ALDL1

From SaturnWiki

1 ALDL Interface Cable

ALDL Interface Cable

Most any monitoring or logging software will need a cable to interface with the ALDL port under the dash. There are two types of ALDL interface on the Saturn S series vehicles. OBD-I, which is described on this page and OBD-II.

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Cable Types

ALDL interface cables for interfacing with software such as EFI-Live, are simple level converters that also combine the TX/RX signal into one wire to communicate with the PCM.

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DIY Cable

Probably the easiest DIY cable is a simple two transistor converter.You can build this circuit for about $5.00 with few parts. There is another cable design, that involves a max232 IC, I will not cover it because, simply put, there is NO situation where the max232 circuit will work and the 2-transistor won't. The max232 circuit is just overkill, and extra complication.

Schematic for a home brew ALDL level converter

Completed circuit on a prototype board (Front)

Completed circuit on a prototype board (Back)

Completed circuit on a prototype board (Back with overlay)

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Parts List

3x 10kohm resistors

3x 1n914 diodes

2x 2n3904 transistor

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Connection

Note:

DO NOT use the ground in the ALDL connector and expect things to go smoothly, that ground is noisy and will kill your ALDL logging session constantly with Checksum errors. Get a good clean ground from the frame somewhere, I personally use the bolt that holds the dash frame just to the left of the ALDL connector. As well I have hardwired the interface into my car, with the serial cable stashed in the glove box.

Note:

On 1995 vehicles you will need to add 2 switching diodes in series between the circuit and the ALDL port, with the bands pointed towards the circuit. This may also help clean up the data on other years as well.

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