HUD digits all light up part way?

[Justin]

 

I had an alt go out on me last week and pulled one off of my other cars for the time being. Since then my HUD has been acting weird. The digits all light up somewhat, still dimmer than the digits that are supposed to be lit up, but not enough of a difference to make it easily legible under any circumstances. Has anyone ever heard of such a thing? The alt I'm running now is working properly, roughly 13.5-14.0. That is all I've changed. This is driving me crazy, it's always worked flawlessly.

[Justin]

Worse now, all digits light up completely, rendering the HUD useless. Is the unit fubared or what? Anyone have a first gen Cutlass HUD for sale?

[GnatGoSplat]

Maybe it just needs resoldering? Couldn't hurt.

 

[Justin]

I thought of that, but what scares me is the fact that it was fine until my alt went out. I'm afraid maybe there was some power spike when it went out that might have screwed something up. I'll try it though, can't hurt.

[GnatGoSplat]

Could just be coincidence. Or it could have zapped something in the power supply circuits. The big problem with modern electronics is you often can't tell by looking if a part has gone bad.

 

[Justin]

Hmm, the power supply theory is interesting, could it be that the hud constantly thinks it's in its start-up self test mode? Maybe power was bleeding over gradually, and now is full-bore? That must be integral to the HUD, like a 1 second deal or whatever.

[GnatGoSplat]

It's possible. I was thinking something in the power supply could be leaking current. The display driver chip probably gets 2 voltages as a reference, one for segment ON and one for OFF. If the OFF reference voltage is too high, it's possible the chip will be unable to turn any segments completely off. They'd just kind of glow like you described originally, and as the condition got worse, they'd start glowing more brightly. Since many VFD panels use voltages as low as -27V, there's usually a DC-DC converter to change +12V to some other voltage like -27V. It's possible there is a DC-DC converter and some part in it is shot. If I get a chance, I might look at my spare HUD to see if there's a circuit like that. I'd definitely try the resoldering thing before trying to diagnose the power supply though. And even before you do that, check to make sure enough voltage is getting to the HUD. It could be a problem with the switch.