Like it says, blower always runs. Battery ran down overnight because the blower ran all night with the key out of the ignition. So he shows up at the house and gives me the rundown while I hook up the Verus.
No codes in any module.
There's a bidirectional test for the blower. Toggling it on and off has no effect. I pull down the HVAC system cover to expose the blower motor and blower motor control processor (over engineered resistor).
2 big fat wires and one skinny mini. Easy! Power-ground-control. (see the diagram below)
I'm already suspecting the processor has shorted internally, so I show him how to disconnect the processor when he gets home so the battery won't run down and tell him to drop it at the shop the next day.
Next day! I know there's no problem with power or ground, so I'm thinking a quick check on the control should show no voltage, and I'll call the processor and get it out the door.
OOPS! No key in the ignition shows 5v on the control wire! That's not what I expected, is the HVAC module staying awake keeping this thing running all night?
Disconnect the 3 wire connector on the processor and the 5v is gone! It can't be coming from the HVAC module, can't be backfeeding through the processor....
So now I want to cut the control wire and see if the fan stays running. (See the pic of the 5v coming out of the processor with the wire cut taking the HVAC controler out of the loop) In my mind, if it stays running it MUST be the processor.
Cut the control wire, fan stays on, time for a processor right?
I have a rolled over Escalade at the yard, so I grab the processor, pop it in and, OOPS AGAIN! Fan stays on! Exact same symptom. What did I miss???
According to the service info, the processor sends 5v to the HVAC module (I assumed the module would send this voltage to the processor), which pulses the 5v signal to ground (which I observed with the scope, but didn't get a capture to share), which pulses ground to the blower.
That means this thing is SUPPOSED to have unswitched power and unswitched ground. Which also must mean it always has 5v on the signal wire. Looks like I may have found the 5v regulator for my pressure transducer ;)
Everyone thinks I blew the call, but I have to be right, there's no other variables. I can prove it with the OHM meter. The ground input and ground output to the blower shows 00.0 OHMs. Should be infinite resistance until the signal wire is pulled to ground.
I'm convinced, time to buy a brand new processor for $100. The used one must have just happened to fail in the same way. Maybe some pattern failure going on here?
Today, the $100 processor shows up, and it's a Durajunk splice in unit... Oh well, I snip the wires, connect the blower and just twist the power and ground wires on to make sure the blower doesn't come right on, and it doesn't! Man it's good to be right, until I solder all the connections, turn the key on, and now OOPS AGAIN, the blower won't turn on at all!
Well, stupid me cut all the wires at once and blew the 40amp fuse.
I was right all along, and the all the test results pointed to the processor, but man, I hate fixing it and it stays broke!
Makes me wonder how many people would have told the customer he needs a HVAC control unit now after the first processor acted exactly the same as the bad one!
I don't think I've ever second guessed myself so much during such a simple diagnosis.
So what did I learn? Have confidence in my diagnosis, and if you're to lazy to disconnect the battery, cut wires one at a time!
[Last edited Jan 27, 2016 03:08:34]