Hello,
My rear defroster is not working on my 2000 Lexus RX300.
Have checked fuses both in engine compartment and behind mini storage bin to left of steering wheel.
Thinking a relay would be burned out, but cannot find any information about it online.
Is it in rear trunk area? If so, which one is it?
I know they sell replacements online so this must be somewhat of a common problem.
Thank you much
Fat Louis

 

Please confirm absence of 12v on the rear defroster lines. Also keep in mind the 12v is a timed event [as I understand] .. so it should be there for about 8 minutes when you activate it.

The reason I mentioned the above is that there can be a physical break in the element, so it is prudent to check for voltage first. Test light would be more appropriate then a voltmeter as voltmeter can show 12v while the wire may have very limited current supply capability. When the wire gets flexed it can rub and create a short or a break. While some companies are notorious for bad wire insulation, Lexus is not known for this. Unless a careless worker forgets to secure the tension relief tie.

Salim

 

Hi Salim
I checked the side view mirrors for heat function and they do not work either.
I will check the lines with my multimeter, but no one has ever touched anything back there and we always carry cargo with the tonneau cover on so I cannot see how it could be damaged. Any ideas on the relay location? Thanks

L

 

There are multiple fuses and relays involved.
Common to mirrors and rear defogger is a 15A heater fuse that powers the separate mirror and rear defogger relays. Since both don't work this seems likely
The defogger relay and fusable link are in the engine block fuse box

 

I found the RR DEF RELAY
It was right under my nose. Labeled on underside of fusebox cover.
Which one is the MIRROR HEATER RELAY?
Sorry, I am not that great at reading the diagram you sent
I tried to use channel locks to pull the RR DEF RELAY but I cracked the case (20 years under the hood takes its toll I guess).
Any way the relay can be shorted and fused to the connector base?
thanks a bunch
Louis

 

I think mirror heater relay is inside in the relay block by the fuses. Bottom right relay.
But from that diagram it seems more likely to be the fuse they have in common

 

As seen in Maxsteel's drawing both relays have the same fuse in question on the power side of the relay coils and the Body ECU pulls the other side of the relay coils to ground.

The Body ECU will not turn on relays until it receives a message on the Multiplex BEAN from the "center cluster integration panel" when button was turned on.
PS- You didn't mention in your first post about missing mirror heat too- this was important to the diagnosis.
Could it be possible the button is broken?

 

HI I borrowed a relay tester from local auto parts store.
I got the both side view mirrors heat when I put it in test mode, but the rear defroster still will not work.
The button for side mirror and rear glass defrost lights up when I push it as well.
I purchased a new used relay from ebay. Should get it today or tomorrow.
I tested my old relay and it has very little resistance between the power prongs.
Does that mean the relay is bad?
I have dismantled the trim on the rear hatch to take a look at the wiring.
All the defroster lines look intact but I need to get another tester to check things out.
Thank you for your insight.
L

 

Good news I was hoping it wasn't a broken button or the cluster.

If you can identify which pins are the coil it can be removed and powered with a 9v battery or a bench top power supply.
The coil might measure only a few ohms- I can't find an ohms spec on it but should be low.

 

Check the mirror defogger relay ... they may be identical/interchangeable parts. Please verify by comparing the stamped number on them.

Kind of obvious to some, but in-circuit and out of circuit ohms readings can be drastically different.

Salim