1997 C5 corvette heater blower intermitent

Hi all from the not so sunny UK!
I'll put an intro to myself in the appropriate section shortly.

Anyway, got a C5 corvette in my workshop with dual climate control and an intermittent heater blower motor behind the glove box.

Here is the wiring diagram..



First thing....disconnect the heater blower fan and connect a test light between the 2 wires. Test light lit, multimeter said 10.8V.
Plugged it back in, no fan.

Probed the tan wire with my picoscope and got a PWM square waveform that went from 0.3 volts up to 5 volts. Didn't check the frequency but duty cycle was about 80% No heater fan operation still

Disconnected the 2 pin connector at the module (tan, black and red wires) and expected 12v. But got zero volts! waveform went flat line but very very noisy.

Plugged it back in, and square waveform appeared again at about 80% duty cycle, but no fan. Moved speed control up and down on HVAC controller and nothing.
Then the waveform suddenly dropped to about 10% duty cycle and the fan started working! No speed control though.

Then it stopped again!

Couple of things here......the diagram shows 12 volts within the HVAC controller, so when i probe the the tan wire and disconnect it from the module, shouldn't I get 12V??

Anybody shed any light on the operation of the circuit here? Looks like a normal potential divider circuit but doesn't behave like it.

Andy.

i think you have a bad blower motor. you said you unhooked the blower motor it lit a test light and plugged it back in had nothing

Does your duty cycle change on the tan wire as you change speeds on the control head? The tan wire is your control wire and should change duty as you adjust the speed on the control unit. You could connect your test light to B+ and tap the tan wire and see if it makes the blower work but first I would try putting a headlamp bulb in place of the motor and going through the speeds. The light should dim or get brighter as you change it. If it doesn't then put it between the pins A(red) and B(black) to make sure your power and ground can handle a load.
I put small spade ends on the end of mine for higher current components like blower motors.

Hi Gav09.
That is interesting.
The duty cycle didn't change as the speed was changed on the head unit. Well, thats not quite correct...it did change but very erratically and with a huge time delay (10 seconds ish). And the fan would only run when the duty cycle dropped down to about 10%. But never any change in fan speed.
I connected my test light to a good ground and tapped the tan wire when the fan wasnt working to pull the duty cycle down and the fan worked everytime.
The factory flowchart (oh dear) says "disconnect the tan wire from the module and with test light to ground touch the tan wire with it. Put fan on full and it should light. It doesn't.
It then says check for continuity in the tan wire from the head unit to the module. If ok, change head unit. Not cheap in this country.
This is all pointing to a fault in the head unit to me, but got to be sure.