I just took my SC300 to be inspected for the first time and failed. Of the list of things that failed me, the power adjust mirrors weren't working.

Going through the forums, it seemed that it could either be the switch or the mirror ECU.

Is there any way I can diagnose the switch or ECU before I throw parts at it. The only functions that work on the switch right now are the window switches, the door lock does not work.

 

The only good way to diagnose it is to first try another donor used switch assembly and see if that works or to try a known working Mirror ECU to swap in.

The Door Lock ECU is a separate module to my knowledge so yours not working would make me personally try another known good working used switch assembly since it's so easy to take out and unplug.

...

Aside if it eventually does come down to the Mirror ECU having an issue what we've always said in the past is to either find a known good working used one or to buy a brand new one from a dealer (or discounted OEM importer). However I am unaware if they are still available brand new any longer.

Longer info just as FYI: Theoretically it may be possible to repair one (after prying open the metal tabs on the outer casing) if getting the exact brand and electrical value capacitors and/or new resistors replaced professionally. No one has tried that to date as far as I know but these are 80's/90's era circuit boards just like any other. Capacitors or resistors failing (or both) and potential electrolytic capacitor leakage onto the board traces are usually the causes of failure with any old electronics from this era and usually addressing those components (with the exact brand and electrical values as per OEM) has a chance of revival. The worst case would be a proprietary microchip having burned out on the board since those are *not* going to be available from a common supplier like Digi-Key or Mouser Electronics. But commonly in vintage computer and most vintage small electronics repair it's old capacitors, old resistors, board trace integrity and other very common components that are examined first when there's a malfunction or failure.

 

I would probably try a new window switch before anything else. The mirrors not working are most likely due to the mirror ecu, but Kahn has documented how much of a PITA that is to get to, whereas the window switch is super easy (and readily available), and you know you've got lock problems that might be connected.

As far as testing, you would need to check voltage on the appropriate wires into and out of the switch.

 

I'll learn how to test the window switch when I get a chance. I already ordered a new window ECU on eBay since it was only $30. A new switch assembly is at least $100.

The reason I think it's the ECU is because I had issues with the main engine ECU and there's evidence of water damage in the car. My tilt and telescopic wheel doesn't function either so I'm guessing that ECU could be out too.

I need to go to a junkyard with an SC300 one of these days for all of these parts but that's about an hour away.

 

So you're the one who bought it... I had that on my watch list for a while, since mine likely needs replacing, and it got bumped to the Ended list the other day.

Last I looked, window switches were like $45-50, but it's been a few years and prices may have gone nuts.

There's no relationship between the three issues.

As for water damage, it would have to be pretty severe to affect where the Mirror ECU is up under the dash.