STREET SMART: A Silver Bullet


Figure 3 Replacing the sensor didn’t keep


the code from coming back, so we can be pretty sure that’s not the problem. A quick check of the wiring sche-


matic (figure 3) shows the ISS has three wires: power feed, sensor sig- nal, and ground (referred to as "Low Reference"). The power feed to the sen- sor also feeds the shift solenoids; since the vehicle still shifts when the code sets, the power was probably okay. Next was the sensor signal; the


ISS is supposed to send a variable frequency signal to pin 3 at the trans- mission control module (TCM). Signal frequency should increase with input shaft speed. In this case it stayed at 3 Hz. But there was a signal there, so the circuit was probably okay, too.


8 The only thing left was the ground


to the sensor from pin 45 at the TCM to pin V at the transmission connector. A quick check showed that the computer wasn’t supplying ground to the ISS sen- sor. Without ground, the sensor won’t create its signal. The technician spliced a chassis


ground into the sensor ground wire, and suddenly the sensor was providing a variable frequency signal. After clear- ing the codes, the technician drove the vehicle; no further problems, and no more code P0171. This is your silver bullet: a ground


wire to the ISS ground circuit. Tie in a ground wire to pin V at the transmis- sion (Make the splice on the inside of the transmission so it’s a clean repair).


GEARS March 2012


This ground wire has fixed several calls to the ATRA HotLine about DTC P0717, without creating any other prob- lems to the system. It’s fast… it’s easy… and it works perfectly. So before you pull it, make sure


you check it. That’s not just smart, that’s street smart!


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