Electrical mystery: Blower only runs if 10A "heater" fuse is removed?

eicca · August 4, 2016

I'm posting this on a few different Lexus forums to reach the largest possible audience and get the most information I can. I just bought my GS400 a few days ago and it's in fantastic shape except for a few general maintenance/wear items and this glitch with the A/C blower. I know I'm not the only one to have this issue but I haven't seen a definitive cause and solution. So I want to solve this issue once and for all for the next person that it will inevitably hit.

So here's the case to solve. With the 10A "heater" fuse still installed in the driver's side fuse box, the blue/black wire that is supposed to feed 12v positive to the blower controller/resistor is grounded whenever the heater relay switches on. I have tested for continuity and it does indeed go to ground.

However, when the 10A "heater" fuse is removed, the blue/black positive wire receives its proper 12v when the heater relay switches on and the blower activates and works perfectly.

I need to know: What is in that "heater" fuse circuit that would cause the 12v positive blower wire to switch between ground and positive? That main hot wire, according to my extensive research, supposedly goes from the resistor/controller to the heater relay and then directly to the 50A HTR fuse and from there to the Battery. Is there something in there that I'm not aware of? What could be affecting the relay's function?

I've got bits and pieces of the factory service manual and wiring diagrams but nothing complete. If anybody can point me in the direction of the official Lexus-issued manual while we're at this that would be great.

Thanks in advance. Let's track this down and solve it once and for all.

ColinBarber · August 4, 2016

here is the wiring diagram from a 2001 model - sounds like the circuit is the same.

Does the heater relay look original? Sounds to me as if it isn't the correct one and is working in reverse - i.e. normally closed is connecting to the heater fuse rather than ground.

eicca · August 5, 2016

Thanks for the diagram. I checked the part number on the relay and it's definitely a Toyota blower relay. It definitely sounds like it's working in reverse but I can't imagine that there's an identical relay that works the other way around.

One more piece to the puzzle: I'm only getting 6v and 0.1 amps at the aforementioned "heater" fuse. And when the car is off I'm getting -6v and -0.1a. That doesn't sound normal. Although when I take the hot wire to the fuse to ground, it's full 12v.

eicca · August 5, 2016

BREAKTHROUGH:

That purple wire (V) running from HTR RELAY pin 5 to BODY ECU NO.1 is supposed to go to ground when the body ecu switches on. Instead, there's a 12v constant current coming from it even when the car is off. I confirmed that this is causing the relay to remain activated all the time EXCEPT when switching the car ON and the 10A "heater" fuse circuit then applies 12v to HTR RELAY pin 3. The two positive currents collide and the relay deactivates, killing the A/C blower. I confirmed that, even with the car off, the HTR RELAY clicks when inserting it into its socket. This explains the revering polarity at the 10A "heater" fuse.

Next step: Where is body ECU #1? I need to confirm that the purple wire is routed correctly (or not) in order to diagnose why it's positive instead of grounded.

I would hope that if the body ECU is bad it would manifest itself with other symptoms but there are zero other problems with the entire car.