some sort. Maybe use the tach signal to drive an NPN transistor and the transistor to drive the factory tach. See attachment, may or may not work. May disrupt your smoke containment vessels!

That is an interesting idea! I wouldn't be surprised if the tach adapter was just a transistor. Too bad i already bought it on summit

I would be willing to try my own transistor, but you are right, it is risky. I would def want some more information on the factory tach signal.

[fastback86]

I wouldn't be surprised if the tach adapter was just a transistor.

I wouldn't be either. The RPM modules for the MSD box are just resistors.

[gxnos]

Well i installed the 8910 adapter, and the tach works great. i tried opening it up to see whats inside, but its all epoxied together. msd wins i guess.

I also discovered that the 2 wires crimped together on the neg side of the coil (stock wiring) is the T off for the tach wire. Would have been good information to know before pulling the cluster to get directly to the tach wire :/

[gxnos]

now that i have been driving it with the msd 6al2 and the 8910 tach adapter, now i have a problem with the tach intermittently going erratic and reading inaccurate.

sometimes as its coming up to about 3k, it starts to shake, then starts bouncing around.

anybody else have this problem and know of a fix? all my wiring is fine, the adapter and box are far away from a heat source. can't really think of anything else besides a bad unit. i know the tach is fine - change back to the stock ignition system and it works fine.

[79Coupe]

So this is no help, BUT I had an MSD 6AL in my 79 coupe. MSD Distributor and I used the tach output off the MSD box. My factory 79 tach worked just fine. It did lag in the upper RPM, but it worked just fine around town.

I now have an MSD box on my 80 Cobra and the tach doesn't work at all. I haven't played with it much, as my house has been absorbing all my time and money. I'll check it out this weekend and see if I can figure it out.

[Marz]

The Tach adapter works when it wants to.

Eventually I just removed the previous owner installed MSD 6AL and tach adapter.

Personally, if it's a stock to mildly built car... the stock Duraspark would be just fine.

[gxnos]

The Tach adapter works when it wants to.

Eventually I just removed the previous owner installed MSD 6AL and tach adapter.

Personally, if it's a stock to mildly built car... the stock Duraspark would be just fine.

That sucks. I like keeping my car original as possible - dont want an aftermarket tach, etc... But its a 500hp turbo car..

I forgot the tach adapter came with a diode and I think the instructions mentioned something about using it if the tach is erratic. I'll check into it.

[C7Z]

Did the diode fix the erratic tach ?

Thanks !

[gxnos]

Did the diode fix the erratic tach ?

Thanks !

its been so rarely intermittent, i havent bothered. now that its cooling off, it hasnt been doing it at all. hmmm...

[1qwkgt]

I not trying to jack the tread but I'm kind of having the same problem. Does anyone know if the 84 GT factory tach the same as a 86 GT factory tach. I know that the ignition systems are different but is the tach.Trying to get my 84 tach to work with a accel 300, TFI ignition, and it's converted to fuel injection. Any thoughts I'm game for trying anything.

[Gears and Crosses]

I hate to add fuel to the fire. I have the same problem (no tach) and I have been searching other sites and they are using Tach adapter 8920. It kinda looks like you can use either one on some Mustangs, but 8910 is wired one way and the 8920 wired another. I am ready to get this adapter but I am confused on which one to get. I have a 86 GT 5 speed but I am using a Mallory HyFire 6 box. Mallory didn't have any adaptors until recently (that I knew off). One of their two adapters is very expensive ($120+). That is why am looking at the MSD adapters.

[rich w]

Hi Guys-Im new to this site-I have a 86Gt-100% stock.
Last Night I too installed a MSD 6al[6420].using the MSD TFI harness[Love that harness!]
Car runs waayy smoother,but now I have no tach!Icalled the MSD tech line and Rick there said I need the MSD 8920 tach adapt.There was no mention on the 8910-My personal research told me the 8920 was the proper unit.
Yes there are 2 wires going into the fact neg.coil-and I believe one of these wires triggers the fact tach-But which one??
They are both green w yellow stripes??
Anyone have an easy way to finding the "tach" trigger wire between these 2 wires?I guess I could cut one,see if the car starts and if it starts I must have the tack wire[the wire I just cut]then I could go ahead and incorperate the 8920 MSD Tach Adapt.
Or if no start I cut the wrong wire-feel like Im getting ready to defuse a bomb here?LOL
Anyways any help would be great!I cant believe we are talking about this and the cars are 25 years old!!!!I first googled this problem expecting a 100 of the same answers and it would be easy-Nope!!
Thanks for any help!Rich!

[Bymguate]

Adding fuel to the fire.....
Im using a vortech btm by msd. Its a 6 wire one instead of 8 wires...which u can use in basically any ignition sistem. This one will retard up to 15 degrees. U do have the knob in which u can select 1, 2 or 3 degrees per psi... Installed it and no tach, neither my shift light works... I used easy conectors so i could swith back to stock easily, so when i plug back to stock tach works shift light works, but my timming wont retard since i eliminated all the vacuum madness in the 2.3 t carb.
You guys think the tach adapter 8920 fix this or its just one of those things im gonna have to learn to live with and basically throw in a tach over the stock tach thats not working?

[Bymguate]

Adding fuel to the fire.....
Im using a vortech btm by msd. Its a 6 wire one instead of 8 wires...which u can use in basically any ignition sistem. This one will retard up to 15 degrees. U do have the knob in which u can select 1, 2 or 3 degrees per psi... Installed it and no tach, neither my shift light works... I used easy conectors so i could swith back to stock easily, so when i plug back to stock tach works shift light works, but my timming wont retard since i eliminated all the vacuum madness in the 2.3 t carb.
You guys think the tach adapter 8920 fix this or its just one of those things im gonna have to learn to live with and basically throw in a tach over the stock tach thats not working?

[Dr. Matt]

Bymguate,

I can't help with your question, but have one for you. Where are you getting your shift light signal while running the stock tach? Did you add a separate RMP switch?

[Bymguate]

Shift lights go with neg signal from the coil