There are several issues which can cause segments not to function:
The power supply can cause segments to flicker randomly. Click here for info on troubleshooting the power supply.
The polarizing film on the front of the LCDs can become faded over time, causing (usually) all segments on the panel to appear invisible / clear and unreadable. See this page for examples:
The LCD panel might be defective. Replacements are available from email@example.com.
The LCD driver IC might be defective.
The elastomeric connector might be dirty or defective.
The board might not be tightened properly.
Testing the LCD segments
Warning! Under no circumstance connect power to the segments to the LCD panel.
By default, the segments are off, which means that light is blocked. When the segment is energized, it will become clear, which allows light to pass through. If the segments are all clear, the LCD polarizing film may have faded. We sell a replacement film to resolve this issue.
It takes only a tiny current to drive the segments. My preferred method of testing LCD panels is to touch the edge connections, making sure not to cut myself. Typically, the static charge on a persons body is sufficient to drive a segment. If not, I'll run my hand through my hair or over a plastic surface, then repeat the test. Move along the row of contacts, watching each segment become clear in turn. Move to the problem area and test the segment(s) which aren't working properly. If the problem segment fails to become clear, the likelihood is that it has become damaged. The fix is to replace the LCD panel. Contact firstname.lastname@example.org for replacement panels.
Make note of which pads on the bottom board correspond to which
If the problem segments are working (becoming clear when a charge is applied), the problem lies elsewhere.
Testing the Driver ICs
It will need to be connected to power while out of the car for this testing procedure. Click here for info on bench testing the cluster.
To get access to the LCD contacts, disassemble the cluster. Remove the bulbs so that you don't break them or burn yourself during this procedure. Then reassemble the two boards, making sure to use something non-conductive as a spacer between the boards.
When you first power on the cluster, a 2-3 second lamp test occurs. All segments become clear, allowing light to pass through and the segment to appear "on".
Measure the voltage of the pads connected to the segments (connect ground to the larger ground pads at the end of each row of LCD contacts). Each segment pad should read about 4.0V, whether the segment is on or off.
Using a Digital Meter, measure the resistance between each segment pad and the larger ground blocks at the ends of each row of LCD contacts.
When the segment is on/clear, the meter should measure a voltage, which usually causes the display to flash on a digital meter.
When a segment is off, the meter should measure somewhere between 1k and 7.5k ohms.
If the problem segments don't behave this way, there may be a problem with the driver IC or its power / data source (the 12 pin board connector). If the problem segments measure correctly as shown above, the problem may lie elsewhere.
Reassembling The Cluster
Reassemble the bottom board using the docs here. Pay attention to the steps about cleaning the LCD contacts and measuring resistance after installing the bottom board.
Reassemble the rest of the cluster using the docs here.