Andy:

It would be interesting to know how the diode fits in the circuit; that is does it have positive or negative voltage across the diode when operating.

If the diode is back-biased (negative voltage) it will look like an open circuit (high resistance). If it has a positive voltage across it, it will have low resistance.

Based on my car, if you want to retard timing a little bit, you go to a LOWER resistance value. I would find out what resistance you had in the original Resistor you removed. I would also ask the dealer why the original resistor was replaced with a diode. Try the original resistor back in the car. If it pings, go to a smaller resistor--you can buy them at Radio Shack or other electronics store. You probably need a thin-film 1% resistor for permanent replacement, but can use any type just to try it out.

Let us know what happens!

Regards

Tom Elerding