Oil pressure light on European Z400J four cyclinder.

Yep that screw just holds on the electrical connector. The actual oil pressure switch is threaded and unscrews.
www.z-power.co.uk/oil-pressure-switch650750

Change that poxy crimp connector for a proper soldered joint. 
As I said I don't know your model but that pressure switch appears to be screwed into the sump which kinda makes the rubber boot important to protect the connection from all the road crap. 

Don't you think it's odd that the electrical pole connection is missing ? Also it's weeping oil slightly which doesn't seem right.

No, 900 and 1000 have the pole type with the switch located under the cam chain tensioner. 650s have your type which on 650's is located behind the ignition plate on righthand side. On yours it's seems to use the 650 type but by the oil filter.
With the ignition on you should be able to touch that connector to the sump case to earth it out which will illuminate the idiot light. With disconnected the idiot light should be off. Doing this will test your wiring and idiot bulb. If it's the same result connected or not then you have a wiring issue also.
Where's it leaking from? The electrical connector thread or the switch to sump thread? 
Remember the switch needs to be earthed to the engine for it to work so you can't just use ptfe tape on the thread. 

Ok, disconnected the wire from the oil pressure switch and earthed it to the battery with a jump lead as I couldn't earth it to the sump and see the idiot light. Oil idiot light comes on when earthed with ignition on and goes out when not earthed. So wiring seems OK does this point to the switch then? Very slight oil weep from the hole that the screw goes into to hold the wire on.

If the wiring is OK it can only be the switch unless, God forbid, you have no oil pressure.
You'll need to drain the oil for either a new switch or check  out the old one. You know your next job!
To check out the old one test you have continuity between the switch body and the screw with the plunger resting and open circuit with the plunger depressed. Check for smooth action whilst checking. 

I'm pretty sure I have oil pressure as its leaking from the rockerbox cover. I had already looked on the Zedpower site and the switch is the thick end of thirty quid with delivery, which is a releif as I was worried it was going to be something cheap instead

wrote:

Thoughts please Gentlemen.

 

The oil pressure switch discussion seems well in hand, so...

I'd like to draw the group's attention to the safety wire on the fastener on the left. Is that the only piece so equipped or are there others?

EDIT: now that my brain is working, I realized that fastener is your sump plug. (The copper washer helped.) Still, not everyone safety wires these things.



 

wrote:

wrote:

Thoughts please Gentlemen.



 

The oil pressure switch discussion seems well in hand, so...

I'd like to draw the group's attention to the safety wire on the fastener on the left. Is that the only piece so equipped or are there others?

EDIT: now that my brain is working, I realized that fastener is your sump plug. (The copper washer helped.) Still, not everyone safety wires these things.

 

              Yes I did notice the wired sump plug after I had bought the bike but I'm not too concerned as it's not the sort of bike you would use for racing. I's the only bolt that has been wired. I just put it down to an enthusiastic DIY er who got bored on a rainy day but I don't suppose I'll ever know why.. If I can find a suitable recepticle for the oil I plan on taking the pressure switch out and having a root around inside it. It'll save me £30 if i can get it going again. Anyone have an exploded diagram?

wrote:

ebay.de/itm/185104307972

 

               Thanks for posting. Is that your ebay listing? There are similar but not the same listings on UK ebay but one of the exact model is always good. I suppose the engines will be same despite the bodywork.

The actual service manual likely has a different cover than the assembly and prep manual. I suspect the service manual covers far more topics, and yes, get one that covers your exact model and year.

I do not recall which model and year you have, but while you are trolling the internet for a service manual, you might take a peek at this one in case it covers yours.

www.kzrider.com/modules/ServiceManuals/Kz400500550_79to83.pdf

P.S. I wonder why somebody went to the trouble of safety wiring that abused drain plug?

wrote:

The actual service manual likely has a different cover than the assembly and prep manual. I suspect the service manual covers far more topics, and yes, get one that covers your exact model and year.

I do not recall which model and year you have, but while you are trolling the internet for a service manual, you might take a peek at this one in case it covers yours.

www.kzrider.com/modules/ServiceManuals/Kz400500550_79to83.pdf

Registration docs say it's a 1983 registered four cyclinder which I dont think the US got. Worst case I can think of about the drain plug is the threads are stripped and it's wired to stop it falling out!