In this case, understanding how the system operates was key to solving the issue.
A 2018 Toyota Sienna minivan was brought to us with the customer complaining that the blower was stuck on high, with a blank HVAC control panel display. The backlighting for the control panel was illuminated but the display itself was blank.
We took a quick look at a wiring diagram to see what the system components looked like and which modules were involved. We found that there was a central controller called an AC Amplifier Assembly plus a front and rear control panel that communicate with the AC Amplifier via a LIN bus. Since the control panels reported to the AC Amplifier and the Amplifier was on the vehicle’s main data bus, the Amplifier was checked for codes. We retrieved codes B14B2 (Lost Communication with front panel LIN) and B B14B3 (Lost Communication with rear panel LIN).
Testing for open and short circuit
We read through the trouble tree for code B14B2 to see what the manufacturer wanted us to test and or replace. Basically, the instructions suggested checking the Front Control Panel for power and ground and testing the LIN circuits for open and short circuits. If this worked out, the next step is to replace the Front Control Panel, if this does not solve the problem, the next solution is to replace the Amplifier.
Our test of the wiring for the Front Control Panel revealed no faults nor did we find anything wrong with the LIN bus. We decided to scope the LIN bus to see what the LIN signature looked like. The LIN bus was shorted to ground and showed a flatline at 0 volts. When the Amplifier was unplugged the LIN bus jumped up to the Bias voltage. We concluded that there was a shortage in the Amplifier, so we ordered and installed a new amplifier. Unfortunately, the new Amplifier did not fix the problem.
At this point, we had to take a step back and re-assess what were doing. What was really needed here was a better description of the operation of the LIN bus as there is an obvious problem with the bus. LIN uses a master and slave type of bus, in this case, the Amplifier is the Master, and the two Control Panels are the slaves. That means that all communication must go through the Master. Most systems that use LIN on a multi-module bus, wire the bus wires together in a parallel circuit external to the Master. This was not the case with our Toyota, since the LIN circuits are connected internally in the Amplifier. Once we realized how the system worked, we knew that we had to look at the rear LIN. These were separate wires going into the Amplifier and were not mentioned in the trouble tree assessment for the B14B2 code.
HVAC system responds
We scoped the rear LIN and found that it was shorted to ground as well. After a quick check to see if this was causing our problem, the rear LIN was disconnected from the Amplifier. As soon as the rear LIN was removed from the Amplifier the Front Control Head came back to life and the HVAC system responded to Control Panel Inputs, confirming that we had a problem in the rear LIN.
At this point, we started checking the wiring to the Rear Control Head and discovered that if we pulled down on the headliner, the LIN bus came back to life. It turned out that mice had chewed through the wiring shorting the LIN bus to ground. Other than the fact the headliner had to be removed to access the wiring, a simple wiring repair was all that was required to fix this issue.
We made a couple of mistakes on this one, if we had chosen the B14B3 code instead of the B14B2 code the trouble tree would have worked, secondly if we had realized that the LIN bus wires were connected internally in the Amplifier, we would have checked the rear LIN earlier. The bottom line here is to make sure you understand all aspects of whatever system you are trying to diagnose.