To test a mechanical dimming mirror:
Since these mirrors are often around 20 years old they don’t always behave as they should. Because of this I’ve found a few things that help determine what’s going on.
Power:
The on/off function is indicated by the green LED above the sensitivity dial. It stays on as long as the unit is powered up. The switches are sometimes intermittent so this will help determine if it’s getting power and that it’s constant. It may be necessary to darken the room to see it since it isn’t very bright. If the power is intermittent it could be the switch but also check the connector where it attaches to the main board. I’ve noticed cold solder joints at this location.
Sensitivity (distance) dial:
The distance label on the front refers to the sensitivity dial which allows adjustment of when the dimming function is activated.
Center LED:
The center LED indicates if the mirror is in dimming mode, not if it’s on or off. It has nothing to do with distance even though the label is right next to it.
Testing the mirror:
Testing it is really simple. Set it down on a table with the mirror facing your direction. Hook up the power and ground using an old harness so that positive and negative won’t cross even if the unit moves. Then turn it on. Observe that the distance LED lights up. The unit may move depending on where it was when it was last powered up. The center LED should be off at this time. Now shine a light on the photoelectric sensor on the front (mirror side) of the unit and watch for the center LED to light up. (Don’t stare at the light in the mirror.) If this doesn’t happen, adjust the distance dial from near (lower numbers) to far (higher numbers) until it does. If the LED does not light up then you may need to turn off or dim the surrounding lights for it to activate. Or you can try covering the other sensor. It’s necessary for there to be a great enough difference between the front and rear sensors. Once the center LED lights up, you should hear the motor and be able to see the mirror move to the dimming position within a short period and then stop. If the center LED lights up but you do not hear the motor or see it move within a short period of time, or hear it running continuously, shut the unit off and read below. Otherwise you may test the reverse function by applying +12V to the reverse lead. With the flashlight in place it’s a good time to check the functionality of the unit by switching the reverse on and off. If the center light never activates, then you know it has a problem detecting the difference in the light.
The reason for watching the LED instead of listening for the motor is that these units often fail in such a way the motor or the components on the main board are over stressed causing further damage. If the center LED lights up and it remains quiet or continuously runs, then something is wrong. I’ve noticed a couple of different scenarios with these mirrors when they fail.
One is that the belts and pulleys are often loaded with dirt and grease. The motor may be loose and actually contact the opposing pulley.
In other cases, the gears have little or no grease and the drive train tends to seize up and hold the motor in place. Even after loosening up the drive train and applying grease it seems the motor still has trouble and may come to a stop. Then the components on the right hand side of the main board tend to quickly over heat. I’ve noticed that these units tend to have a more secure mount for the motor and that the contacts shown as A and B have more corrosion. I’m assuming that the slop in the motor mount and belt along with the grease allowed the motor and output circuits to survive a less stressful life than these drier ones I’ve found.
Electro-chromatic:
These mirrors have no internal moving parts and the later ones are variable depending on the difference in the amount of light between the sensors.
Testing electro-chromatic:
The electro-chromatic is easy to test. Set it down on a table with the mirror facing your direction. Hook up the power and ground using an old harness so that positive and negative won’t cross even if the unit moves. Then turn it on by depressing the switch in the center. The switch itself should light up. Shine a light on the photoelectric sensor on the front (mirror side) of the unit and watch for the mirror to darken. Don’t stare at the light in the mirror. It’s easy to see the mirror darken by observing at an angle. If the mirror does not darken then you may need to turn off or dim the surrounding lights for it to activate. Or you can try covering the other sensor. It’s necessary for there to be a great enough difference between the front and rear sensors.
Mounting to the window:
It should also be noted that there are differences in the size of the mounting tabs used to attach the mirror to the window. So if you’re purchasing one keep in mind to buy one that’s similar or be ready to switch what’s attached to your window. The older mirrors typically use the smaller style but this is NOT always the case. The flared style often does not use a bolt to secure it to the mounting tab, but is locked into place via a spring lock. These are typically found on newer vehicles.
I started a list:
Year/model..........Body(Curved/ Square)..........Wiring (Plug-in/Lead sticking out.)
1986 IROC
..............____
...........................Plug-in
1987 GTA
..............Curved
.......... . 
