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Gentex Rear View Mirror Upgrade

     UPDATED: July 14, 2018                       CONTACT       
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If you have any comments or if you can improve this information, please feel free to email.

This mirror came standard on a large number of early to mid-2000s GM vehicles, mainly Suburbans, Tahoes, etc.  The mirror was made by Gentex and there are millions of them.  If you're a frequent salvage yard visitor, finding something like this may not be difficult.  Otherwise, you can easily find a number of them available on eBay. 
This one is the Gentex 177.
The Gentex 177 can be found like this one above with Compass and Temperature display, but be careful, since some have a COMPASS ONLY (no Temp display).  If you want both Compass and Temperature, make sure you choose the correct one. 
Also there seems to be some available with a RED/AMBER number display instead of BLUE/GREEN, like this one.
eBay prices seem to vary between $30 and $50 (and higher).

I got mine for under $40 shipped, including the outside TEMP SENSOR and 7-POLE PLUG PIGTAIL. Be sure to choose one that comes WITH BOTH of these items.
There appears to be a couple different types of sensors. The one I got is the TOP sensor shown above. I don't think there is a difference in how they function.  The sensor should be mounted somewhere outside your car. The usual location seems to be under the front of the car as far from engine or radiator heat as possible.

This mirror mounts on a standard wedge mount with a metal button that's glued to the windshield. 
So YES, this will be a significant departure from originality in your 240.
  If this bothers you, then this mod may not be for you.  Stay with your old mirror and suffer!

I briefly considered trying to modify the mirror mount to fit the Volvo roof mount, but the effort did not appear to be worth it.  So I mounted the new mirror to the windshield.

The metal wedge mount windshield button is usually not included when you buy a mirror on eBay. I got mine separately for a couple dollars.

<<< Your original 240 mirror mounts to the roof and will have a plastic cover like this.  Your original mirror can be pulled off with a tug.  If comes off that way as a safety feature to protect your face when it collides into the mirror in a crash.

<<< You can place that plastic cover back on to cover the hole. As you can see here I have a radar detector, so I decided to attach a mount for it where that cover used to be.  I'm not using the old plastic cover.

I then glued the metal wedge mount button on the windshield.  Make sure you glue it on the correct side and NOT upside down. 

Before gluing the button, I took some time to check positioning of the mirror by holding it up to the windshield.  That metal button is going to be permanent, so make sure it's where you want it to be.  I chose to place the top of the button about 2 inches below the top of the windshield.

Failures to Stick.
This seems to happen a lot.  I used Permatex windshield mirror adhesive (found at my local auto parts store) and followed the instructions perfectly. It says to allow 30 minutes drying time before mounting the mirror. The button popped off while I was turning the set screw on the mount. 
I used a wire brush wheel on my bench grinder to clean the hard glue off of the button. Then I tried again, this time allowing several hours for drying. Got the mirror mounted and as soon as I tried adjusting it, the button popped off again!  The glue was having no problem sticking to the button, but it was not sticking well to the windshield glass.

So on my THIRD try I used some 180 grit sandpaper to rough up the glass first.

Then I glued again and allowed the glue to dry OVERNIGHT.  That worked.

<<< Here's the result.  There's a single set screw in the mounting base that tightens to snug the mount against the metal button. It's a Torx T-20 screw. Make it snug only.  No need to twist hard, especially if your hoping to keep the mirror on the windshield.

Overall this was an easy installation and well worth the effort in my opinion.

Below I'll go over the wiring, programming and a couple other things.  Keep reading.

<<< Here's the outside view.

This connector plugs into the back of the mirror. It has 7 poles (and will probably have 7 wires), but you will only need to use 4 of them. 
Pole #1 is the closest one to the center of the mirror.
Black/White or Pink
+12V switched (key in first ON position)

not used

not used

not used
Temp Sensor (polarity not important)
Temp Sensor

Here are some simple instructions for setting up the mirror after installation.
It covers setting the temperature display (Fahrenheit or Centigrade). It also covers setting the compass zone and compass calibration.

Click Image (PDF document, 280 kb)


<<< The compass and temperature display in this pic doesn't tell the complete story. This photo was taken in my garage.  It's nice and visible when it's not too bright outside, but it can be really hard to see in bright daylight conditions.  Others have complained about this, but it does not seem to be a very common complaint that I could find when searching, so I don't know if all Gentex 177 mirrors are like this or just some of them. While the mirror does automatically dim, the compass/temp display DOES NOT automatically change in brightness if it's darker or brighter outside.  I think some later mirror models do have that feature, but this mirror was CHEAP and the newer models are EXPENSIVE.

Some exploratory surgery was needed. 
I wanted to see if the brightness could be improved. 

This part is totally up to you.
If you break it, don't come crying to me.

<<< Separating the front frame from the mirror body is easy.  Wedge a screwdriver in there and slowly twist until you get a POP. Then slowly work around the rim twisting and popping the frame until you've gone all the way around and it comes loose.
The mirror and circuit board will come out together.
<<< Here's the back of the circuit board.  You can see that it's attached to the mirror body by a small harness. 

<<< To free the circuit board from the mirror body, simply unplug that small plug.  It just pulls out.
While being CAREFUL with the two small wires still connecting the circuit board to the mirror, separate the circuit board from the mirror and turn the circuit board over like this.

Here's the front.

That dark blue rectangle is the compass/temperature display. 

<<< That is a piece of dark blue film which is held on with some soft glue on the right and left edges. I inserted a micro screwdriver to gently lift it. Then I could get my fingernail in there and I slowly pulled it off. 

<<< Here you can see the display without the dark blue film over it.  Now I know you want to know if the display is still blue/green. The answer is YES.  The color hasn't changed, but this does help make it brighter. Keep reading.
<<< With the mirror again placed over the circuit board, this is what you can now see through the window. 

That may seem undesireable, but once it's in the car it's almost invisible. And it completely disappears when the display lights up.

Reassemble your mirror the same way it came apart.
<<< Final installation is complete. Compare the OLD photo on the LEFT with the NEW photo on the RIGHT.  The display brightness has been improved.  It's much easier to see now in bright daylight. Mission complete!


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