There are a lot of horror stories about getting to that upper mounting bolt but i found a work-around so you dont have to touch that bolt at all. The trick is to replace ONLY the Circuit Board (inside the ECM). You dont need to remove the ECM from the car. There are 5-Tabs on the backside of the ECM housing [see pic]... you simply lift those up, remove the cover and unscrew the PCB. And you can do the whole job in about 30-minutes.

BEFORE YOU START...
If you are unsure as to whether or not its your Mirror Control ECM or if the Door-Switches (or wiring) are bad, you can follow the diagnostic procedures as outlined in the Power_Mirrors_Troubleshooting.pdf. One point though... if both mirrors stopped functioning AT THE SAME TIME, then you can almost be certain the problem is the Mirror ECM.
If you have a replacement ECM on hand, you can easily test it by simply (key off) unplug your existing 2 ECM connectors and then plug them into the replacement ECM, ground the ECM housing, and then turn the key on and test the mirrors. If both mirrors now function correctly, then follow the steps below to swap the PCBs. BUT If one or both of the mirrors still dont work then refer to the Troubleshooting Matrix on PG-7 of the Power_Mirrors_Troubleshooting.pdf.

TOOLS...
14mm socket w/ratchet and/or breaker bar, 10mm combination wrench, std phillips P-1 screwdriver, flat screwdriver.
Special tools...
A magnetic flexible phillips OR a right-angle driver OR a 2-sided micro-ratchet driver with a short phillips insert bit driver (see pic). One of those 3 will will make the job a lot easier.
NOTE: PCBs are fragile and susceptible to static electricty damage. Handle carefully and ground yourself often.
-----------------------
5 Steps To Swap The ECM PCBs
***KEY-OFF for all these steps***

1.) REMOVE THE DRIVERS SEAT (its only a 5-minute job!)...
a) Lift the backrest release lever and let the seat move to its most forward position.
b) Remove the headrest (the little button on the headrest bracket).
c) Remove the plastic covers on the 3 bolts on the backside of the chair frame [see pic]
d) Remove the 3 14mm bolts... dont use power tools on these bolts (sensitive fine-thread).
e) Move the seat all the way to the backward position.
f) Remove the 2 front plastic bolt covers and then the 2-14mm bolts.
g) Tilt the chair up in the front and disconnect the power connector. Push the remaining floor-wiring inside that Hvac floor vent (just to get it out of the way).
h) Remove the chair from the car... Note that it is 60-70 lbs? so if you are young and have a good back then just pick it up and remove it otherwise, sort of "walk it" up to the door scuff plate and then pick it up. DONT PICK IT UP BY THE PLASTIC TRIM AROUND THE BASE OF THE CHAIR (the trim will crack!). Reach underneath for that steel tubing right behind the trim part.
FYI...there is a 1" long floorboard guide-pin attached to the underside of the inside seat-track (dont bend it).

2.) PRE-REMOVAL PRECEDURES...
a) Move the Tilt-Wheel to its highest position.
b) Remove the 10A Dome Fuse From Relay-Block #2 (Fuse-box next to car battery)
c) Remove the Drivers side Undercover (remove 3 screws, twist 1/4 turn each to remove the diag port & footlight).

3.) REMOVING THE COVER FROM THE MIRROR ECM HOUSING AND SWAPPING THE PCBs...
a) Unplug the 2 Connectors to the ECM.
b) Remove the lower bracket mounting bolt.
c) Pull the rear support bracket out of the firewall bracket.
*NOTE FOR NEXT STEP... Its Not Really Necessary but If You Want To Disconnect & Move The Wire Loom That Blocks The Front Of The ECM Cover [see pic] Then You Need To FIRST Disconnect The Neg Battery Cable...
d) With a small screwdriver, locate the 5 Tabs on the backside of the ECM housing and lift them up. Remove the cover for access to the PCB.
e) Remove the 4 corner PCB screws (go for the 2 right-side screws first)... WARNING: twist or torque the housing only enough as necessary to get at the PCB screws; you dont want to warp or distort the housing ('cause then you would have to replace it and that would ruin your whole day).
f) With the screws out, remove the PCB and replace with the known good one (ground yourself before handling). Reinstall the screws (dont over-tighten... just 2 fingers on the screwdriver is plenty).
g) Replace the ECM cover and bend the Tabs back in place (get it lined up correctly... practice by putting the cover on the ECM housing from which you removed the good PCB). Re-connnect the ECM (and any other) Connectors.

4.) WITH THE PCB NOW REPLACED, REVERSE THE ABOVE PROCEDURES TO FINISH THE PROJECT.
***Reinstalling Seat***
a) Get that 1" long guidepin in the pin hole and the seat drops into perfect place.
b) The factory manual calls for a dry (no grease) torque spec of 27ft-lb on the seat bolts. If you're **** like me, you use a torque-wrench on everything but the gas-cap. If you dont have a torque-wrench then just get them sufficiently snug.

5.) TEST THE MIRRORS But Note... If you have previously moved the mirrors manually then you may want to manually move them back to Center first, otherwise you may not get full side-to-side or up-down movement with the new ECM-PCB.

AND YOU ARE DONE!



5 Tabs to lift up to get at PCB

Special Tools

Seat-Bolts Rear Cover - Twist screwdriver

5 Tabs to lift up to get at PCB

 

I have a mirror ECU coming soon so this is very timely... Thanks for providing this and the troubleshooting document.

 

I have a spare mirror ecu to install out of a 98, my car is 95, is that compatible?

