converting a 1990 to a 1991-1994 ecu system

Converting from 1990 ECU to 1991-1994

This Conversion does work. I have it working in my car with NO side effects. There are shortcuts so you don't have to replace all the parts but I don't recommend it nor will I tell people what they are.

Foreword: The wiring that I'll be talking about isn't stereo wiring. Do it right the first time since it's a major part of the car and any short can cause an accident or death from loss of control. I take no responsibility for anything that goes wrong from people following these instructions. I've spent hours and hours researching this and am passing on the knowledge so others don't have to do this again. 3 things to remember for a great joint: 1) heat the wires then flow the solder. 2) use some silicone then put the heat shrink on 3) cover the wiring back up with the wire loom and electrical tape.

Parts Needed:

1991-1994 Comparable ECU (from same model type)
1991-1994 Coil Pack and harness
1991-1994 Ignition Power Transistor (located below coil pack on intake manifold) with harness
1991-1994 Gauge Console (If you feel comfortable taking the gauge assembly apart you can replace just the tach)

1. Deal with the ecu wiring first. The only difference between the 1990 and the 1991-94 ecu pin outs is pin 6 and 14. Just swap these two pins by cutting and soldering and heat shrinking. Check this pin out guide so you can see what wires I'm talking about.

2. Next thing is to go under the hood. Remove the old coil pack and ignition power transistor with mount. The new ones will bolt on with their mounts, no modification is needed for that.

3. Look at the differences between the 1990 and 1991-94 wiring. No new wires are needed, only the ground wire and Tach signal wire change where they end up. Here are the separate wiring schematics for the 1990 and 1991-94.

4. Remove the wire loom holding all the wires together up to the main wire loom. You'll wrap these back up after you're done but it makes it a lot easier to work with them separated. Some of the wires will not be used anymore, they do a loop inside the main wire loom and don't actually go anywhere. We'll just connect directly between the new power transistor harness and the new coil pack harness. See the wiring diagram for how to connect everything.

5. Make sure all the connections are tight, you should be able to pull on both sides of the joint without it breaking. Put plastic wire loom over the wires and use electrical tape to seal it all up. You don't want to have moisture or oil get into the wiring.

6. Remove the dash to get at the gauge console. I'm not going to walk you through this. It's only about 10 screws total and if you can't do this on your own you shouldn't be touching the wiring. Nuff said.

7. Start it up.

Pictures:

Click on images for full size pics.

Compare the 1990 (bottom) with the 91-94 coil pack and power transistor module.
Notice the harness differences.

The tach adapter is part of the coil pack on the 1990 model, it's in the power transistor module in the 1991-94.
1990 model is on the bottom.

1990 power transistor module (top) compared to 1991-94. Notice the harness differences.

The new 1991-94 Coil pack harness (left) and power transistor module harness (right)

All the coil pack wires directly connect to the power transistor module wires. (match the colors)
The black wire with white stripe will connect to original wiring also to make a 3 wire pigtail. See wiring diagram for more info.

This is the old noise filter. This isn't needed anymore since there's a noise filter built into the new coil pack.

Here's a picture of the wiring completed before the heat shrink tubing is put on.

I opted to open the wire loom all the way back to the noise filter harness so I could solder all the wires together there. If you don't want to do that much work you can just cut the noise filter off , solder the wires all together then plug the harness back in. Make double sure that these wires are well insulated, you'll really bugger things up if these short on anything else.

Here's a close up of the three wires that must be joined together since the noise filter won't be used anymore.
They are all white wires, one has a ground strap insulation around it though and is in a brown sheathing. See the picture if you don't understand what I'm describing.

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This page was last edited December 10, 2001 02:07 AM