Auto Xray, Innova and a few others make OBD-I readers for our cars. Look for something that can view PIDs. Amongst those read is ECT.
You can also simply check voltage readings at the ECT and interpolate them. The ECT uses a thermistor so dont be surprised to see voltage and resistance be related (not inversely). I typically check the readings at about 100*F (my ambient temp) and then at 180*F (operating temps).
One quick question: You don't have the aftermarket temp sender in the t-stat housing do you? If so, it can be normal for it to jump around because the stat affects the reading.
You can also simply check voltage readings at the ECT and interpolate them. The ECT uses a thermistor so dont be surprised to see voltage and resistance be related (not inversely). I typically check the readings at about 100*F (my ambient temp) and then at 180*F (operating temps).
One quick question: You don't have the aftermarket temp sender in the t-stat housing do you? If so, it can be normal for it to jump around because the stat affects the reading.