Thats not good...... I know on Dad's older truck and van (94 E-150 and 97 LD F-250 (a 7 lug one, looks like an F-150) has the LCD display, and he has several bulbs out on both, we are just waiting on warmer weather to pull the dash out of both.... and he has the issue with not being able to see the display in the dark, but the display still works.
In this case, with the vacuum florescent display, if part of it is not displaying, either the display has started to fail, there is a connection issue inside the instrument cluster, or there is a drive issue (communication issue between the PCM and the instrument cluster).
dirtndrums,
There is a way to test the instrument cluster. If you push in on the trip odometer, hold it in and turn on the ignition. Once the ignition is on, hold the button until the word test (or what you can read) shows up in the odometer. You push the odometer button you can step through all the functions...
Take a look at this page:
Odometer Trick
it shows all different things you can see, one of the first 2 tests are the GUAGE test, which does a full sweep on all the gauges, and lights all segments of the odometer. There is also a bulb test, that lights all the "idiot lights" in the dash.....
I would try to run the tests and see if the cluster itself can light all the segments of the odometer.... if so.... I would say the computer is not communicating to the cluster.... if it does not light, I would say there is an issue in the cluster itself.... I believe the test is contained in the cluster, not the PCM......
Another nice thing, is..... it shows some of the other input to the cluster, such as the tac display in a digital layout, .... take a look at the link.... it has all the different displays.... I have this printed out, and in the truck.... it also will allegedly show any codes in the PCM.....
I actually plan on doing the associated tac 1200 mod, for high idle..... that will be nice on the winter for heat, and summer for the ac....