OBD0MPFI ECUs have two main designs that affect how the ECU is chipped.

• 90-91 ECUs have an external 38256 (same as 27 C256) ROM. This can be removed and replaced with any of the usual candidates.
• 88-89 ECUs have an Oki83 C154 processor in them and lack the external ROM so easy to replace in the later ECUs.
• 88-91 USDM PM8 (HF) ECUs all use the Oki83 C154 design, even in 90-91.

All is not lost, however. There are THREE ways to chip a 88-89 ECU, and neither are terribly hard:

NOTE to PM7-B020 owners: As this topic suggests, pin 31 of the MCU should be disconnected from the PCB before it is grounded to enable the external ROM chip. Failing to do so may result in solid CEL and intermittent ECU operation. This issue was found only on PM7-B020 and should be considered on other ECUs only if the normal "jumpering" doesn't work AND there are no faults in adding the socket. This should also not be a problem if you're adding a daughter-board for the MCU, but yet again, be careful.

1. Replace the 40 pin MCU with another Intel8051-compatible MCU that has Internal ROM like the Oki83 C154 and program the MCU with an A5-free program. Requires modification of stock programs to remove the weird use of A5!!! Requires further mods to the program to make it tuly Intel8051 compatible] Note: As of 12/Jan/04, this has not been finished.(Current project sez Dave) Requires a programmer that can program the Intel8051 chip you choose to use. My method of choice. (Dave)

2. Replace the 40 pin MCU with a daughterboard that has a socket for an EPROM, a 74 HC373 Latch, a socket for the original MCU and _EA setup for an external ROM instead of internal ROM. Does not require modification of code - stock program will work fine because Oki83 C154 is still used. Does require a circuit board, additional parts and significantly more soldering. See also here

3. This is the easiest way.. You use the same programs as a 90-91.. And install a socket with 6 to 10 inches of wire to each pin. then follow the chart to install the wires from the eprom socket to the board.

Pin 31 of M83C154 to pin 20 of M83C154 (Jumper pin) to swicth to the eprom code.. If you will, Pin 31 on the MCU is the EA (External Addressing) ping, and when you tie it to Pin 20 (GND) you force the MCU to execute from external ROM.

Another possible way of doing this step involves creating a socket for your new chip that sits on top of the External RAM chip. This approach greatly reduces the wires that need to be used, but the special socket board itself gets a bit triky and may present you with clearance problems ( two chips and a socket on top of each other may stand so tall that you may not be able to mount the top cover of the ECU). Also, this approach is NOT recommended for people new to soldering. It involves some skills with the iron and a lot of caution. The idea is to mount a socket ( ZIF or not) on a small prototyping board and add 2 single-row headers on the bottom of the board. The headers get soldered to the External RAM chip, and your newly burned chip goes into the socket. Some of the header pins have to be cut, but you can use a rule of thumb here. Leave only the pins that the table below says are connected to the XRAM. Or just look at the attached pictures to see which ones are missing. If you get stumbled upon something, Take out the datasheets and triple-check. One more thing i highly recommend is after you're done, check the circuit with a continuity tester, but use the table above! The two tables ARE be electricly identical.

Before you start, examine the photographs I did while chipping my PM6. This will help you get a better picture of what the final thing should look like. I made a small cut on the proto-board to denote PIN1 as is done on most ICs. Also, the below mentioned connection from EEPROM Pin 28 to Pin 1 was made after the photos were shot, so You will not see it.

Just to make it clear, Pin 20 of the EEPROM is connected to EEPROM pin 14 which (as the table suggests) is connected to RAM pin12 and therefor it is connected to GND. EEPROM Pin 28 is connected to EEPROM pin 1 and they both are connected to the MCU pin40 which is the +5V Vcc.

And don't forget to connect "Pin 31 of M83C154 to pin 20 of M83C154 (Jumper pin) to swicth to the eprom code..."

Attachment?:	Modify:	Size:	Date:	Who:	Comment:
DSC00663.jpg	mod	65387	10 May 2005 - 13:47	1net	Socket Board ( Top)
DSC00662.jpg	mod	67627	10 May 2005 - 13:48	1net	Socket Board ( Bottom)
DSC00667.jpg	mod	99145	10 May 2005 - 13:49	1net	Soldered Socket Board 1
DSC00672.JPG	mod	125029	10 May 2005 - 13:50	1net	Soldered Socket Board 2
DSC01263_small.jpg	mod	157074	17 Aug 2005 - 06:45	1net	Soldered Socket Board 3 ( all done)