MP2510 excess toner/dev on copies - gritty

[JayPaul]

2 months ago I had to look at the 2510 for 'gritty copies'. They were fairly gritty, each page had a thin layer of toner/dev on it that just rubs off onto your fingers. The fuser is ok and doing its job properly.
They are not one of our actual customers its their own machine and they buy their own toners and pcu's etc.
In the end I said get this fake pcu out and buy a real one in and I made sure their toner was original and not compatible.
Few days later they phone up saying its gritty again, I go down there and notice that the transfer roller gear is pretty worn away as well and wont be turning well so they got a new one from us. That was the end of it.
Now last week suddenly they demand a free call out because their copies are still gritty and aparantly the problem was never fixed (not sure why it takes them 6 weeks to realise that, bit suspicious).
So I go out there again but this time its barely even doing it, you can slightly feel some excess on pages but only if you are pedantic.
I look at the pcu drum and its visibly covered in dev/toner. I check hopper and other areas, nothing seems to be out of place. I tell them to replace the pcu and the guy that sold it to them said hes fine with swapping it for another new one next day. Whilst on site I put my 1027 pcu into machine and its perfectly fine, great quality copy and customer says 'its still gritty' but when I feel it, its completely normal not gritty at all. I tell them thats literally the best it can produce it cant be any better than that.
Next day their new pcu comes and by about 2pm they phone again saying it statred ok in the morning but now its gritty again!
What on earth is the problem then? Grounding issue, maybe the charge is being told the wrong voltage?
Because they arent under contract I cant put any new parts in it so I have to find a problem thats easily fixable... any ideas?

[sandmanmac]

There seems to be a lot of PCU's being installed yet no mention of running the Dev inititalization (SP2801) for any of them.
I'd also see that Dev bias is set to default (SP2201_1 / 2201_2)

[JayPaul]

Dev initialisation done on PCU by me, except on my PCU which I put in just to show them what copies should look like after a new PCU is installed. As I still need that for one of my customers then it will be initialised (if that made sense)

[slimslob]

Check the hot roller for toner build up. If it is pulling developer, you can check that with a magnet. If the grit is drawn to the magnet, it is developer. If it is pulling developer, then there is a problem in the high voltage system.

[zed255]

Make sure the quenching LED strip is working properly. I once ripped one of the LEDs off with the tip of my vacuum and had dev pulling too. The strip and is wired in series, so removing one element kills the whole thing