27" iMac 2009 only boots to black screen, have to PRAM reset each time

[Boomish69]

So as the title says I have a 27" iMac 2009 that only boots to black, if I reset the PRAM then it boots fine and runs fine all day!

Any suggestions?

I've googled to find lots of similar problems but mostly to do with failing video cards, as the machine runs fine I don't think it's the card, it seems like the battery has gone on the motherboard so it's not retaining the resolution. Only other thing is it could be a RAM thing, it's got 16 MB of ram but the machine passes all the hardware tests including the official Apple service test I managed to get hold of.

Appreciate and feedback I'd really like to sort this thing out, the only other thing I thought of was upgrading the video card, I've seen some Radeon HD 6770's on Ebay.

 

[Brian Y]

Instead of PRAM'ing, have you tried just switching off and on again?

Sounds like a dodgy graphics card connection. Only way to really tell is to check the diagnostic LEDs which is something you don't want to be doing at home unless you've done it before (running an iMac while open is very dangerous).

 

[Boomish69]

Instead of PRAM'ing, have you tried just switching off and on again?

Sounds like a dodgy graphics card connection. Only way to really tell is to check the diagnostic LEDs which is something you don't want to be doing at home unless you've done it before (running an iMac while open is very dangerous).

Thanks Brian, yep turning off then on results in the black screen, hence I have to PRAM. All LED's are fine as is the diagnostics..

 

[Brian Y]

Thanks Brian, yep turning off then on results in the black screen, hence I have to PRAM. All LED's are fine as is the diagnostics..

Certainly odd!

Do you get any video signal to an external display whilst the internal one is blank? It *could* be power/signal not getting to the LCD.

 

[Prise]

Many years ago on a PC build, I had the identical issue. Every time I turn it off, I'd have to reset the PRAM on the motherboard.

In my case, the culprit was the motherboard with the battery was faulty, and kept draining it too quickly.

I'm not sure about the iMacs, but it could be a motherboard/battery issue. Though, I'm not sure if Macs require the periodic battery replacements.

 

[Boomish69]

Certainly odd!

Do you get any video signal to an external display whilst the internal one is blank? It *could* be power/signal not getting to the LCD.

Thanks for the reply guys, yes very odd, must be battery, I can restart no probe's and it runs ok all day & night fine, but if I switch off then on I have to PRAM reset.
First thing I tried was external monitor Brian, and nope no video, so maybe it's a video board issue? I just am not sure , I look into the battery first. and see how it goes.