ECU PROM Replacement

!!! CAUTION !!! - Read all instructions carefully before starting work. Failure to properly execute some instructions could damage components and result in the vehicle being immobilized. Then you gotta do that most feared of all do-it-yourself jobs....call the tow truck, schlep it to the shop, and ask someone else to fix your screw-up! :^)

'Nuff said.....Now, on with the show!

Tools needed:

1. Phillips screwdriver (preferably magnetic, if not you may need a magnetic retrieval tool)

2. Normal Phillips screwdriver

3. Precision blade screwdriver (small jewelers type, or something very similar in size)

4. Slip joint pliers

5. Socket wrench

6. At least 12" of socket extensions

7. 10mm socket

8. 8mm socket

9. 8mm combination wrench

10. Anti-static mat or strap

11. Suitable supply of Guinness

My D90 has the factory A/C kit, the one that takes up a huge amount of passenger foot space. This kit has a large plastic vent/outlet (four total vents) box fixed to the lower dash area that spans the whole vehicle. The ECU is located behind all this kit. If your ECU is under the passenger seat stop here and commence drinking. If you are changing the fuel filter you can also use most of this procedure to gain access to the fuel relay so you can depressurize the fuel system.

Disassembly:

1. Disconnect the battery.

2. Remove the small plastic plugs that cover the screw access hole in the A/C vent box.

3. Remove the vent grates, gently push in on the sides and they will pop out with a bit of persuasion. This so you can easily see the screwheads for removal.

4. Using the magnetic screwdriver remove the screws through the access holes. If you don’t have a magnetic driver be careful or you may drop the screws down in the vent box and they can be hard to get out.

5. Locate the small plastic cover on the lower left of the evaporator box. At the bottom of the cover is a screw, remove it. Now you can lift the whole vent box structure out of the way.

6. You will now have exposed three 10mm locknuts across the top of the evaporator. Remove these, be careful not to drop them or the washers down to the bottom of the box!

7. Locate the small plastic cover to the right of the evaporator box. This one covers the Fault Code display, it is held on by two screws, remove them and place the cover out of the way

8. There is a small strap of metal that runs across the bottom of the evaporator box. Follow this across to the right and it will lead to an 8mm sheet metal screw that secures the evaporator box to the bulkhead. Using the long socket extensions remove this screw.

9. !!! CAUTION !!! - Now the evap box is loose and can be moved. Be very gentle when you pull it off the studs at the top as the only thing holding the evaporator up are the metal pipes, BE CAREFULL! Once the box is free of the studs at the top it will want to sag, allow it to sag just enough so you can access the sheet metal screws along the bottom of the dash. I used a folded up towel to support the box from underneath.

10. Remove the sheet metal screws along the bottom of the dash. Don’t forget the one to the far right and the one at the upper right.

11. With the screws removed you can now remove the metal plate with the studs used to hang the evaporator unit. It will slide right out.

12. You may want to slide the Fault Code Display box off its mounting tab and let it hang down out of the way. This will give you a little more room to work with the ECU.

13. Use the 8mm combo wrench to remove the two bolts and washers used to support the front end of the ECU.

14. Now the ECU is free to slide out. The ECU plug is held in by the clip on the right. The plug has a tab that fits into a slot on the left hand side of the ECU.

Replacing the EPROM:

!!! CAUTION !!! - Only do this if you have a good solid earth ground to work with. You will need the anti static strap or at a minimum be able to touch something that is grounded, I recommend your kitchen sink. Touch the ground several times during the procedure to make sure you have dissipated all static charge from your body.

1. Remove the screws holding the ECU cover on. DO NOT use the magnetic screwdriver for this!

2. Once the cover is off you will have exposed the circuit board. The PROM you are replacing is under a small plastic cover. It should be the only one like this. The PROM will only fit in one socket, as the other chips are either bigger or smaller than the replacement.

3. Using the slip joint pliers carefully grasp the plastic cover across the narrow part and gently rock the cover off.

4. With the cover off and the PROM exposed use the jewelers blade screwdriver to GENTLY GENLTY GENTLY pry the PROM up out of its socket. Note which way the lettering on the PROM is oriented, as the new PROM has to be oriented the same way!
   !!! CAUTION !!! - Be careful not to scratch any of the printed circuits near the PROM socket!

5. Making sure each of the little prongs is in the proper socket hole gently push the new PROM down all the way into the socket.

6. Replace the plastic cover.

7. Replace the ECU cover making sure the rubber gasket is seated properly.

Reassembly:

This is just the reverse of taking apart with one exception. With everything still apart, fit the ECU harness plug back into the ECU. Reconnect the battery and start the engine. If she starts and runs fine, stop the motor, disconnect the battery and reassemble everything.

If she will not start (like mine wouldn’t, DOOH!) it is possible you put the PROM in backwards (I managed to do this without even drinking one beer!) Don’t worry the PROM should be OK, just remove and install the correct way and accomplish the above test again. If it still doesn’t work I have no more ideas other than to put the old chip back in and get drunk.