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This page is lifted from the Technical Info page at AKM
Electronics. Please
contact us
if you have questions
Here are the topics covered here:
1. How is the under dash connector wired to the PCM
in the OBDI LT1 cars?
2. What happens if my PCM halts programming in the middle of a
programming sequence with LT1_Edit?
3. How do I make a bench top programming setup for my LT1?
4. How do I build one of your interfaces? Where's the schematics?
5. Where's your schematic for a Meziere electric water pump with a
blown fuse / bad relay LED and bad ground LED?
How is the under dash connector wired to the PCM in
the OBDI LT1 cars?
This answer is targeted towards folks doing LT1 retrofits (Monte
Carlos / Street Rods / El Caminos / S10 Pickups, etc)
OK, grab your voltmeter, wire snippers, and soldering iron, we're
gonna have some fun. You need to have three wires going from the ALDL
connector to the PCM. One of these wires needs to be a ground, another
needs to be for serial data, and the third needs to be for mode
select.
For the older rectangular 12-pin style, the under dash connector
has the following pinout:
F E D C B A
G H J K L M
A = Ground
B = Mode Select
M = Serial Data
(on some older cars E = Serial Data, my cable has "E" and "M" tied
together anyway)
For the OBDII style connector (16-pin D shaped), it has the
following pinout:
01 02 03 04 05 06 07 08
09 10 11 12 13 14 15 16
05 = Ground
06 = Mode Select
09 = Serial Data
(it's also helpful to have 04 = Power Ground and 16 = +12V wired in
as well)
On to the PCM now. The PCM has the serial data on Blue 30 (D30).
This needs to go to "M", "E", or 09, depending on your connector
above. It's a tan wire on my 1994 PCM.
Mode select needs to go to Blue 20 (D20). It's a White/Black wire
on my 1994.
ALDL Ground needs to be tied in to Blue 1 (D1). This ground wire
actually winds its way through the chassis to get to the PCM, but it
wouldn't hurt to run a jumper wire directly to it. The better your
ground, the better your ALDL communications reliability.
Mode select typically isn't used on our cars for the laptop based
scan tools or the LT1_Edit program. It's for diagnostic modes that
only the Tech2 will allow. So, it's good to wire up if you think one
day you'll hit it with a Tech2, but you don't have to have it wired
up.
What happens if my PCM halts programming in the
middle of a programming sequence with LT1_Edit?
It has happened (very rare, but it does), it is a risk we take.
LT1_Edit, when programming, erases one flash chip first, programs it,
then erases the second flash chip, and then programs the second one.
So, if your programming sequence halts during the first 50%, the first
chip has been erased and your PCM will not boot (to start the car). If
your programming sequence halts during the second 50%, the second chip
has been erased and the PCM will not boot. Your PCM is not dead, it
can be revived, but it needs surgery. The erased chip will need to be
removed from the PCM board and reprogrammed with an flash chip
programmer.
Andrew at AKM Electronics is able and willing to fix PCMs for a
small fee as a favor to AKM customers.
Feel free to email Andrew directly, but please own one of his
cables.
To make sure your PCM won't lock up, you need to disable ALL power
saving features of your computer (both in the laptop BIOS and in
Windows), disable the screen saver, make sure all automotive
accessories are turned off (A/C, Radio, etc), and make sure your
laptop battery is sufficiently charged. And don't crank up your home
stereo and play the drums on your steering wheel while programming or
reading - that may cause data errors (right LJ?).
How do I make a bench top programming setup?
Here's another handy tool if you're working with more than one PCM
(as I am). Or if your motor is out, but you want to do some PCM work.
Cost is under $5 for the harness parts, and about $32 for the power
supply (you can use a car battery, but only temporarily, as it will be
drained). Very easy to build.
First, collect the components. You can get everything from
RadioShack.com (hey, I never leave the house anymore to shop!). Now,
you can buy these as a whole connector at the store, but if you're
doing it over the 'net, here are the part numbers and descriptions:
(1) Crimp type cable mount D-sub, 25 pin Female, 910-4770, $.62 /
ea
(15) Gold Pin Sockets for above plug and for PCM pins, 910-4772,
$.09/ea
(1) Plastic hood for D-sub connector, 910-1536, $.72/ea
(1) 13.8 VDC Regulated Power Supply, 910-0442, $31.95/ea
About 8-10 feet of 24 or 26 gauge wire
Now, I just tore apart some old equipment that I had laying around,
yanked a 15 volt regulated power supply from it, and put two diodes in
series to drop the voltage going in to the PCM down to about 14 volts.
You need the voltage higher than 12 volts, but lower than 15. So 13.8
regulated is a good target.
What you'll need to do is to make a wiring harness using the gold
pin sockets. There are four required for the ground (0V) wires, and
four required for the hot (+12V) wires. I did them in a daisy chain -
strip the end of the wire, crimp and solder a pin on there, cut the
wire a few inches up, stripped *both* pieces of wire, crimped and
soldered both of those pieces to another socket, and kept going until
I'd covered the 4 pins I needed.
For the ground wire, I made the first jump from D1 (Blue 1) to C32
(Gray 32), about an 8 inch piece of wire. Next, we jump from C32 to
A18 (Red 18) using about 4 inches of wire. Next we loop from A18 to A2
(the adjacent pin) with a loop of about 1.5" of wire. From there, we
go to the ground terminal of the power supply.
For the +12V wire, my first terminal is at B15 (Black 15). From
there, I jump with about 1.5" of wire over to B31 (the adjacent pin),
and from there about 1.5" of wire to B30 (right next door!), and then
have about a 4 inch piece of wire jumping to D3 (Blue 3). From D3 I
jump to the positive terminal of the power supply.
Now for the ALDL Interface. You need at least two wires - ground
(Pin 1 on my converters), and serial data (Pin 5 on my converter, D30
(Blue 30) on the PCM. You may need to provide 12V, but most converters
were shipped with port power option enabled, and all desktops should
support the port power option.
That clear as mud? ;) Email me if you have any questions!
How do I build one of your interfaces? Where's the schematics?
See my Assembly
Notes Page for all the gory details! That's old stuff, though.
Also check out the New
Style Converter page, no more jumper wires everywhere! I don't
sell them like on the 1/29/00 Page anymore...
Where's your schematic for a Meziere electric water pump with a
blown fuse / bad relay LED and bad ground LED?
Check this diagram out:
Meziere Install
Schematic
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