Morning all, another issue with my 3.5 EFI V8 - this time the oil pressure light is refusing to work at all. It doesn’t come on any time.

I’ve swapped the pressure switch, no joy. Also checked the bulb in the dash, it’s not blown. I’ve wiggled the plugs at the back of the binnacle as well, to no avail.

Are there any other obvious options I’ve missed or does this sound like there’s a broken wire somewhere?

Thanks

I'd guess a broken/corroded wire and/or connection somewhere

The switch has a single live when ignition on wire going to it & acts as an earth when closed to complete the circuit.
When engine running the switch contacts are held apart by the pressure & the light is out.

Take the wire off the switch & with the ignition on touch the wire to a good earth point on the engine (or direct to battery earth terminal with a long test lead - which could be a long length of wire with a small crocodile clip at each end).
See if the light comes on.
If it doesn't work your way back through the system until you find a point where it does.

You could use a mutimeter to see if you have power into the bulbholder, again don't forget to have the ignition on.

ETA. Did you check the bulb by connection to a battery? If not & it was just a visual check of the filament then check with a battery.

Thanks Ratae, good gen once again! I tested the little bulb and it does work, wasn’t just a visual check.

I’ll get on with the rest of it after I’m off shift tomorrow.

Well... it wouldn’t be the easy option, would it?

Testing with a multimeter between the oil px switch plug and earth, the voltage is showing anywhere between 8 and 10 volts (battery is fully charged).

Using a test bulb in the same position results in negative lighting of said bulb! So obviously the supply to the switch is duff.

I’ve traced the brown and white wire back from the px switch to the barrel connector plug that lives near the front left wing, the plug is all fine - the pin output is the same as the final plug output. The wire then continues on back into the area behind the bonnet...

I’ve got a few questions -

Does anyone know where the wire disappears to after that and if there is any easy (ish) way to access it?

After spending days taking my Defender’s dash completely apart chasing a short recently I’m not particularly keen on repeating the experience so soon into my Classic RR ownership experience - would anyone know where I could run a new wire from and to in order to get this light working again.

Many thanks in advance.

I don't know if this is any good to you but the OP light ceased to illuminate when switching on the ignition of my '87 3.5 efi.
Renewing the pressure switch in the block failed to cure the problem & further investigation revealed a worn contact on the back of the board where the bulb makes contact when inserted & twisted to lock into place. A little judicious soldering on the contacts of the bulb (to improve the contact) results in a light that works most of the time.
In my case I imagine a full cure would entail complete strip-down of the binnacle, something I'm not up for

What voltage shows at the socket of the dash pressure warning bulb?

To access the underdash wiring remove Panel 1 in this link.
http://new.lrcat.com/#!/1230/77150/77151/6138
Screws accessible by putting your head in the driver's footwell & looking up. The dimmer switch is attached to the panel but has a decent length wire & a connector so you can detach & remove the panel altogether.
Usually takes me a couple of minutes.