 

Glad to help... Ive taken a fair amount of free advice from ClubLexus over the years so i thought i would add something back. And its not a hard repair... im 71 years old and can get it done without too much trouble.

 

the factory part # (89430-24020) is the same for all the mirror ECU's from 92 to 98 sc400's so there should be no problem.

 

Thank you for the helpful information. Good to see fresh DIY's here on the forum.

 

Just got a '95 SC400, and both outside mirrors don't work. I ordered a used mirror ECU and plugged it in, and now both mirrors will move to the right, and down, but not up or left. I popped out the door switch assembly and checked the continuity of the mirror joystick, and all directions are ok, both left and right. Looking at the troubleshooting guide, I tried checking the left/right switch also, but could never get any readings. So, what do you think is the issue? Bad mirror ECU? Mirror motors? I haven't done further testing yet, but next would probably be the mirror motors.

Thanks in advance!
Mike

 

It could be that the used mirror ECU has some issues of its own. Possibly bad capacitors.

Aside, something no one has ever talked about doing is to have the capacitors in these Mirror ECU boards replaced with brand new capacitors of the same brand and electrical value type. Potentially resistors also. Probably hardly anyone has ever asked a repair company about doing this for these ECUs. For folks who don't work on delicate electronics themselves, maybe it's something that Tanin Auto Electronix or SIA could potentially be asked about.

The genuine capacitors and resistors that Toyota used, once identified by inspecting the old ones on the logic board, are purchasable by a anyone from either Digi-Key or Mouser Electronics.

In all likelihood that is probably what has happened with SC Mirror ECUs failing over the years which might suggest that most of these logic boards are possible to repair. It can be done for main engine ECUs most of the time so why not these as well or really any of the several ECUs throughout the car?

Chips on ANY logic board can eventually fail over time (look to the vintage 1970's-1990's personal computer hobbyist restoration community for examples of this) but it tends to be a rarer phenomenon than failed capacitors, failed resistors and other easy to replace components such as those.

I know this isn't a direct help to your current issue, rmikebaker, but if you can't get another used Mirror Control ECU to work (has the part number been discontinued by Lexus already?) then this would be the next community DIY repair attempt we'd all try out.

 

So, I've discovered some things on this car that don't make sense, so hopefully the smart folks here can help me out.

After staring at the troubleshooting guide (attached to this thread) for the mirrors a little more, I decided to try jumpering the necessary terminals in the switch connector to see if I could elicit some up or left movement of the mirrors. I started mapping out what terminals needed to be connected, and found that terminal 11 in the connector has no wire or terminal. According to the continuity tables, terminal 11 is needed for left and up movement of the mirrors, but not only that, it's needed for right and down movement, which does work. WTF?? HELP!!

 

Following up on my mirror issues... A friend of mine works at Toyota and got me some good schematics and troubleshooting info, but it's only added to the mystery. Here's the schematic he gave me, which I used to do some continuity testing from the blue switch connector in the door, to the "B" connector under the dash that connects to the mirror ECU. I couldn't get a meter reading when testing pin 12 of the switch to pin 3 of the "B" connector. That would completely explain why I don't get any left or up movement of the mirrors, on either side. So.... I pulled the carpet back to get at the IF2 connector, to see if I could isolate which "half" of the circuit was at fault. Doing some more continuity testing, I couldn't get a meter reading on EITHER path - pin 12 of the switch to pin 19 of IF2, or pin 19 of IF2 to pin 3 of connector "B". That seems really odd. Maybe I'm just not hitting the right pins, but I've been able to check other paths successfully. Not sure what my next step is, other than to maybe take the door apart and try to run a new wire all the way through, but that seems like a challenge.

Looking for some guidance from all the wiring gurus

 

I finally caved, took the door apart, and pulled the wiring harness all the way back through. I was then able to get a solid wire run through so I can pull a new wire to replace the "gray" wire. Man what a pain in the rear.

 

Thank you, @boughtnew, for providing all of this useful information, including the Lexus Service Manual attachment. I not only used it to fix the power mirrors on my 1995 SC300, but I also made an entire YouTube playlist that shows the entire troubleshooting process. You can see it here:

https://youtube.com/playlist?list=PLgQiYGh8k3FUdmTYzaUejXybC3e07WHEU&si=L3gL08aqyB_2QxRe

Thanks again!
Paul

 

The OP hasn't been on here in a year and a half, but thanks for posting this alternative method of replacing the Mirror ECU. However, I have a hard time believing anyone could do this job in 30 minutes, regardless of age. Maybe OP is a very small person... I'm not particularly big at 5'9" and 165 lbs, and I can't fit both arms up in there at the same time. Trying to remove those tiny PCB screws one-handed is a solid hour by itself. There's no way I was getting them back in one-handed.

So, I went with Plan B: twisting the heck out of the old rear panel until the upper mount fatigued enough to break free. I don't love leaving sharp stuff behind, but then my hands are currently proof that there's already plenty of sharp stuff up there! Also, I probably got a bit lucky and it eventually snapped right through the upper bolt's hole...which probably means the bolt wasn't very tight in there. I cut the upper mount off the new panel, so there's nothing up high getting in the way now.

I also appreciate the advice to plug in the new Mirror ECU and verify it's functioning properly before all the trouble to remove & replace.

@KahnBB6 unfortunately, there's nothing on the old board that looks suspicious as far as bulging or leaking capacitors. Of course, my passenger mirror was fully functional and my dr's mirror only lacked up/down control, so maybe it's some small corruption that isn't obvious to my untrained eye.