Yeah, the TSRMs are different after 89. They don't provide some information but the pre 89 skips some things too. For example the 87 has very little about engine lubrication. Since the AFM on N/A cars is a potentiometer vane unit the following applies only to the Karman Vortex AFM used on turbo cars:
THA is referenced to E2 while the Ks signal is referenced to E1. (E1 is chassis ground while E2 is sensor ground). Be sure you're referencing the right pin when you measure.
Electrically the AFM is two distinct circuits: Air flow and Air temp. There is no internal connection between the two. Vc is the power supply for the AFM, Ks is the Karman frequency, and E1 is the Karman power supply ground. THA and E2 are separate and should be measured as such. THA requires no power supply from the ecu as it's just a thermistor.
When you measure E1 and Vc you're measuring the internal circuit that supplies power to the entire Karman section of the AFM and the optical emitter (the led that shines on the internal mirror). When you measure E1 and Ks you're measuring the optical detector, the photo transistor that reports to the ecu what the mirror is doing. This is the signal that represents air flow.
Your AFM should test out with the resistance values specified in the manual but another way is to test it on the car. THA and E1 are simple enough, just ohm across them and see if you get a value that corresponds to the temp. Plug the AFM in, start the car, and measure the voltage between Vc and E1. Should be around 5 vdc. Again, this is the power supply for the Karman section of the AFM and it's common to most of the car's sensors. Because the ecu is digital it supplies everything with 5 volts and without this the AFM will be dead. (The same goes for the throttle position sensor).
Next measure the Ks signal using the frequency function of your meter or a scope. If using a scope you'll see a square wave with an amplitude of 5 volts and the frequency varying with rpm. At idle the amount of air flowing through the AFM should generate about 15-20 hertz. At 2500 rpm it should be around 30-40 hertz.
So in summary:
1) Ohm THA and E2. The resistance must fall within spec for the temp..
2) Check for 5 volts between Vc and E1
3) Check for around 20 hertz (at idle) between Ks and E1, increasing in frequency with rpm.
If you have this the AFM is working. It's best to use a scope to see how clean the Ks signal is but if the car runs and the frequency checks out the odds are everything is as it should be. Of course, you must consider that if the engine is doing something goofy for other reasons the airflow though the AFM will be effected. Hope all that wasn't too confusing.
