T he days of taking a rebuilt unit off the shelf and just throwing it at the vehicle without much concern is becoming increasingly more difficult. You have to be con- cerned with whether the internal electronics are compatible with the vehicle’s system and how the unit mounts as well. I receive many calls on the ATSG Tech Line pertaining to interchangeability of parts, units, etc. In this instance the way the call goes is: We just finished putting this shelf unit in and now we have these @#&%* codes that we didn't have be- fore! In this article, the inter- changeability concern is gear ratio errors and the related codes they store in the Ford AX4S and AX4N transaxles. Keep in mind that this holds true for other makes and models as well. For example, the 4T60-E has FOUR different speed-sensor- rotor tooth counts. Install the wrong one and is that vehicle’s computer confused or what? In the AX4S and the AX4N the drive and driven sprockets determine gear ratio. These units have TWO different drive and driven sprocket tooth counts (See Figure 1), which seems to be a well hidden fact based on the number of calls we get concerning this problem. The two combinations are a 37/36 and a 38/35, and the problem is that either combo can vary between the same model car. For example, a regular Taurus passenger car with a 3.8 engine will have a dif- ferent tooth count from the same car and engine in a police, taxi or municipal application. The chart in Figure 2, page 34, indicates which model AX4S transaxle matches up to which en- gine, and the chart in Figure 3, page 34,  is for AX4N. The transaxle I.D. code is on the white tag usually found on the top of the bellhousing. Unfortunately, that tag sometimes is missing. In that event, refer to the chart in Figure 4, page 35, for the correct combinations. It’s a time saver if you tag that rebuilt unit with the necessary in- formation before it hits the shelf, like maybe which type of lockup solenoid is in it (you know what happens; don't want to go there), which engine it was mated to, sprocket tooth count, etc. These sprocket tooth counts de- termine gear ratio in these transaxles. Mismatching these sprocket tooth counts can cause code B28 or PO741 to be stored, Shift Pointers P  R  N  D  D  2  1 AX4S/AX4N Interchange/ Inter-trouble By Pete Luban Figure 1 Driven Sprocket Drive Sprocket Printed in the September 1997 Transmission Digest