The basic secret to doing this is hooking up the stock tach adapter, located on the coil pack, before the MSD modifies the signal. By doing this the stock tach will work and the MSD will function correctly.
Below is one of the ways that this can be done.
To start out 1990 DSM's have some frankly messed-up wiring in them. This is why a lot of things changed when the 2nd year of DSMs came out for the 1991 model year. Installing a MSD DIS-2 ignition have caused many a headache when trying to get the stock tach to work right. There have been various posts from people that have been told by MSD to put in resistors and diodes and everything short of a flux capacitor to make them work. It actuality it isn't all that hard!
First of all you will absolutely NEED the MSD tach adapter (MSD# 8912). Without this piece the stock tach just frankly won't work.
At some point you are going to have to take the coil pack out of the car to do a little modification to it. There are 4 bolts that hold the stock coil pack in so make sure to get them all before tugging.
It may seem impossible but you can wiggle the coil pack out of the intake manifold without pulling the whole fuel rail and wiring. Loosening the fuel rail will make it a bit easier though. Below you can see a picture of the 1990 coil pack. You can tell a 90 coil pack because it will have a 4 pin connector where the later years have a 3 pin.
The 4th pin on the 1990's is the output from the stock tach adapter. The stock tach adapter is a little black box that says TACHO INTERFACE on it, this can be seen in the circled area of the third picture. There are 3 wires the come off this tach adapter, they should be red, white, and yellow. This box takes the 2 signals sent to the coils and combines them to one signal which is then sent to the stock tach via the yellow wire.
Follow the red and white wires from the tach adapter back until they come to the connector on their respective coil, as seen with the red and white arrows. You can simply cut the red and white wires as close to the coil as you can or what I did was take the little black plastic covers off that exposed the terminal where they are wired in and cut even closer. The green arrow points to one of the covers that can come off to expose the terminals. I had a spare coil pack so I spent the extra time just doing it as close to the terminals as possible and put some shrink tubing over the connectors so it looked all perty.
Soldier or crimp connect longer wires onto the tach adapter wires (red and white cut wires) so you can reconnect them later.
Now for the fun part of installing the MSD wiring harness. I'm going to assume that you have already found a suitable location to mount it and wiring such things as main power, ground, igntion kill, etc are dealt with accordingly.
Wiring up the harness is pretty straight forward. What you are going to be doing is cutting wires on the ECU harness that comes up to the coil pack, then the MSD harness is going to intercept those cut wires and run into the DIS2 and back out. It is just like intercepting the airflow signal on a S-AFC install. I didn't take down the wire colors that are on the connector from the ECU but I will give you wire colors that are on the coil pack and it isn't hard to just look at the other side of the connector to see what colors the wires are. There are 3 wires that are going to be dealt with for the harness install, main power, trigger 1, trigger 2.
Starting with the main power to the coil pack you can see it in the upper right hand corner of the coil pack plug as the coil pack would sit in the car. It is black with a white tracer. Pull back the black tape or covering on the harness that runs to the ECU and see which color that is. It would probably be beneficial at this part to just remove all the covering from that segment of harness until it runs into the larger harness. Cut this wire between the connector and the part where this piece of harness goes into the larger loom.
Now you need to figure out which side goes where! Well, the MSD instructions are real easy to follow. You have 2 red wires and 2 brown/orange wires. This is just MSD's way of running a thicker wire without actually having to do it. The wires of the same color are going to be hooked to the same place. The RED wires hook to the side of the cut harness going towards the ECU. The BROWN/ORANGE wires go towards the coil pack. Just remember brown ALWAYS goes on the side closest to the coil pack. Soldier, crimp, twist-tie, whatever to join the wires. I suggest you soldier as it will give a better connection and my save some headaches down the road if something corrodes.
Next you will be dealing with the trigger wires. These are just the same as the power wire. You can find them as they are the red and white wires that are on the COIL PACK connector or the ones on the side closest to the timing belt. Again cut about 1/2 down and connect in one of the matching MSD wires. Just remember that if you use GREEN to intercept trigger 1 then use the GREEN/BROWN wire on the other side of that cut trigger 1 wire. It doesn't matter at all if you use the green or the white for trigger 1 or trigger 2 just as long as you use the corresponding brown wire on the other end.
Now you've done that and you are wondering where to tie the stock tach adapter back in. You want to wire the tach adapter in before the MSD intercepts the signal from the ECU harness. You can use the same connections that were used on trigger 1 and 2 were the WHITE and GREEN wires connect to the ECU side of the harness or you can tap into the MSD harness anywhere that suits. Just remove a section of insulation and connect the white and red wires from the stock tach adapter to the white and green wires on the MSD harness. Again, it doesn't matter which trigger wires you grab to go to the stock tach adapter.
On mine I ended up going about 6" further down the MSD harness and soldering into the green and white wires as it better suited my placement of the stock tach adapter on the firewall (I removed my stock tach adapter from the coil pack for another reason prior).
Now make sure the MSD Tach Adapter is plugged in (it mounts in bullet connectors near the MSD unit itself) and everything should work fine. I was orginally hoping that this setup would allow the stock tach adapter to work without the use of MSD Tach Adapter seeing how it got a signal from the ECU before the DIS2 intercepted it... but this was not the case.
Aaron Litt - MrMoo
Photos courtesy of Jerry Ledford