In sci.electronics.repair John Robertson wrote:

If your machine engine is anything like the Laserjet 4/5 I have those
service manuals and can send them to you - at least you would have an
idea about how the fuser is put together on another model...

I don't see how those would be close, the 4's and 5's were single toner
cartridge mono printers and the 3600n is a 4 cart color. It's easily 2x
taller and much more complex with the paper path and toner loading.

I don't have a manual either but I beleive on a 3600 the fuser is a user
replacement part. There is some way of getting the old one out and new one
in without disassembly of the printer itself. It looks like a toner
cartridge, a little smaller.

Even if the OP thinks it's getting warm, it still is likely to be defective.
Those things, if they don't get to temp within a period of time, it'll show
as a failure, too much current draw from it coming on and off too rapidly is
also shown as a failure.

If it's showing a fuser failure on the lcd panel, the best move is to
replace it and see what happens. Personally I think the OP sort of made a
mistake trying to get it going via the purchase of the toner carts. If it
was the other way around, had good toner carts but a fuser error, it would
of been worth the risk.

Those printers don't get better with age. Even if a replacement fuser is
around $100(USD), likely it'll start having paper feed problems because of
the rollers and depending on the number of prints already on it, whatever
they use for the "drum" is going to start streaking or print bands.

It's not ancient, unless you call 10 years that, but those aren't your
mothers laserprinters either. I'm just saying all those (the 3500 and 3600)
are probably at the end of their lifespan now.

One thing with those that always drove me nuts is the fact that even if one
color cart (like yellow) runs out, the whole printer locks up until it's
replaced, even if you just want to do a black and white invoice. There is a
way around that, if you prepare it in advance, but then the printer lives in
this "test mode" which gets annoying.

One other comment about the carts he purchased, unless someone is looking
for them, they are worthless. I just tossed a 3500N last month for all the
above reasons and had 10 carts. 2 brand new in the box from HP direct, the 4
that were in the printer (half to 100% full) and 4 spent ones.

After checking with a half dozen "we buy old toner and ink cart" places, 3
of them offer nothing and didn't want any of them, two others offered
nothing but would send a label (recycle y'all) and only one place offered
nothing for the spent ones, 50 cents for the still usable ones and get this,
a whole dollar for the unopened factory ones.

-bruce