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Tim Mohr
Tim Mohr, ASE MASTER
Category: Lexus
Satisfied Customers: 15969
Experience:  30 YEARS EXPERIENCE, FORIEGN AND DOMESTIC, GAS AND DIESEL
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I was responding to the post below. i am having the same

Customer Question

i was responding to the post below. i am having the same thing happen and when i ground the purple wire to the body of the care everything works fine
If you are 100% certain that the red wire with a blue tracer that goes to the back of this junction block and to pin number three of the HTR relay has 12 V and the relay that you have is good this means that the of violet or purple wire pin number five is not being grounded by the body control computer. With the relay plugged in and if you ground this purple wire the relay should click and the blower controller should work. If this is the case then there is either a problem in the body ECU or this violet or purple wire has an open connection between the heater relay and the body computer. About the only other possibility is there is something over riding the system not allowing the body ECU to turn this heater relay on. If that is the case then the system gets a little more complicated or shall I say a lot more complicated to diagnose in this type of format. The red wire with a blue tracer is fed 12 V from the 10 amp heater fuse. There is only one junction or splice connector between there and the heater relay. Check these things out. I am done for the evening I will follow up with you tomorrow. Things have been really busy lately so it's hard to stay from the computer constantly.
Read more: http://www.justanswer.com/lexus/56iul-lexus-gs400-lexus-gs400-1999-problem.html#ixzz3nKgeYiN3
Submitted: 2 years ago.
Category: Lexus
Expert:  Tim Mohr replied 2 years ago.

Hi, my name is ***** ***** you ground the purple wire at the relay and everything works?

Customer: replied 2 years ago.
i grounded the purple wire from where it connects to the relay at the back
Customer: replied 2 years ago.
everything works fine
Expert:  Tim Mohr replied 2 years ago.

ok, then you have a bad ECM, can you go to the BCM and ground that wire at the BCM and see if everything would too?

Customer: replied 2 years ago.
what exactly is the ecm and bcm
Expert:  Tim Mohr replied 2 years ago.

the ECM is the main computer, the BCM is the body control module, the computer for the body electrical

Customer: replied 2 years ago.
i changed the bcm and it worked fine until it went out again. everything else works fine so what if i manually ground the purple wire so the blower can work? will that harm anything or cause any problems with the electrical things in the future?
Expert:  Tim Mohr replied 2 years ago.

if you ground the wire permantly, the relay will always be engaged and it can run the battery down,Tim

Customer: replied 2 years ago.
if i get an ecm will i have to get new keys programmed and also where is the ecm located
Expert:  Tim Mohr replied 2 years ago.

The ECM is located on the left front of the engine compartment, it is direcrly behind the drivers side headlamp, and yes, the keys will need to be reprogrammed in to the new ecm,Tim

Customer: replied 2 years ago.
sorry for all the questions but i just want to make sure that this ground problem is lead to the ecm in your opinion. i have switched out the grey box which is called the multiplexer before and it started working but after 2 months it stopped. are you saying the the ecm controls the ground that goes to the multiplexer?
Expert:  Tim Mohr replied 2 years ago.

No, the ecm does not ground the relay, the BCM grounds the relay, body control module,Tim

Customer: replied 2 years ago.
ok, tim, i appreciate your help but i am stuck and want to see if you could help me trouble shoot this problem a little more before i take it to lexus dealership or bu the ecm. knowing what we know that the relay is not being activated due to lack of ground what else could it be
? i am frustrated with this and need to trouble shoot a little more before i spend more money. please help
Expert:  Tim Mohr replied 2 years ago.

the only other thing that can cause this is a bad ground going to the BCM, you can check all the grounds at the BCM, if they are good, it can only be a bad BCM,Tim

Customer: replied 2 years ago.
Body control module aka multiplexer which is behind the fuse box?
Expert:  Tim Mohr replied 2 years ago.

Yes, it is also called a mutiplexer by some,Tim

Customer: replied 2 years ago.
TIM im not ready to give up on this yet. i need some more advice on what this problem could be
Expert:  Tim Mohr replied 2 years ago.

did you check all the grounds at the BCM?

Customer: replied 2 years ago.
Not quite but i did put another multiplexer box which fits behind the fuse block in and the same thing happened so either both box are bad which i doubt or something else is the problem.
Customer: replied 2 years ago.
How can i find which wire or where the ground is located on the box
Customer: replied 2 years ago.
this is the multiplexer/bcm you are referring to correct?
Customer: replied 2 years ago.
Please help me with tryingbto fix my problem. Thanks.