Digital Display Redux - a different approach
I thought I'd add some new information (I think) to this thread which comes and goes (just like the display itself). My son has a 95 9C1 and the standard fix worked fine for him. Remove the display, clean up the VFD unit contacts, a little solder, back in biz. For my 95 SS, no such luck. I've read of a few other folks not having success with this particular fix as well. After a few months of guessing my speed and mileage, I decided to take the bigger hammer to it. Here's what I found. Might be useful for others. Be ESD aware ... both the VFD and the parent board have sensitive devices on them.
Under the VFD display (once removed) there is a small circuit board attached to the main dash board itself (commonly called a daughter board). Sorry the shot is not a wider angle, but you can't miss it.
This is the power supply for the little itty bitty cathode wires in the VFD display (you can just see them through the glass face if you look really really close). This power supply heats those wires and emits electrons which strike the display segments ... and lights them up. Mess up this supply and while everything else works (like cleaning the VFD connections for example), it simply won't light up.
If you flip the dash board over (remove the clear plastic cover), you can see the 7 solder pins that hold this daughter board to the dash panel. Those connections will look fine (promise). Sorry for the clip leads, that's how I analyzed the problem ... those two pins provide the 5V differential 37.5kHz singals needed to heat up the cathode wires in the VFD display.
OK, carefully remove this daughter board. The pins are undersized so it will come out easily if you use a solder-sucker or clean solderwick, it will practically fall out. Although heat will not destroy these pins, do not try and heat and pry multiple pins (I promise you will destroy it). Once removed, you can almost see the problem ... though it is very, very slight (daughter board flipped over here, pins now facing up).
Bad design. Solder is electrical, not mechanical. This daughter board has more mass than can be supported by the solder. The vibration and the shaking of the car (who me??) stresses the joint (daughter board side). These pins are barely pressed in and soldered poorly for this purpose (a very common failure for "hanging" daughter boards). Were this board horizontal ... less of an issue. Hanging out in space ... nope.
Here's a microscope view of one of the pins down at the base of the solder fillet. The fuzziness in the foreground is the pin itself (it's fine). But notice the very very small fracture line around the pin. All 7 pins have this exact failure to some degree (hence an intermittent problem). If you tap the pin, you can see the crack open and close.
The crack is not at the pin site (hence not an oxidation or flux issue). This crack is farther out around the pin, in the solder, at the hole size. The pin is undersized. Proper manufacture would have this pin press fit (very tightly) or screws to hold the daughter board stable. So be it.
Fix is simple, resolder and clean off the flux. You might be tempted to drop a blob of silicone under the board when you resolder it (to keep it stable), but I suggest you just fix it again in 17 years. If you glue that daughter board in, you'll ruin it next time.
There is no way this failure is just my car. The rest of the soldering is solid as a rock. This daughter board construction is trash. If yours is working today and you live anywhere with temperature extremes ... you will see this failure sooner or later. It's not chemical or salt or bad batch, it's mechanical.
Way more info than you need, but if you know someone with some soldering skills, this can be done without too much fuss. You can see the cracks in the solder with a magnifying glass.
For those struggling with an even deeper problem, here are some notes to save you hours of chasing the wrong kind of tail (shoot me).
If you want to power the dash out of the car, pay attention and double check (use a current controlled supply or fuse the supply at 400mA), you can light up the display as follows:
12V C2 pin A5
Ground C2 pin A6
Tie C1 pin A4 to 12V (this is the dimmer control).
C1 is on the drivers side of the display, C2 on the passenger's side (careful, you are looking at the back side !!). Note that the connectors are not in the same orientation.
The following transistor could easily be bad. If so, the display will either not dim, or not light. I believe this is the dimmer control that you see as a transistor in the shop manual for the cluster circuit (at least that's what it does). I didn't study it, not my failure, but that makes sense to me. It throttles the 12V to the VFD cathode supply (the daughter board) to "dim" the heater voltage.
Reference the daughter board pictured previously with the two orange clip leads attached. If you connect an oscilloscope (low speed is fine), you will have the following characteristics.
Each signal is with respect to ground, both are 180 degrees out of phase. You do not need the VFD in the circuit to see these waveforms, but loaded down it could make a difference to your particular problem (use 4 ohms, 2 watt to test if you want a load). You cannot see anything with a voltmeter (the frequency is too high). Unloaded, the circuit will draw about 150mA, loaded with a 4 ohm resistor, 280mA, with the actual VFD display ... about 350mA. Use the displays and these current values to diagnose (currents are with respect to a 12.8V supply).
If you see anything else ... it's not working. It could show a lower voltage peak to peak depending on the dimmer transistor's state of happiness, but it should still look like this. My problem ... both singals were in phase (on top of each other) when not working (no heat for the cathode wires).
Couple of thoughts. The daughter board is a very simple design. I would be surprised if any of it's components are fried in any way. You can short the output and it's not going to care. The dimmer transistor acts as a current limiter to boot. Look for cracked solder joints at the pins of the daughter board. You can check the dimmer transistor with a voltmeter (it's a simple bias circuit).
The VFD connection to the dash circuit board (the black standoffs) is a rediculous design (engineering in a toilet). While the lower connector (cathode heater circuit from the daughter board) has two small pins that help align it, the upper connector does not. It is really easy to get the pins in between the pads. I used a small mirror to center the VFD at both connectors before screwing it down.