94 Roadmaster ALDL A/B jumper runs fan

[Towroma]

Trying to access OBD I codes. Jumping A and B does not cause SERVICE ENGINE light to flash. When ignition is turned on, the cooling fan runs. What could this mean?

[maxwedge]

Could be obd 1.5 which needs a scanner or the wrong terminals are jumped.

[Towroma]

The underdash connector has the standard 12-pin OBD 1 configuration. I jumped the two rightmost pins on the top row. The SERVICE ENGINE light flashes once very briefly, then nothing more. The cooling fan (only one electric fan with tow package) runs continually with the pins jumped. I'm baffled.

[Blue Bowtie]

Both conditions are correct. The 1994 and 95 B- and D-Bodies require a scanner to access stored codes. The old method of jumpering the ALDL to force the MIL to strobe out the stored codes is not applicable. My '94 Impala acts exactly the same way.

Also, even with the older ECMs, once in diagnostic mode (ALDL jumpered) the electric cooling fan will operate. That is normal all the way back to my 1986 Trans Am, and probably some even older ECMs with the 8K PROMs and no CALPAK.

Further, many places which offer free code scanning (like Auto Zone and other chains) will be unable to scan your vehicle since it is not truly OBD-II. A Tech II, Snap On MT2500, and some Actron scanners will be able to access the ECM. Some PC software programs with a proper adapter can also access the ECM data.