Ford PATS transceiver scope diagnosis
Customer stated thate vehicle would not crank. Verified no crank symptom and noticed security light flashing rapidly. Found codes in PCM as follows. B1681 | Passive Anti Theft System (PATS) Transceiver Mod Signal Not Received B2103 | Passive Anti Theft System (PATS) Transceiver Circuit Fault. Cleared codes and B1681 returned. Neither of the two keys in vehicle worked.
Tested the power and ground at PATS transceiver module and it check OK.
Scoped the two data communication wires between the PATS transceiver module and the PCM. Found that the PCM was properly pinging the PATS module but the PATS module was not returning any information back to PCM.
Read the description below of how the PCM and the PATS module communicate.
The yellow scope channel is connected to the Gray/Orange wire which is the circuit that the PCM outputs 12 volts to the PATS module with. The PATS module is then supposed to toggle this circuit to ground to send the key code information back to the PCM. The green scope channel is connected to the White/Light Green wire which is a 12 volt output from the PATS transceiver to the PCM. The PCM then toggles this circuit to ground to request key code information from the PATS transceiver module. The first scope capture shows two ignition cycles from OFF to ON. This two way communication between the PATS transceiver and the PCM is designed to happen immediately after key on to allow the vehicle to crank and start. As you can see in this scope capture. The Green trace shows that the PCM is toggling a request for the key code information to the PATS transceiver. However the yellow trace just remains stuck at 12 volts. The PATS transceiver is not toggling this circuit to ground to send the key information back to the PCM. So we condemned the PATS transceiver and got authorization from the customer to replace it.
We installed the new PATS transceiver module and cleared the stored codes and the vehicle cranked and started on both keys. The vehicle would not crank until I cleared the stored codes. You can see on the scope capture above (showing one ignition cycle from OFF to ON) that we have the PCM request on the green trace and then the PATS transceiver response on the yellow trace. On this vehicle the PCM stores the PATS key codes and the PATS transceiver is simply the sensor to relay the key code to the PCM. So this means that we did not have to relearn the PATS key codes after replacing the PATS transceiver. Also the PATS transceiver is not a programmable module on this vehicle so that did not require any programming. The PCM still had those key codes stored and it was as simple as replacing the PATS transceiver, clearing the codes, and testing for both keys to start the car.
Interesting. I would have bet money a 2004 Ford would be start-stall with a PATS issue. Thanks TK!
Hi Geoff: This is an "E" type. It inhibits starter engagement. HTH, Guido
I have the chart. It's a good example of why we shouldn't ever "remember" how things work, but look it up.
Timothy, I would also check if the PCM update has been done for PATS issues. The issue is when the battery goes dead or low it will forget the keys. Just to make the job complete.
Hi John, Thanks for the tip. If my memory serves me right, I believe I had checked for an update with IDS and it was current. It has been a few days ago, so I hope I'm not getting mixed up with another Ford.
Are the scope channels reversed in the after picture?. In the before yellow is the Data TX and green is the request. In the after yellow is active before green and green has a data packet.
I am fairly sure I had the channels hooked up the same way the second time as the first time.
Thanks for sharing. So the PATS transceiver doesn't need programming. Interesting
The transceiver essentially is an antenna for the module that holds the keys. Changing the antenna wouldn't effect the key ID's written to the module.
Thank you for posting! Very interesting to me....luckily you had 2 keys and could confirm the likelihood of 2 keys going bad at once is slim. (speeds diag process) Reading the service operation information you posted, it immediately made me think "Why" does this vehicle have an external transponder device with coding? It seems to only be a gateway for the key signal to go to the PCM. Why not…