2019 Toyota Highlander HVAC Fan Blower Runs Full Speed All The Time
A local body shop called me in to look at a late model Toyota SUV. It had been damaged on the Left Rear quarter panel and deployed one airbag. The concern was any time the battery was connected, the HVAC blower fan would run full blast. The first thing I did was check for codes. No codes in HVAC. I then took a look at the wiring diagram. The fan blower module has three wires. B+ (power all the time) Ground and a PWM signal. In checking each lead disconnected, there was 12.8v on the B+, 3mv on the Ground and a square wave on the small lead. I figured that there was a bit of potential difference in the place I grounded my scope and the ground at the blower. I was not too worried about that. The first couple of captures were a little different than I expected. The bottom of the waveform was chopped. I asked the shop if they had another Highlander to test against. They had one. The square wave was cleaner. I did not like the waveform in the first but I suspected the issue was the fan module. I had the customer remove the fan from the second Highlander. I plugged it into the target vehicle. The fan sped up and slowed down as commanded on head unit. With this fan connected the waveform was nice and square but elongated. The fan module was condemned. This vehicle has 8500 miles on it. I suppose the air bag deployment could have destroyed the circuit. It could also be a defect that could lead to a big national back order. It seems a little odd to have a part that could fail to run all the time being connected to B+. It seems you would want a switched lead. Maybe the design will be changed in the near future.
There were some GM models that did that in the 80's or 90's. "Blower module" was the failure there iirc. Everything old is new again. :-)
Are you saying that with the blower connector removed there was a pattern on the SI circuit?
Hi Jim, I think auto correct got a hold of your question. With the blower motor connector removed there is a pulse on the SI circuit. I am not sure of how it works internally but it seems like there would be a transistor inside the circuit of the fan module. It would use the signal and B+ (or GND) to vary fan speed. I think I will see if I can get the fan motor to take apart if they will give…
I am thinking the pulse control device in the fan motor was leaking power to have it turning and the signal shows the rise due to the leak. There does seem to be some negative risk, as you point out, where the 50A fuse supply to the fan motor is not switched. Maybe the worst is the fan staying on and draining the battery energy? It seems to me that Toyota engineering is not what it once was.
You need to specify front or rear blower. On the front I know for sure that the SI line voltage is set by the blower motor. The AC Amplifier sets the Duty Cycle % to control speed. Since the motor will not respond to anything above limit A or below limit B the unit may be hot at all times. This is basically identical to the newer GM blower motors. GM will set some codes while Toyota does not…
Hi Jim, it was the front fan that was being diagnosed. The rear was behaving.
Was there any welding done during the repair? It would not surprise me to see one of the transistors damaged during the welding process.
Mark, Not sure on the welding. This shop is pretty careful about disconnecting the battery but anything is possible.
Disconnecting the battery is not enough. I am sure your body shop knows this but they should disconnect the battery and then install a large capacitor on the system to protect the system from transient voltage. amazon.com/OTC-3386-Antiz… Most of the critical or expensive control units are pretty well protected but it would not surprise me of something relatively non critical as a…