From my old days of repairing Commodore Amiga computers, I remembered that we used to inexpensively make a dual ROM switch just by soldering two chips together and fitting a switch on a couple of pins. I figured the same would be possible with the BMW Motronic unit.
The new chip arrived and was a standard copyable EPROM (a 27C256, 120ns to be exact). I made a copy of it and a copy of the factory chip and made a simple switch to choose between the chips. The switch is hidden underneath the dashboard so that it can't easily be switched whilst the engine is running (which could do something bad).
How to make a dual-chip switch
This will probably only work if your after-market chip is a 27C256 EPROM. Some makers use non-copyable chips and I have no idea if this will work with them. I take no responsibility if you or this modification damage your chip(s) or your car in any way.
You will need the following tools:
• Soldering iron (and solder)
• Socket or spanner to remove the Motroonic unit
• Torx type screwdriver to dismantle Motronic unit (I can't remember the size)
You will also need the following parts:
• A SPDT switch, any type you want (I used a toggle switch)
•Three wires (I used three wires off a computer type ribbon cable), length will depend on where you want to fit the switch
• Optional - one or two blank 27C256 120ns (or faster) EPROM chips - if you want to use copies of your chips (You will also need access to an EPROM programmer)
• Two 22Kohm 1/4 watt resistors
Optional - a 28 pin IC socket
Follow these steps:
1. Remove the Motronic unit and take off the lid held on by four Torx screws.
2. Remove the EPROM chip which may be held in place by a plastic clip.
3. Bend up straight pin 22 of both EPROMs (or socket).
4. Place one chip (or socket) on top of the other. You may want to bend all the pins straight first so that the chips stay in place by themselves.
5. Solder all pins (except the bent pin 22's) of the top chip/socket to the other chip.
6. Twist the leads of one end both 22Kohm resistors together and solder this end to pin 28 of either EPROM.
7. Solder the other lead of one resistor to pin 22 of one of the chips.
8. Solder the other lead of the other resitor to pin 22 of the other chip.
9. Solder a piece of wire to pin 22 of the socket and the other end of the wire to the centre pin of the switch.
10. Solder another piece of wire to pin 22 of one chip and the other end to either of the outer pins of the switch.
11. Solder another piece of wire to pin 22 of the other chip and the other end of the wire to the other outer pin of the switch.
12. Plug the two chips into the socket.
13. The switch is completed so now all you need to do is fit it into the Motronic unit, run the wires out of the unit so that they don't short out, and find somewhere to mount the switch. You should ensure that insulation (heatshrink or electrical tape) is used to ensure that nothing shorts out with the switch.
I wouldn't recommend that you switch between chips with the engine running. I am not sure what the result is but it may damage something.
One other thing I have noticed is that the 318is seems to tune itself to fuel quality, etc and store this information. When you disconnect the battery (or just the Motronic unit) it seems to forget these settings and run rough for the first few miles. It may be a good idea to reset the Motronic unit when switching chips by disconnecting the battery negative lead or pulling the Motronic fuse for a few minutes.
(Thanks to Alvaro from Spain for the attached diagram)
Comments