Cabin Blower stopped

It looks like the logic for the PWM is negative going, such that +5V at S is the off state, and 0V is the on state. That would make sense of your second set of readings. A high K-ohm input reading at the BMC would be about normal. So far so good. There is a possibility that the A/C output is just an open collector output, and requires the pull down impedance of the BMC to make a proper looking waveform.

 

I'm getting a bit confused. Page 7 of 7 in the repair manual pages attached above suggests that the square wave peak-peak is supposed to be 11V. OP measures 5V peak-peak. How does this point to the BMC being the failed component?

The repair manual says that if the measurement of 11V is good, then replace the BMC. If no good, then replace the A/C amplifier. As far as I can tell, the OP's measurement is "no good".

Since the OP is borrowing a scope, I suggest the OP ensure the scope is operating as expected:
1) If the scope's probe calibration signal output specifies peak-peak amplitude, make sure that square wave signal displays a vertical displacement as specified.
2) Use a known good voltage source (say, a new 9V battery) and make sure the scope trace displaces ~4.5 divisions when the vertical input sensitivity is set to 2V/division, for example. (Actually, a new 9V battery will produce ~9.6V so to be precise, the scope trace should displace ~4.8 divisions @ 2V/division vertical sensitivity.)

Also, if OP needs to probe a wire for a voltage measurement, rather than cut the insulation, I suggest inserting a small needle into the wire to pierce the insulation. Then attach your probe to the needle. That might be less intrusive to the wire.

 

I believe you can remove the visible clip on the right-side of the lower center panel. There are six hidden claws and clips that hopefully will release as you tug at the panel.

OK. Unless you find an error in the scope measurement, it sounds like the A/C amplifier is at fault.

 

Sounds like the scope is working fine. Usually a scope is rated for ~3% or so error on DC voltage measurements; AC voltage measurements depend upon the frequency of the measured signal vs. the vertical channel bandwidth of the scope. That should not be an issue with the very low frequency signals we are dealing with here.

What kind of scope are you using, BTW?

 

Interesting that you see a waveform period of 200 microseconds, vs. the service manual's illustration which shows a two-second period.

If you set the horizontal time/division to 500 milliseconds/division as per the manual, what do you see? (This is the same as 0.5 seconds/division) You may have to set the trigger to NORM, not AUTO.

If there is a stable image, can you take a photo of the scope image and post?

OK, this is spec'd at 200 MHz bandwidth, two channel model. That is certainly more than sufficient for your testing purposes here. However, this is one of the first digital scopes made by Tektronix so watch out for display artifacts that may be caused when the horizontal time/division control is incorrectly set compared to the signal's actual period.

 

Here is the waveform that you should be seeing, attached.

 

I was looking at my copy of the repair manual, and the scope picture shows 50 usec/division (50 microseconds) with the waveform period covering four divisions. This is different than the docs posted by noisebeam and nh7o.

Noisebeam is actually finding a waveform with 200 microsecond period, which appears valid based upon my repair manual. The only remaining question is why the p-p amplitude is only 5V vs. 11V per the manual.

 

To reconcile these two measurement findings with the repair manual, I hypothesize that the air conditioner amplifier is at fault. My guess is that the output which noisebeam is measuring is faulty because it has an open-collector inverter which has shorted and the input of that inverter (operating at TTL 5V logic voltage levels) is being passed directly to the output. This would explain why the waveform shape is opposite to what the manual depicts, and the voltage is 5V instead of 11V.

If the OP does not find any fault with his measurement technique when he measures directly at the A/C amplifier output, I would feel confident that part is at fault.

The OP might want to check here to see if a used A/C amp can be purchased at a reasonable price:
04 and newer Prius Parts Cars

My manual is for the 2005 model year. My guess is that when the manual was uploaded to techinfo.toyota.com there may have been some editing which in this case was done in error. It makes much more sense for the control pulse-width-modulation signal to be operating at 5 kHz instead of 0.5 Hz.

Also, you should have no doubt that you can fix the problem yourself. You already know that the problem is either with the A/C amplifier or the blower motor controller. So worst case, you can buy and install both and call it a day.

However, as stated above, I would say there is a 95% likelihood the A/C amplifier failed if you confirm the scope measurements directly at the A/C amplifier output. Please make sure of the earlier measurements (described in the seven page manual excerpt) which confirm the presence of battery voltage at both components and that the wiring harnesses are providing good ground connections.

 

I agree that there is a possibility of a typo in the manual, as the low frequency does not make sense. However, the tech using the logic pulser in the video does indeed seem to be getting a low pulse rate. That may be due to the way the pulser itself registers input. So I'll be curious to know what the real answer is.

 

The video is of a 2010 Prius, and apparently Toyota implemented a design change for G3, where the BMC is integrated into the fan housing to give the BMC the benefit of the cooling air flow.