Which tach are you using?
Did you set up the tach output correctly?
I have used as low as 480 ohm resistors to pull-up. Some PCMs don't work with 1K ohm for unknown reasons.
Be aware that using low resistor values put more stress to the sink circuit of the PCM, it may damage the tach output.
I have done it to a few swaps and they are working ok for more than 2 years, but use low resistor values only as a last resort.
If you don't want to risk your PCM, use a converter like this:
http://www.summitracing.com/parts/dak-sgi-8
I'm searching for a cheaper alternative, no luck so far