Tacho oil pressure light

[john milner]

This is regarding a 2003 K series 1.8 VVC.

I am going to fit a 20PSI low pressure switch. I already have a Stack mechanical oil pressure gauge so the pressure light in the tacho (Caerbont) currently does nothing. I know I could fit an additional lamp but if possible for neatness I would like to make use of the original one in the tacho. I have been through several wiring diagrams and I cannot figure out how to do it.

The redundant gauge plug has green, black and white/brown wires. I think the W/N is to the old sensor and the green and black are for power. That would mean the W/N must somehow connect to the tacho via something else (e.g. ECU).

I also note that the Caterham wiring diagram incorrectly states GB rather than WN.

Can it be done easily without changing voltages etc?

[Jonathan Kay]

The redundant gauge plug has green, black and white/brown wires.

Agree about Green and Black. Have you tried taking White/Brown to earth and to 12V and seeing what the light does?

What sort of electrical oil pressure sender do you now have installed? Is it earthed through the body or terminal? What is its resistance (with the feed disconnected) when the engine is stopped and when running?

Jonathan

PS: Standard function of Green/ Brown is "Switch to reverse lamp"!

[revilla]

Hi John,

Mine is also a 2003 K Series 1.8 VVC and you idea got me thinking, so I've had a look at the tacho wiring (in the publishes wiring diagram, not actually on the car yet). I can't tally up the wiring shown with what you are describing. Below is an extract from the wiring diagram showing the tachometer wiring. I've traced the wire colours and where they go to:

So as well as the green supply and black ground wires, there is a brown and yellow alternator warning light, white and black tacho signal, red and white illumination, green and white right indicator, green and red left indicator and blue and white main beam. The tacho doesn't actually have an oil pressure warning light. What it does have is a redundant shift light which was wired to the ECU on the Supersport. This is driven from a wire marked as black on the wiring diagram although I believe it should actually be black and blue, at least this is the colour of the wire it connects to in the engine loom. I believe that the output from ECU is a switched earth (in a Rover it is used to activate the relay for the air conditioning pump clutch), so you could wire the oil pressure switch from this wire to earth to light the "shift light" as an oil pressure warning.

I've never tried lighting this LED on mine so I can't really comment for sure but the thing that would worry me would be whether it would be bright and attention-grabbing enough for an oil pressure warning light; if you're going to fit such a warning you really want it to grab your attention so you can shut down within seconds.

Andrew

[john milner]

Thanks my memory must be going. I could have sworn that when I fitted the Stack gauge ten years ago that I lost a very low pressure oil light and the tacho was the only place I could think of with LEDs. I should have counted the LEDs before asking a stupid question. My next idea was to use the gear change light. I know it is not ideal but I'm not a racer and something is better than nothing. It will be interesting to see if I can ever get it to flash when cornering.

[Richard Price]

I've used the redundant shift light in the tacho as a low oil pressure warning light, however, it's a long time since I did it, and don't remember the detail.

one thing that I do recall, is that I needed to use a relay to swap the mode! Without the relay, the light was on when the pressure was healthy, and off when the pressure was low.....

[john milner]

Thanks. I would have thought the switch would be closed with no/low pressure and open with the right amount of pressure. To need a relay to reverse the operation this may be the other way around.

I will need to cut the change light wire and see if it connects to + or -. If as I now suspect the gear change light is designed to operate with + from the ECU then this could be a pain to wire and may explain the need for a relay or alternatively a pressure switch with two terminals.