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A/C blower only works with 10A "heater" fuse removed


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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 rub. 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?

 

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.

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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.

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3 hours ago, Trevor Catt said:

Sounds like a poor earth to the circuit...i would check that first

 

...You mean like how I just barely noticed that there is no ground wire from the chassis to the negative battery terminal.......?

I'll be damned to utter miserable hell if that fixes it. I've easily spent a cumulative 24hrs researching what the effing problem could be.

And it would explain why I've got circuits backfeeding all over the place. Voltage readings from relay pins that aren't supposed to be hot and that kind of thing.

Time for a run to Ace Hardware. Will report back.

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Nope never mind. There's already a chassis ground in place somewhere because adding a new one made zero difference.

Back to the damn drawing board.

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ONE STEP CLOSER. The problem is a 12v constant feed coming up from a line that is supposed to be ground.

Screen Shot 2016-08-04 at 21.33.07.png

Explanation:

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.

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.

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  • 9 months later...
 

Any updates on this?? seems like you are on to something here, I'm having the same issue.

I never figured out the actual cause of the problem, and others reported that replacing the body ECU doesn't always fix it, and nothing else is wrong to this day. But I have good news: I figured out a solution.

That purple ground wire mentioned earlier runs to pin 12 of junction 2F (bottom left plug) in the passenger fuse box. For whatever reason, like I mentioned, it's feeding 12v positive when it's supposed to be switched to ground by the body ECU. This seemed redundant to me. Why couldn't it just go straight to ground? All I did was cut that purple wire and send it to a new ground from the cable wrapping side. DO NOT GROUND THE END THAT COMES OUT OF THE PLUG or you will blow your ECU-B2 fuse. In the pic, you can see that I cut the purple wire and twist-spliced it onto a bit of green wire I had laying around. I hook that green wire to ground on a bolt somewhere above the glove box. TADA! Everything works perfect now. Easy fix, hopefully I can save you two weeks of prodding with a multimeter.

IMG_2476.JPG

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